[v2] Improve ptrace-error detection on Linux targets

  Changes from v1:

- Addressed Pedro's comments re. internal organization and gdbserver
  support.

- Addressed Eli's comments (doc fixes).

- New ways of detecting what's wrong with ptrace.



In Fedora GDB, we carry the following patch:

  https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-attach-fail-reasons-5of5.patch

Its purpose is to try to detect a specific scenario where SELinux's
'deny_ptrace' option is enabled, which prevents GDB from ptrace'ing in
order to debug the inferior (PTRACE_ATTACH and PTRACE_ME will fail
with EACCES in this case).

I like the idea of improving error detection and providing more
information to the user (a simple "Permission denied" can be really
frustrating), but I don't fully agree with the way the patch was
implemented: it makes GDB link against libselinux only for the sake of
consulting the 'deny_ptrace' setting, and then prints a warning if
ptrace failed and this setting is on.

My first thought (and attempt) was to make GDB print a generic warning
when a ptrace error happened; this message would just point the user
to our documentation, where she could find more information about
possible causes for the error (and try to diagnose/fix the problem).
This proved to be too simple, and I was convinced that it is actually
a good idea to go the extra kilometre and try to pinpoint the specific
problem (or problems) preventing ptrace from working, as well as
provide useful suggestions on how the user can fix things.

Here is the patch I came up with.  It implements a new function,
'linux_ptrace_restricted_fail_reason', which does a few things to
check what's wrong with ptrace:

  - It dlopen's "libselinux.so" and checks if the "deny_ptrace" option
    is enabled.

  - It reads the contents of "/proc/sys/kernel/yama/ptrace_scope" and
    checks if it's different than 0.

For each of these checks, if it succeeds, the user will see a message
informing about the restriction in place, and how it can be disabled.
For example, if "deny_ptrace" is enabled, the user will see:

  # gdb /bin/true
  ...
  Starting program: /usr/bin/true
  warning: Could not trace the inferior process.
  Error:
  warning: ptrace: Permission denied
  The SELinux 'deny_ptrace' option is enabled and preventing GDB
  from using 'ptrace'.  Please, disable it by executing (as root):

    setsebool deny_ptrace off

  During startup program exited with code 127.

In case "/proc/sys/kernel/yama/ptrace_scope" is > 0:

  # gdb /bin/true
  ...
  Starting program: /usr/bin/true
  warning: Could not trace the inferior process.
  Error:
  warning: ptrace: Operation not permitted
  The Linux kernel's Yama ptrace scope is in effect, which can prevent
  GDB from using 'ptrace'.  Please, disable it by executing (as root):

    echo 0 > /proc/sys/kernel/yama/ptrace_scope

  During startup program exited with code 127.

If both restrictions are enabled, both messages will show up.

This works for gdbserver as well, and actually fixes a latent bug I
found: when ptrace is restricted, gdbserver would hang due to an
unchecked ptrace call:

  # gdbserver :9988 /bin/true
  gdbserver: linux_ptrace_test_ret_to_nx: Cannot PTRACE_TRACEME: Operation not permitted
  gdbserver: linux_ptrace_test_ret_to_nx: status 256 is not WIFSTOPPED!
  gdbserver: linux_ptrace_test_ret_to_nx: failed to kill child pid 2668100 No such process
  [ Here you would have to issue a C-c ]

Now, you will see:

  # gdbserver :9988 /bin/true
  gdbserver: linux_ptrace_test_ret_to_nx: Cannot PTRACE_TRACEME: Operation not permitted
  gdbserver: linux_ptrace_test_ret_to_nx: status 256 is not WIFSTOPPED!
  gdbserver: linux_ptrace_test_ret_to_nx: failed to kill child pid 2668100 No such process
  gdbserver: Could not trace the inferior process.
  Error:
  gdbserver: ptrace: Operation not permitted
  The Linux kernel's Yama ptrace scope is in effect, which can prevent
  GDB from using 'ptrace'.  Please, disable it by executing (as root):

    echo 0 > /proc/sys/kernel/yama/ptrace_scope

  linux_check_ptrace_features: waitpid: unexpected status 32512.
  Exiting

(I decided to keep all the other messages, even though I find them a
bit distracting).

If GDB can't determine the cause for the failure, it will still print
the generic error message which tells the user to check our
documentation:

  There might be restrictions preventing ptrace from working.  Please see
  the appendix "Linux kernel ptrace restrictions" in the GDB documentation
  for more details.

This means that the patch expands our documentation and creates a new
appendix section named "Linux kernel ptrace restrictions", with
sub-sections for each possible restriction that might be in place.

The current list of possible restrictions is:

  - SELinux's 'deny_ptrace' option (detected).

  - YAMA's /proc/sys/kernel/yama/ptrace_scope setting (detected).

  - seccomp on Docker containers (I couldn't find how to detect).

It's important to mention that all of this is Linux-specific; as far
as I know, SELinux, YAMA and seccomp are Linux-only features.

I tested this patch locally, on my Fedora 30 machine (actually, a
Fedora Rawhide VM), but I'm not proposing a testcase for it because of
the difficulty of writing one.

WDYT?

gdb/doc/ChangeLog:
yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (Linux kernel ptrace restrictions): New appendix
	section.

gdb/ChangeLog:
yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>
	    Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdbsupport/gdb-dlfcn.c (gdb_dlopen): Add argument 'dont_throw'
	Don't throw if it's true.
	* gdbsupport/gdb-dlfcn.h (gdb_dlopen): Add optional argument
	'dont_throw'.  Update comment.
	* inf-ptrace.c (default_inf_ptrace_me_fail_reason): New
	function.
	(inf_ptrace_me_fail_reason): New variable.
	(inf_ptrace_me): Update call to 'trace_start_error_with_name'.
	* inf-ptrace.h (inf_ptrace_me_fail_reason): New variable.
	* linux-nat.c (linux_nat_target::attach): Update call to
	'linux_ptrace_attach_fail_reason'.
	(_initialize_linux_nat): Set 'inf_ptrace_me_fail_reason'.
	* linux-nat.h (linux_nat_target) <ptrace_me_fail_reason>: New
	method.
	* nat/fork-inferior.c (trace_start_error_with_name): Add
	optional 'append' argument.
	* nat/fork-inferior.h (trace_start_error_with_name): Update
	prototype.
	* nat/linux-ptrace.c: Include "gdbsupport/gdb-dlfcn.h" and
	"nat/fork-inferior.h".
	(selinux_ftype): New typedef.
	(linux_ptrace_restricted_fail_reason): New function.
	(linux_ptrace_attach_fail_reason): Add optional 'err'
	argument.  Call 'linux_ptrace_restricted_fail_reason'.
	(linux_ptrace_me_fail_reason): New function.
	(linux_child_function): Handle ptrace error.
	* nat/linux-ptrace.h (linux_ptrace_attach_fail_reason): Update
	prototype.
	(linux_ptrace_me_fail_reason): New function.
	* target-delegates.c: Regenerate.
	* target.h (struct target_ops) <ptrace_me_fail_reason>: New
	method.
	(target_ptrace_me_fail_reason): New define.

gdb/gdbserver/ChangeLog:
yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>

	* linux-low.c (linux_ptrace_fun): Call
	'linux_ptrace_me_fail_reason'.
---
 gdb/doc/gdb.texinfo        | 138 +++++++++++++++++++++++++++++++++++++
 gdb/gdbserver/linux-low.c  |   3 +-
 gdb/gdbsupport/gdb-dlfcn.c |   4 +-
 gdb/gdbsupport/gdb-dlfcn.h |   7 +-
 gdb/inf-ptrace.c           |  19 ++++-
 gdb/inf-ptrace.h           |  10 +++
 gdb/linux-nat.c            |   7 +-
 gdb/nat/fork-inferior.c    |   4 +-
 gdb/nat/fork-inferior.h    |   7 +-
 gdb/nat/linux-ptrace.c     |  86 +++++++++++++++++++++--
 gdb/nat/linux-ptrace.h     |  15 +++-
 11 files changed, 282 insertions(+), 18 deletions(-)
  

On Mon, Aug 26, 2019 at 1:32 PM Sergio Durigan Junior
<sergiodj@redhat.com> wrote:
> @@ -30,11 +32,70 @@
>     of 0 means there are no supported features.  */
>  static int supported_ptrace_options = -1;
>
> -/* Find all possible reasons we could fail to attach PID and return these
> -   as a string.  An empty string is returned if we didn't find any reason.  */
> +typedef int (*selinux_ftype) (const char *);
> +
> +/* Helper function which checks if ptrace is probably restricted
> +   (i.e., if ERR is either EACCES or EPERM), and returns a string with
> +   possible workarounds.  */
> +
> +static std::string
> +linux_ptrace_restricted_fail_reason (int err)
> +{
> +  if (err != EACCES && err != EPERM)
> +    {
> +      /* It just makes sense to perform the checks below if errno was
> +        either EACCES or EPERM.  */
> +      return {};
> +    }
> +
> +  std::string ret;
> +  gdb_dlhandle_up handle = gdb_dlopen ("libselinux.so", true);

I would dlopen libselinux.so.1 instead so that this works even without
the -dev package.

> +
> +  if (handle.get () != NULL)
> +    {
> +      selinux_ftype selinux_get_bool
> +       = (selinux_ftype) gdb_dlsym (handle, "security_get_boolean_active");
> +
> +      if (selinux_get_bool != NULL
> +         && (*selinux_get_bool) ("deny_ptrace") == 1)
> +       string_appendf (ret,
> +                       _("\n\
> +The SELinux 'deny_ptrace' option is enabled and preventing GDB\n\
> +from using 'ptrace'.  Please, disable it by executing (as root):\n\
> +\n\
> +  setsebool deny_ptrace off\n"));
> +    }
> +
> +  FILE *f = fopen ("/proc/sys/kernel/yama/ptrace_scope", "r");
> +
> +  if (f != NULL)
> +    {
> +      char yama_scope = fgetc (f);
> +
> +      fclose (f);
> +
> +      if (yama_scope != '0')
> +       string_appendf (ret,
> +                       _("\n\
> +The Linux kernel's Yama ptrace scope is in effect, which can prevent\n\
> +GDB from using 'ptrace'.  Please, disable it by executing (as root):\n\
> +\n\
> +  echo 0 > /proc/sys/kernel/yama/ptrace_scope\n"));
> +    }
> +
> +  if (ret.empty ())
> +    ret = _("\n\
> +There might be restrictions preventing ptrace from working.  Please see\n\
> +the appendix \"Linux kernel ptrace restrictions\" in the GDB documentation\n\
> +for more details.\n");
> +
> +  return ret;
> +}
> +
> +/* See declaration in linux-ptrace.h.  */
>
>  std::string
> -linux_ptrace_attach_fail_reason (pid_t pid)
> +linux_ptrace_attach_fail_reason (pid_t pid, int err)
>  {
>    pid_t tracerpid = linux_proc_get_tracerpid_nowarn (pid);
>    std::string result;
> @@ -50,6 +111,11 @@ linux_ptrace_attach_fail_reason (pid_t pid)
>                       "terminated"),
>                     (int) pid);
>
> +  std::string ptrace_restrict = linux_ptrace_restricted_fail_reason (err);
> +
> +  if (!ptrace_restrict.empty ())
> +    result += "\n" + ptrace_restrict;
> +
>    return result;
>  }
>
> @@ -68,6 +134,14 @@ linux_ptrace_attach_fail_reason_string (ptid_t ptid, int err)
>      return string_printf ("%s (%d)", safe_strerror (err), err);
>  }
>
> +/* See linux-ptrace.h.  */
> +
> +std::string
> +linux_ptrace_me_fail_reason (int err)
> +{
> +  return linux_ptrace_restricted_fail_reason (err);
> +}
> +
>  #if defined __i386__ || defined __x86_64__
>
>  /* Address of the 'ret' instruction in asm code block below.  */
> @@ -321,7 +395,11 @@ linux_grandchild_function (void *child_stack)
>  static int
>  linux_child_function (void *child_stack)
>  {
> -  ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) 0, (PTRACE_TYPE_ARG4) 0);
> +  if (ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) 0,
> +             (PTRACE_TYPE_ARG4) 0) != 0)
> +    trace_start_error_with_name ("ptrace",
> +                                linux_ptrace_me_fail_reason (errno).c_str ());
> +
>    kill (getpid (), SIGSTOP);
>
>    /* Fork a grandchild.  */
> diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
> index fd2f12a342..04ada53bf6 100644
> --- a/gdb/nat/linux-ptrace.h
> +++ b/gdb/nat/linux-ptrace.h
> @@ -176,13 +176,26 @@ struct buffer;
>  # define TRAP_HWBKPT 4
>  #endif
>
> -extern std::string linux_ptrace_attach_fail_reason (pid_t pid);
> +/* Find all possible reasons we could fail to attach PID and return
> +   these as a string.  An empty string is returned if we didn't find
> +   any reason.  If ERR is EACCES or EPERM, we also add a warning about
> +   possible restrictions to use ptrace.  */
> +extern std::string linux_ptrace_attach_fail_reason (pid_t pid, int err = -1);
>
>  /* Find all possible reasons we could have failed to attach to PTID
>     and return them as a string.  ERR is the error PTRACE_ATTACH failed
>     with (an errno).  */
>  extern std::string linux_ptrace_attach_fail_reason_string (ptid_t ptid, int err);
>
> +/* When the call to 'ptrace (PTRACE_TRACEME...' fails, and we have
> +   already forked, this function can be called in order to try to
> +   obtain the reason why ptrace failed.  ERR should be the ERRNO value
> +   returned by ptrace.
> +
> +   This function will return a 'std::string' containing the fail
> +   reason, or an empty string otherwise.  */
> +extern std::string linux_ptrace_me_fail_reason (int err);
> +
>  extern void linux_ptrace_init_warnings (void);
>  extern void linux_check_ptrace_features (void);
>  extern void linux_enable_event_reporting (pid_t pid, int attached);
> --
> 2.21.0
>
  

> From: Sergio Durigan Junior <sergiodj@redhat.com>
> Cc: Pedro Alves <palves@redhat.com>,
> 	Eli Zaretskii <eliz@gnu.org>,
> 	Ruslan Kabatsayev <b7.10110111@gmail.com>,
> 	Sergio Durigan Junior <sergiodj@redhat.com>
> Date: Mon, 26 Aug 2019 14:32:05 -0400
> 
> gdb/doc/ChangeLog:
> yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>
> 
> 	* gdb.texinfo (Linux kernel ptrace restrictions): New appendix
> 	section.

OK for this part.

Thanks.
  

On Monday, August 26 2019, Christian Biesinger wrote:

> On Mon, Aug 26, 2019 at 1:32 PM Sergio Durigan Junior
> <sergiodj@redhat.com> wrote:
>> @@ -30,11 +32,70 @@
>>     of 0 means there are no supported features.  */
>>  static int supported_ptrace_options = -1;
>>
>> -/* Find all possible reasons we could fail to attach PID and return these
>> -   as a string.  An empty string is returned if we didn't find any reason.  */
>> +typedef int (*selinux_ftype) (const char *);
>> +
>> +/* Helper function which checks if ptrace is probably restricted
>> +   (i.e., if ERR is either EACCES or EPERM), and returns a string with
>> +   possible workarounds.  */
>> +
>> +static std::string
>> +linux_ptrace_restricted_fail_reason (int err)
>> +{
>> +  if (err != EACCES && err != EPERM)
>> +    {
>> +      /* It just makes sense to perform the checks below if errno was
>> +        either EACCES or EPERM.  */
>> +      return {};
>> +    }
>> +
>> +  std::string ret;
>> +  gdb_dlhandle_up handle = gdb_dlopen ("libselinux.so", true);
>
> I would dlopen libselinux.so.1 instead so that this works even without
> the -dev package.

Ah, right, I forgot about this.  I fixed it in my local copy, thanks for
pointing it out.
  

Hi Sergio,

This is looking quite good to me.  Some comments below.

On 8/26/19 7:32 PM, Sergio Durigan Junior wrote:
> Changes from v1:
> 
> - Addressed Pedro's comments re. internal organization and gdbserver
>   support.
> 
> - Addressed Eli's comments (doc fixes).
> 
> - New ways of detecting what's wrong with ptrace.
> 
> 
> 
> In Fedora GDB, we carry the following patch:
> 
>   https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-attach-fail-reasons-5of5.patch

This link will soon be dead, right?  (thinking about the commit log)
Maybe point at some reference other than master.

> Its purpose is to try to detect a specific scenario where SELinux's
> 'deny_ptrace' option is enabled, which prevents GDB from ptrace'ing in
> order to debug the inferior (PTRACE_ATTACH and PTRACE_ME will fail
> with EACCES in this case).
> 
> I like the idea of improving error detection and providing more
> information to the user (a simple "Permission denied" can be really
> frustrating), but I don't fully agree with the way the patch was
> implemented: it makes GDB link against libselinux only for the sake of
> consulting the 'deny_ptrace' setting, and then prints a warning if
> ptrace failed and this setting is on.
> 
> My first thought (and attempt) was to make GDB print a generic warning
> when a ptrace error happened; this message would just point the user
> to our documentation, where she could find more information about
> possible causes for the error (and try to diagnose/fix the problem).
> This proved to be too simple, and I was convinced that it is actually
> a good idea to go the extra kilometre and try to pinpoint the specific
> problem (or problems) preventing ptrace from working, as well as
> provide useful suggestions on how the user can fix things.
> 
> Here is the patch I came up with.  It implements a new function,
> 'linux_ptrace_restricted_fail_reason', which does a few things to
> check what's wrong with ptrace:
> 
>   - It dlopen's "libselinux.so" and checks if the "deny_ptrace" option
>     is enabled.

Update reference to "libselinux.so.1" here too.

> 
>   - It reads the contents of "/proc/sys/kernel/yama/ptrace_scope" and
>     checks if it's different than 0.
> 
> For each of these checks, if it succeeds, the user will see a message
> informing about the restriction in place, and how it can be disabled.
> For example, if "deny_ptrace" is enabled, the user will see:
> 
>   # gdb /bin/true
>   ...
>   Starting program: /usr/bin/true
>   warning: Could not trace the inferior process.
>   Error:
>   warning: ptrace: Permission denied

Curious, that:

 warning:
 Error:
 warning:

looks a bit odd, specifically the "Error:" line.  Do you know where is
that coming from?

>   The SELinux 'deny_ptrace' option is enabled and preventing GDB
>   from using 'ptrace'.  Please, disable it by executing (as root):

I'm really not a fan of these "Please, ".  Kind of sounds like
gdb is begging, to me.  I'd rather use an informational tone, like:

   The SELinux 'deny_ptrace' option is enabled and preventing GDB
   from using 'ptrace'.  You can disable it by executing (as root):

> 
>     setsebool deny_ptrace off
> 
>   During startup program exited with code 127.
> 
> In case "/proc/sys/kernel/yama/ptrace_scope" is > 0:
> 
>   # gdb /bin/true
>   ...
>   Starting program: /usr/bin/true
>   warning: Could not trace the inferior process.
>   Error:
>   warning: ptrace: Operation not permitted
>   The Linux kernel's Yama ptrace scope is in effect, which can prevent
>   GDB from using 'ptrace'.  Please, disable it by executing (as root):

Ditto.

> 
>     echo 0 > /proc/sys/kernel/yama/ptrace_scope
> 
>   During startup program exited with code 127.
> 
> If both restrictions are enabled, both messages will show up.
> 
> This works for gdbserver as well, and actually fixes a latent bug I
> found: when ptrace is restricted, gdbserver would hang due to an
> unchecked ptrace call:
> 
>   # gdbserver :9988 /bin/true
>   gdbserver: linux_ptrace_test_ret_to_nx: Cannot PTRACE_TRACEME: Operation not permitted
>   gdbserver: linux_ptrace_test_ret_to_nx: status 256 is not WIFSTOPPED!
>   gdbserver: linux_ptrace_test_ret_to_nx: failed to kill child pid 2668100 No such process
>   [ Here you would have to issue a C-c ]
> 
> Now, you will see:
> 
>   # gdbserver :9988 /bin/true
>   gdbserver: linux_ptrace_test_ret_to_nx: Cannot PTRACE_TRACEME: Operation not permitted
>   gdbserver: linux_ptrace_test_ret_to_nx: status 256 is not WIFSTOPPED!
>   gdbserver: linux_ptrace_test_ret_to_nx: failed to kill child pid 2668100 No such process
>   gdbserver: Could not trace the inferior process.
>   Error:
>   gdbserver: ptrace: Operation not permitted
>   The Linux kernel's Yama ptrace scope is in effect, which can prevent
>   GDB from using 'ptrace'.  Please, disable it by executing (as root):
> 
>     echo 0 > /proc/sys/kernel/yama/ptrace_scope
> 
>   linux_check_ptrace_features: waitpid: unexpected status 32512.
>   Exiting
> 
> (I decided to keep all the other messages, even though I find them a
> bit distracting).

Yeah, the other messages are implementor-speak, showing gdb function
names.  We should ideally clean this all up.

> 
> If GDB can't determine the cause for the failure, it will still print
> the generic error message which tells the user to check our
> documentation:
> 
>   There might be restrictions preventing ptrace from working.  Please see
>   the appendix "Linux kernel ptrace restrictions" in the GDB documentation
>   for more details.
> 
> This means that the patch expands our documentation and creates a new
> appendix section named "Linux kernel ptrace restrictions", with
> sub-sections for each possible restriction that might be in place.
> 
> The current list of possible restrictions is:
> 
>   - SELinux's 'deny_ptrace' option (detected).
> 
>   - YAMA's /proc/sys/kernel/yama/ptrace_scope setting (detected).
> 
>   - seccomp on Docker containers (I couldn't find how to detect).
> 
> It's important to mention that all of this is Linux-specific; as far
> as I know, SELinux, YAMA and seccomp are Linux-only features.
> 
> I tested this patch locally, on my Fedora 30 machine (actually, a
> Fedora Rawhide VM), but I'm not proposing a testcase for it because of
> the difficulty of writing one.
> 
> WDYT?
> 
> gdb/doc/ChangeLog:
> yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>
> 
> 	* gdb.texinfo (Linux kernel ptrace restrictions): New appendix
> 	section.
> 
> gdb/ChangeLog:
> yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>
> 	    Jan Kratochvil  <jan.kratochvil@redhat.com>
> 
> 	* gdbsupport/gdb-dlfcn.c (gdb_dlopen): Add argument 'dont_throw'
> 	Don't throw if it's true.
> 	* gdbsupport/gdb-dlfcn.h (gdb_dlopen): Add optional argument
> 	'dont_throw'.  Update comment.
> 	* inf-ptrace.c (default_inf_ptrace_me_fail_reason): New
> 	function.
> 	(inf_ptrace_me_fail_reason): New variable.
> 	(inf_ptrace_me): Update call to 'trace_start_error_with_name'.
> 	* inf-ptrace.h (inf_ptrace_me_fail_reason): New variable.
> 	* linux-nat.c (linux_nat_target::attach): Update call to
> 	'linux_ptrace_attach_fail_reason'.
> 	(_initialize_linux_nat): Set 'inf_ptrace_me_fail_reason'.
> 	* linux-nat.h (linux_nat_target) <ptrace_me_fail_reason>: New
> 	method.
> 	* nat/fork-inferior.c (trace_start_error_with_name): Add
> 	optional 'append' argument.
> 	* nat/fork-inferior.h (trace_start_error_with_name): Update
> 	prototype.
> 	* nat/linux-ptrace.c: Include "gdbsupport/gdb-dlfcn.h" and
> 	"nat/fork-inferior.h".
> 	(selinux_ftype): New typedef.
> 	(linux_ptrace_restricted_fail_reason): New function.
> 	(linux_ptrace_attach_fail_reason): Add optional 'err'
> 	argument.  Call 'linux_ptrace_restricted_fail_reason'.
> 	(linux_ptrace_me_fail_reason): New function.
> 	(linux_child_function): Handle ptrace error.
> 	* nat/linux-ptrace.h (linux_ptrace_attach_fail_reason): Update
> 	prototype.
> 	(linux_ptrace_me_fail_reason): New function.
> 	* target-delegates.c: Regenerate.
> 	* target.h (struct target_ops) <ptrace_me_fail_reason>: New
> 	method.
> 	(target_ptrace_me_fail_reason): New define.
> 
> gdb/gdbserver/ChangeLog:
> yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>
> 
> 	* linux-low.c (linux_ptrace_fun): Call
> 	'linux_ptrace_me_fail_reason'.
> ---
>  gdb/doc/gdb.texinfo        | 138 +++++++++++++++++++++++++++++++++++++
>  gdb/gdbserver/linux-low.c  |   3 +-
>  gdb/gdbsupport/gdb-dlfcn.c |   4 +-
>  gdb/gdbsupport/gdb-dlfcn.h |   7 +-
>  gdb/inf-ptrace.c           |  19 ++++-
>  gdb/inf-ptrace.h           |  10 +++
>  gdb/linux-nat.c            |   7 +-
>  gdb/nat/fork-inferior.c    |   4 +-
>  gdb/nat/fork-inferior.h    |   7 +-
>  gdb/nat/linux-ptrace.c     |  86 +++++++++++++++++++++--
>  gdb/nat/linux-ptrace.h     |  15 +++-
>  11 files changed, 282 insertions(+), 18 deletions(-)
> 
> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
> index e1bc8143e6..43f749c6f1 100644
> --- a/gdb/doc/gdb.texinfo
> +++ b/gdb/doc/gdb.texinfo
> @@ -182,6 +182,9 @@ software in general.  We will miss him.
>                                  @value{GDBN}
>  * Operating System Information:: Getting additional information from
>                                   the operating system
> +* Linux kernel @code{ptrace} restrictions:: Restrictions sometimes
> +                                            imposed by the Linux
> +                                            kernel on @code{ptrace}
>  * Trace File Format::		GDB trace file format
>  * Index Section Format::        .gdb_index section format
>  * Man Pages::			Manual pages
> @@ -44682,6 +44685,141 @@ should contain a comma-separated list of cores that this process
>  is running on.  Target may provide additional columns,
>  which @value{GDBN} currently ignores.
>  
> +@node Linux kernel ptrace restrictions
> +@appendix Linux kernel @code{ptrace} restrictions
> +@cindex linux kernel ptrace restrictions, attach
> +
> +The @code{ptrace} system call is used by @value{GDBN} on GNU/Linux to,
> +among other things, attach to a new or existing inferior in order to
> +start debugging it.  Due to security concerns, some distributions and
> +vendors disable or severily restrict the ability to perform these

typo: severily -> severely


> +operations, which can make @value{GDBN} malfunction.  In this section,
> +we will expand on how this malfunction can manifest itself, and how to
> +modify the system's settings in order to be able to use @value{GDBN}
> +properly.
> +
> +@menu
> +* The GDB error message::               The error message displayed when the
> +                                        system prevents @value{GDBN} from using
> +                                        @code{ptrace}
> +* SELinux's @code{deny_ptrace}::        SELinux and the @code{deny_ptrace} option
> +* Yama's @code{ptrace_scope}::          Yama and the @code{ptrace_scope} setting
> +* Docker and @code{seccomp}::           Docker and the @code{seccomp}
> +                                        infrastructure
> +@end menu
> +
> +@node The GDB error message
> +@appendixsection The @value{GDBN} error message
> +
> +When the system prevents @value{GDBN} from using the @code{ptrace}
> +system call, you will likely see a descriptive error message
> +explaining what is wrong and how to attempt to fix the problem.  For
> +example, when SELinux's @code{deny_ptrace} option is enabled, you can
> +see:
> +
> +@smallexample
> +$ gdb program
> +...
> +(@value{GDBP}) run
> +Starting program: program
> +warning: Could not trace the inferior process.
> +Error:
> +warning: ptrace: Permission denied
> +The SELinux 'deny_ptrace' option is enabled and preventing GDB
> +from using 'ptrace'.  Please, disable it by executing (as root):
> +
> +  setsebool deny_ptrace off
> +
> +During startup program exited with code 127.
> +(@value{GDBP})
> +@end smallexample
> +
> +Sometimes, it may not be possible to acquire the necessary data to
> +determine the root cause of the failure.  In this case, you will see a
> +generic error message pointing you to this section:
> +
> +@smallexample
> +$ gdb program
> +...
> +Starting program: program
> +warning: Could not trace the inferior process.
> +Error:
> +warning: ptrace: Permission denied
> +There might be restrictions preventing ptrace from working.  Please see
> +the appendix "Linux kernel ptrace restrictions" in the GDB documentation
> +for more details.
> +During startup program exited with code 127.
> +(@value{GDBP})
> +@end smallexample
> +
> +@node SELinux's deny_ptrace
> +@appendixsection SELinux's @code{deny_ptrace}
> +@cindex SELinux
> +@cindex deny_ptrace
> +
> +If you are using SELinux, you might want to check whether the
> +@code{deny_ptrace} option is enabled by doing:
> +
> +@smallexample
> +$ getsebool deny_ptrace
> +deny_ptrace --> on
> +@end smallexample
> +
> +If the option is enabled, you can disable it by doing, as root:
> +
> +@smallexample
> +# setsebool deny_ptrace off
> +@end smallexample
> +
> +The option will be disabled until the next reboot.  If you would like
> +to disable it permanently, you can do (as root):
> +
> +@smallexample
> +# setsebool -P deny_ptrace off
> +@end smallexample
> +
> +@node Yama's ptrace_scope
> +@appendixsection Yama's @code{ptrace_scope}
> +@cindex yama, ptrace_scope
> +
> +If your system has Yama enabled, you might want to check whether the
> +@code{ptrace_scope} setting is enabled by checking the value of
> +@file{/proc/sys/kernel/yama/ptrace_scope}:
> +
> +@smallexample
> +$ cat /proc/sys/kernel/yama/ptrace_scope
> +0
> +@end smallexample
> +
> +If you see anything other than @code{0}, @value{GDBN} can be affected
> +by it.  You can temporarily disable the feature by doing, as root:
> +
> +@smallexample
> +# sysctl kernel.yama.ptrace_scope=0
> +kernel.yama.ptrace_scope = 0
> +@end smallexample
> +
> +You can make this permanent by doing, as root:
> +
> +@smallexample
> +# sysctl -w kernel.yama.ptrace_scope=0
> +kernel.yama.ptrace_scope = 0
> +@end smallexample
> +
> +@node Docker and seccomp
> +@appendixsection Docker and @code{seccomp}
> +@cindex docker, seccomp
> +
> +If you are using Docker (@uref{https://www.docker.com/}) containers,
> +you will probably have to disable its @code{seccomp} protections in
> +order to be able to use @value{GDBN}.  To do that, you can use the
> +options @code{--cap-add=SYS_PTRACE --security-opt seccomp=unconfined}
> +when invoking Docker:
> +
> +@smallexample
> +$ docker run --cap-add=SYS_PTRACE --security-opt seccomp=unconfined
> +@end smallexample
> +
>  @node Trace File Format
>  @appendix Trace File Format
>  @cindex trace file format
> diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
> index 3113017ae6..1e0a5cbf54 100644
> --- a/gdb/gdbserver/linux-low.c
> +++ b/gdb/gdbserver/linux-low.c
> @@ -971,7 +971,8 @@ linux_ptrace_fun ()
>  {
>    if (ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) 0,
>  	      (PTRACE_TYPE_ARG4) 0) < 0)
> -    trace_start_error_with_name ("ptrace");
> +    trace_start_error_with_name ("ptrace",
> +				 linux_ptrace_me_fail_reason (errno).c_str ());
>  
>    if (setpgid (0, 0) < 0)
>      trace_start_error_with_name ("setpgid");
> diff --git a/gdb/gdbsupport/gdb-dlfcn.c b/gdb/gdbsupport/gdb-dlfcn.c
> index 921f10f3d8..9e5a992c17 100644
> --- a/gdb/gdbsupport/gdb-dlfcn.c
> +++ b/gdb/gdbsupport/gdb-dlfcn.c
> @@ -58,7 +58,7 @@ is_dl_available (void)
>  #else /* NO_SHARED_LIB */
>  
>  gdb_dlhandle_up
> -gdb_dlopen (const char *filename)
> +gdb_dlopen (const char *filename, bool dont_throw)
>  {
>    void *result;
>  #ifdef HAVE_DLFCN_H
> @@ -66,7 +66,7 @@ gdb_dlopen (const char *filename)
>  #elif __MINGW32__
>    result = (void *) LoadLibrary (filename);
>  #endif
> -  if (result != NULL)
> +  if (dont_throw || result != NULL)
>      return gdb_dlhandle_up (result);
>  
>  #ifdef HAVE_DLFCN_H
> diff --git a/gdb/gdbsupport/gdb-dlfcn.h b/gdb/gdbsupport/gdb-dlfcn.h
> index 6a39d38941..a8ddbc03da 100644
> --- a/gdb/gdbsupport/gdb-dlfcn.h
> +++ b/gdb/gdbsupport/gdb-dlfcn.h
> @@ -32,10 +32,11 @@ struct dlclose_deleter
>  typedef std::unique_ptr<void, dlclose_deleter> gdb_dlhandle_up;
>  
>  /* Load the dynamic library file named FILENAME, and return a handle
> -   for that dynamic library.  Return NULL if the loading fails for any
> -   reason.  */
> +   for that dynamic library.  If the loading fails, return NULL if
> +   DONT_THROW is true, or throw an exception otherwise (default
> +   behaviour).  */
>  
> -gdb_dlhandle_up gdb_dlopen (const char *filename);
> +gdb_dlhandle_up gdb_dlopen (const char *filename, bool dont_throw = false);
>  
>  /* Return the address of the symbol named SYMBOL inside the shared
>     library whose handle is HANDLE.  Return NULL when the symbol could
> diff --git a/gdb/inf-ptrace.c b/gdb/inf-ptrace.c
> index 4a8e732373..b6cfd803cf 100644
> --- a/gdb/inf-ptrace.c
> +++ b/gdb/inf-ptrace.c
> @@ -94,6 +94,22 @@ inf_ptrace_target::remove_fork_catchpoint (int pid)
>  #endif /* PT_GET_PROCESS_STATE */
>  
>  
> +/* Default method for "inf_ptrace_me_fail_reason", which returns an
> +   empty string.  */
> +
> +static std::string
> +default_inf_ptrace_me_fail_reason (int err)
> +{
> +  return {};
> +}
> +
> +/* Point to "inf_ptrace_me_fail_reason", 

Did you mean "Pointer"?  This reads strange because 
"inf_ptrace_me_fail_reason" is the name of the pointer itself,
so how to point _to_ it?

which implements a function
> +   that can be called by "inf_ptrace_me" in order to obtain the reason
> +   for failure.  */
> +
> +std::string (*inf_ptrace_me_fail_reason) (int err)
> +  = default_inf_ptrace_me_fail_reason;
> +
>  /* Prepare to be traced.  */
>  
>  static void
> @@ -101,7 +117,8 @@ inf_ptrace_me (void)
>  {
>    /* "Trace me, Dr. Memory!"  */
>    if (ptrace (PT_TRACE_ME, 0, (PTRACE_TYPE_ARG3) 0, 0) < 0)
> -    trace_start_error_with_name ("ptrace");
> +    trace_start_error_with_name ("ptrace",
> +				 inf_ptrace_me_fail_reason (errno).c_str ());
>  }
>  
>  /* Start a new inferior Unix child process.  EXEC_FILE is the file to
> diff --git a/gdb/inf-ptrace.h b/gdb/inf-ptrace.h
> index 98b5d2e09e..7cdab9af89 100644
> --- a/gdb/inf-ptrace.h
> +++ b/gdb/inf-ptrace.h
> @@ -83,4 +83,14 @@ protected:
>  
>  extern pid_t get_ptrace_pid (ptid_t);
>  
> +/* Pointer to "inf_ptrace_me_fail_reason", which implements a function

Ah, here it's "pointer".  But still reads strange.  I think the comment
in the .c file should just say the usual "See inf-ptrace.h.".

> +   that can be called by "inf_ptrace_me" in order to obtain the reason
> +   for a ptrace failure.  ERR is the ERRNO value set by the failing
> +   ptrace call.
> +
> +   This pointer can be overriden by targets that want to personalize
> +   the error message printed when ptrace fails (see linux-nat.c, for
> +   example).  */
> +extern std::string (*inf_ptrace_me_fail_reason) (int err);
> +
>  #endif
> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
> index 945c19f666..b5a9eaf72e 100644
> --- a/gdb/linux-nat.c
> +++ b/gdb/linux-nat.c
> @@ -1191,8 +1191,9 @@ linux_nat_target::attach (const char *args, int from_tty)
>      }
>    catch (const gdb_exception_error &ex)
>      {
> +      int saved_errno = errno;
>        pid_t pid = parse_pid_to_attach (args);
> -      std::string reason = linux_ptrace_attach_fail_reason (pid);
> +      std::string reason = linux_ptrace_attach_fail_reason (pid, saved_errno);
>  
>        if (!reason.empty ())
>  	throw_error (ex.error, "warning: %s\n%s", reason.c_str (),
> @@ -4696,6 +4697,10 @@ Enables printf debugging output."),
>    sigemptyset (&blocked_mask);
>  
>    lwp_lwpid_htab_create ();
> +
> +  /* Set the proper function to generate a message when ptrace
> +     fails.  */
> +  inf_ptrace_me_fail_reason = linux_ptrace_me_fail_reason;
>  }
>  
>  
> diff --git a/gdb/nat/fork-inferior.c b/gdb/nat/fork-inferior.c
> index 68b51aa814..72ac623e20 100644
> --- a/gdb/nat/fork-inferior.c
> +++ b/gdb/nat/fork-inferior.c
> @@ -591,7 +591,7 @@ trace_start_error (const char *fmt, ...)
>  /* See nat/fork-inferior.h.  */
>  
>  void
> -trace_start_error_with_name (const char *string)
> +trace_start_error_with_name (const char *string, const char *append)
>  {
> -  trace_start_error ("%s: %s", string, safe_strerror (errno));
> +  trace_start_error ("%s: %s%s", string, safe_strerror (errno), append);
>  }
> diff --git a/gdb/nat/fork-inferior.h b/gdb/nat/fork-inferior.h
> index 1d0519fb26..7e6b889210 100644
> --- a/gdb/nat/fork-inferior.h
> +++ b/gdb/nat/fork-inferior.h
> @@ -98,9 +98,10 @@ extern void trace_start_error (const char *fmt, ...)
>    ATTRIBUTE_NORETURN ATTRIBUTE_PRINTF (1, 2);
>  
>  /* Like "trace_start_error", but the error message is constructed by
> -   combining STRING with the system error message for errno.  This
> -   function does not return.  */
> -extern void trace_start_error_with_name (const char *string)
> +   combining STRING with the system error message for errno, and
> +   (optionally) with APPEND.  This function does not return.  */
> +extern void trace_start_error_with_name (const char *string,
> +					 const char *append = "")
>    ATTRIBUTE_NORETURN;
>  
>  #endif /* NAT_FORK_INFERIOR_H */
> diff --git a/gdb/nat/linux-ptrace.c b/gdb/nat/linux-ptrace.c
> index c1ebc0a032..599d9cfb55 100644
> --- a/gdb/nat/linux-ptrace.c
> +++ b/gdb/nat/linux-ptrace.c
> @@ -21,6 +21,8 @@
>  #include "linux-procfs.h"
>  #include "linux-waitpid.h"
>  #include "gdbsupport/buffer.h"
> +#include "gdbsupport/gdb-dlfcn.h"
> +#include "nat/fork-inferior.h"
>  #ifdef HAVE_SYS_PROCFS_H
>  #include <sys/procfs.h>
>  #endif
> @@ -30,11 +32,70 @@
>     of 0 means there are no supported features.  */
>  static int supported_ptrace_options = -1;
>  
> -/* Find all possible reasons we could fail to attach PID and return these
> -   as a string.  An empty string is returned if we didn't find any reason.  */
> +typedef int (*selinux_ftype) (const char *);
> +
> +/* Helper function which checks if ptrace is probably restricted
> +   (i.e., if ERR is either EACCES or EPERM), and returns a string with
> +   possible workarounds.  */
> +
> +static std::string
> +linux_ptrace_restricted_fail_reason (int err)
> +{
> +  if (err != EACCES && err != EPERM)
> +    {
> +      /* It just makes sense to perform the checks below if errno was
> +	 either EACCES or EPERM.  */
> +      return {};
> +    }
> +
> +  std::string ret;
> +  gdb_dlhandle_up handle = gdb_dlopen ("libselinux.so", true);
> +
> +  if (handle.get () != NULL)

No need for .get() here.  This:

  if (handle != NULL)

works the same.

(I'd write nullptr throughout instead of NULL in new code.)

> +    {
> +      selinux_ftype selinux_get_bool
> +	= (selinux_ftype) gdb_dlsym (handle, "security_get_boolean_active");
> +
> +      if (selinux_get_bool != NULL
> +	  && (*selinux_get_bool) ("deny_ptrace") == 1)
> +	string_appendf (ret,
> +			_("\n\
> +The SELinux 'deny_ptrace' option is enabled and preventing GDB\n\
> +from using 'ptrace'.  Please, disable it by executing (as root):\n\
> +\n\
> +  setsebool deny_ptrace off\n"));
> +    }
> +
> +  FILE *f = fopen ("/proc/sys/kernel/yama/ptrace_scope", "r");
> +

Use gdb_fopen_cloexec ?

> +  if (f != NULL)
> +    {
> +      char yama_scope = fgetc (f);
> +
> +      fclose (f);
> +
> +      if (yama_scope != '0')
> +	string_appendf (ret,
> +			_("\n\
> +The Linux kernel's Yama ptrace scope is in effect, which can prevent\n\
> +GDB from using 'ptrace'.  Please, disable it by executing (as root):\n\
> +\n\
> +  echo 0 > /proc/sys/kernel/yama/ptrace_scope\n"));
> +    }
> +
> +  if (ret.empty ())
> +    ret = _("\n\
> +There might be restrictions preventing ptrace from working.  Please see\n\
> +the appendix \"Linux kernel ptrace restrictions\" in the GDB documentation\n\
> +for more details.\n");
> +
> +  return ret;
> +}
> +
> +/* See declaration in linux-ptrace.h.  */
>  
>  std::string
> -linux_ptrace_attach_fail_reason (pid_t pid)
> +linux_ptrace_attach_fail_reason (pid_t pid, int err)
>  {
>    pid_t tracerpid = linux_proc_get_tracerpid_nowarn (pid);
>    std::string result;
> @@ -50,6 +111,11 @@ linux_ptrace_attach_fail_reason (pid_t pid)
>  		      "terminated"),
>  		    (int) pid);
>  
> +  std::string ptrace_restrict = linux_ptrace_restricted_fail_reason (err);
> +
> +  if (!ptrace_restrict.empty ())
> +    result += "\n" + ptrace_restrict;
> +
>    return result;
>  }
>  
> @@ -68,6 +134,14 @@ linux_ptrace_attach_fail_reason_string (ptid_t ptid, int err)
>      return string_printf ("%s (%d)", safe_strerror (err), err);
>  }
>  
> +/* See linux-ptrace.h.  */
> +
> +std::string
> +linux_ptrace_me_fail_reason (int err)
> +{
> +  return linux_ptrace_restricted_fail_reason (err);
> +}
> +
>  #if defined __i386__ || defined __x86_64__
>  
>  /* Address of the 'ret' instruction in asm code block below.  */
> @@ -321,7 +395,11 @@ linux_grandchild_function (void *child_stack)
>  static int
>  linux_child_function (void *child_stack)
>  {
> -  ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) 0, (PTRACE_TYPE_ARG4) 0);
> +  if (ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) 0,
> +	      (PTRACE_TYPE_ARG4) 0) != 0)
> +    trace_start_error_with_name ("ptrace",
> +				 linux_ptrace_me_fail_reason (errno).c_str ());
> +
>    kill (getpid (), SIGSTOP);
>  
>    /* Fork a grandchild.  */
> diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
> index fd2f12a342..04ada53bf6 100644
> --- a/gdb/nat/linux-ptrace.h
> +++ b/gdb/nat/linux-ptrace.h
> @@ -176,13 +176,26 @@ struct buffer;
>  # define TRAP_HWBKPT 4
>  #endif
>  
> -extern std::string linux_ptrace_attach_fail_reason (pid_t pid);
> +/* Find all possible reasons we could fail to attach PID and return
> +   these as a string.  An empty string is returned if we didn't find
> +   any reason.  If ERR is EACCES or EPERM, we also add a warning about
> +   possible restrictions to use ptrace.  */
> +extern std::string linux_ptrace_attach_fail_reason (pid_t pid, int err = -1);

If ERR is an errno number, then it's a bit odd to use -1 for default,
since errno == 0 is the traditional "no error" number.  Pedantically, I believe
there's no garantee that a valid error number must be a positive integer.

But, why the default argument in the first place?  What calls this
without passing an error?

>  
>  /* Find all possible reasons we could have failed to attach to PTID
>     and return them as a string.  ERR is the error PTRACE_ATTACH failed
>     with (an errno).  */
>  extern std::string linux_ptrace_attach_fail_reason_string (ptid_t ptid, int err);
>  
> +/* When the call to 'ptrace (PTRACE_TRACEME...' fails, and we have
> +   already forked, this function can be called in order to try to
> +   obtain the reason why ptrace failed.  ERR should be the ERRNO value
> +   returned by ptrace.
> +
> +   This function will return a 'std::string' containing the fail
> +   reason, or an empty string otherwise.  */
> +extern std::string linux_ptrace_me_fail_reason (int err);
> +
>  extern void linux_ptrace_init_warnings (void);
>  extern void linux_check_ptrace_features (void);
>  extern void linux_enable_event_reporting (pid_t pid, int attached);
> 

Thanks,
Pedro Alves
  

On Thursday, August 29 2019, Pedro Alves wrote:

> Hi Sergio,
>
> This is looking quite good to me.  Some comments below.

Thanks for the review, Pedro.

> On 8/26/19 7:32 PM, Sergio Durigan Junior wrote:
>> Changes from v1:
>> 
>> - Addressed Pedro's comments re. internal organization and gdbserver
>>   support.
>> 
>> - Addressed Eli's comments (doc fixes).
>> 
>> - New ways of detecting what's wrong with ptrace.
>> 
>> 
>> 
>> In Fedora GDB, we carry the following patch:
>> 
>>   https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-attach-fail-reasons-5of5.patch
>
> This link will soon be dead, right?  (thinking about the commit log)
> Maybe point at some reference other than master.

Yeah.  I will provide a link to a commit.

>> Its purpose is to try to detect a specific scenario where SELinux's
>> 'deny_ptrace' option is enabled, which prevents GDB from ptrace'ing in
>> order to debug the inferior (PTRACE_ATTACH and PTRACE_ME will fail
>> with EACCES in this case).
>> 
>> I like the idea of improving error detection and providing more
>> information to the user (a simple "Permission denied" can be really
>> frustrating), but I don't fully agree with the way the patch was
>> implemented: it makes GDB link against libselinux only for the sake of
>> consulting the 'deny_ptrace' setting, and then prints a warning if
>> ptrace failed and this setting is on.
>> 
>> My first thought (and attempt) was to make GDB print a generic warning
>> when a ptrace error happened; this message would just point the user
>> to our documentation, where she could find more information about
>> possible causes for the error (and try to diagnose/fix the problem).
>> This proved to be too simple, and I was convinced that it is actually
>> a good idea to go the extra kilometre and try to pinpoint the specific
>> problem (or problems) preventing ptrace from working, as well as
>> provide useful suggestions on how the user can fix things.
>> 
>> Here is the patch I came up with.  It implements a new function,
>> 'linux_ptrace_restricted_fail_reason', which does a few things to
>> check what's wrong with ptrace:
>> 
>>   - It dlopen's "libselinux.so" and checks if the "deny_ptrace" option
>>     is enabled.
>
> Update reference to "libselinux.so.1" here too.

Done.

>> 
>>   - It reads the contents of "/proc/sys/kernel/yama/ptrace_scope" and
>>     checks if it's different than 0.
>> 
>> For each of these checks, if it succeeds, the user will see a message
>> informing about the restriction in place, and how it can be disabled.
>> For example, if "deny_ptrace" is enabled, the user will see:
>> 
>>   # gdb /bin/true
>>   ...
>>   Starting program: /usr/bin/true
>>   warning: Could not trace the inferior process.
>>   Error:
>>   warning: ptrace: Permission denied
>
> Curious, that:
>
>  warning:
>  Error:
>  warning:
>
> looks a bit odd, specifically the "Error:" line.  Do you know where is
> that coming from?

Not offhand; I'll investigate and come back with the results.

>>   The SELinux 'deny_ptrace' option is enabled and preventing GDB
>>   from using 'ptrace'.  Please, disable it by executing (as root):
>
> I'm really not a fan of these "Please, ".  Kind of sounds like
> gdb is begging, to me.  I'd rather use an informational tone, like:
>
>    The SELinux 'deny_ptrace' option is enabled and preventing GDB
>    from using 'ptrace'.  You can disable it by executing (as root):

Fair enough.  Changed as requested.

>> 
>>     setsebool deny_ptrace off
>> 
>>   During startup program exited with code 127.
>> 
>> In case "/proc/sys/kernel/yama/ptrace_scope" is > 0:
>> 
>>   # gdb /bin/true
>>   ...
>>   Starting program: /usr/bin/true
>>   warning: Could not trace the inferior process.
>>   Error:
>>   warning: ptrace: Operation not permitted
>>   The Linux kernel's Yama ptrace scope is in effect, which can prevent
>>   GDB from using 'ptrace'.  Please, disable it by executing (as root):
>
> Ditto.

Changed.

>>     echo 0 > /proc/sys/kernel/yama/ptrace_scope
>> 
>>   During startup program exited with code 127.
>> 
>> If both restrictions are enabled, both messages will show up.
>> 
>> This works for gdbserver as well, and actually fixes a latent bug I
>> found: when ptrace is restricted, gdbserver would hang due to an
>> unchecked ptrace call:
>> 
>>   # gdbserver :9988 /bin/true
>>   gdbserver: linux_ptrace_test_ret_to_nx: Cannot PTRACE_TRACEME: Operation not permitted
>>   gdbserver: linux_ptrace_test_ret_to_nx: status 256 is not WIFSTOPPED!
>>   gdbserver: linux_ptrace_test_ret_to_nx: failed to kill child pid 2668100 No such process
>>   [ Here you would have to issue a C-c ]
>> 
>> Now, you will see:
>> 
>>   # gdbserver :9988 /bin/true
>>   gdbserver: linux_ptrace_test_ret_to_nx: Cannot PTRACE_TRACEME: Operation not permitted
>>   gdbserver: linux_ptrace_test_ret_to_nx: status 256 is not WIFSTOPPED!
>>   gdbserver: linux_ptrace_test_ret_to_nx: failed to kill child pid 2668100 No such process
>>   gdbserver: Could not trace the inferior process.
>>   Error:
>>   gdbserver: ptrace: Operation not permitted
>>   The Linux kernel's Yama ptrace scope is in effect, which can prevent
>>   GDB from using 'ptrace'.  Please, disable it by executing (as root):
>> 
>>     echo 0 > /proc/sys/kernel/yama/ptrace_scope
>> 
>>   linux_check_ptrace_features: waitpid: unexpected status 32512.
>>   Exiting
>> 
>> (I decided to keep all the other messages, even though I find them a
>> bit distracting).
>
> Yeah, the other messages are implementor-speak, showing gdb function
> names.  We should ideally clean this all up.

Agreed.  I can propose a patch later to clean them.

>> 
>> If GDB can't determine the cause for the failure, it will still print
>> the generic error message which tells the user to check our
>> documentation:
>> 
>>   There might be restrictions preventing ptrace from working.  Please see
>>   the appendix "Linux kernel ptrace restrictions" in the GDB documentation
>>   for more details.
>> 
>> This means that the patch expands our documentation and creates a new
>> appendix section named "Linux kernel ptrace restrictions", with
>> sub-sections for each possible restriction that might be in place.
>> 
>> The current list of possible restrictions is:
>> 
>>   - SELinux's 'deny_ptrace' option (detected).
>> 
>>   - YAMA's /proc/sys/kernel/yama/ptrace_scope setting (detected).
>> 
>>   - seccomp on Docker containers (I couldn't find how to detect).
>> 
>> It's important to mention that all of this is Linux-specific; as far
>> as I know, SELinux, YAMA and seccomp are Linux-only features.
>> 
>> I tested this patch locally, on my Fedora 30 machine (actually, a
>> Fedora Rawhide VM), but I'm not proposing a testcase for it because of
>> the difficulty of writing one.
>> 
>> WDYT?
>> 
>> gdb/doc/ChangeLog:
>> yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>
>> 
>> 	* gdb.texinfo (Linux kernel ptrace restrictions): New appendix
>> 	section.
>> 
>> gdb/ChangeLog:
>> yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>
>> 	    Jan Kratochvil  <jan.kratochvil@redhat.com>
>> 
>> 	* gdbsupport/gdb-dlfcn.c (gdb_dlopen): Add argument 'dont_throw'
>> 	Don't throw if it's true.
>> 	* gdbsupport/gdb-dlfcn.h (gdb_dlopen): Add optional argument
>> 	'dont_throw'.  Update comment.
>> 	* inf-ptrace.c (default_inf_ptrace_me_fail_reason): New
>> 	function.
>> 	(inf_ptrace_me_fail_reason): New variable.
>> 	(inf_ptrace_me): Update call to 'trace_start_error_with_name'.
>> 	* inf-ptrace.h (inf_ptrace_me_fail_reason): New variable.
>> 	* linux-nat.c (linux_nat_target::attach): Update call to
>> 	'linux_ptrace_attach_fail_reason'.
>> 	(_initialize_linux_nat): Set 'inf_ptrace_me_fail_reason'.
>> 	* linux-nat.h (linux_nat_target) <ptrace_me_fail_reason>: New
>> 	method.
>> 	* nat/fork-inferior.c (trace_start_error_with_name): Add
>> 	optional 'append' argument.
>> 	* nat/fork-inferior.h (trace_start_error_with_name): Update
>> 	prototype.
>> 	* nat/linux-ptrace.c: Include "gdbsupport/gdb-dlfcn.h" and
>> 	"nat/fork-inferior.h".
>> 	(selinux_ftype): New typedef.
>> 	(linux_ptrace_restricted_fail_reason): New function.
>> 	(linux_ptrace_attach_fail_reason): Add optional 'err'
>> 	argument.  Call 'linux_ptrace_restricted_fail_reason'.
>> 	(linux_ptrace_me_fail_reason): New function.
>> 	(linux_child_function): Handle ptrace error.
>> 	* nat/linux-ptrace.h (linux_ptrace_attach_fail_reason): Update
>> 	prototype.
>> 	(linux_ptrace_me_fail_reason): New function.
>> 	* target-delegates.c: Regenerate.
>> 	* target.h (struct target_ops) <ptrace_me_fail_reason>: New
>> 	method.
>> 	(target_ptrace_me_fail_reason): New define.
>> 
>> gdb/gdbserver/ChangeLog:
>> yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>
>> 
>> 	* linux-low.c (linux_ptrace_fun): Call
>> 	'linux_ptrace_me_fail_reason'.
>> ---
>>  gdb/doc/gdb.texinfo        | 138 +++++++++++++++++++++++++++++++++++++
>>  gdb/gdbserver/linux-low.c  |   3 +-
>>  gdb/gdbsupport/gdb-dlfcn.c |   4 +-
>>  gdb/gdbsupport/gdb-dlfcn.h |   7 +-
>>  gdb/inf-ptrace.c           |  19 ++++-
>>  gdb/inf-ptrace.h           |  10 +++
>>  gdb/linux-nat.c            |   7 +-
>>  gdb/nat/fork-inferior.c    |   4 +-
>>  gdb/nat/fork-inferior.h    |   7 +-
>>  gdb/nat/linux-ptrace.c     |  86 +++++++++++++++++++++--
>>  gdb/nat/linux-ptrace.h     |  15 +++-
>>  11 files changed, 282 insertions(+), 18 deletions(-)
>> 
>> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
>> index e1bc8143e6..43f749c6f1 100644
>> --- a/gdb/doc/gdb.texinfo
>> +++ b/gdb/doc/gdb.texinfo
>> @@ -182,6 +182,9 @@ software in general.  We will miss him.
>>                                  @value{GDBN}
>>  * Operating System Information:: Getting additional information from
>>                                   the operating system
>> +* Linux kernel @code{ptrace} restrictions:: Restrictions sometimes
>> +                                            imposed by the Linux
>> +                                            kernel on @code{ptrace}
>>  * Trace File Format::		GDB trace file format
>>  * Index Section Format::        .gdb_index section format
>>  * Man Pages::			Manual pages
>> @@ -44682,6 +44685,141 @@ should contain a comma-separated list of cores that this process
>>  is running on.  Target may provide additional columns,
>>  which @value{GDBN} currently ignores.
>>  
>> +@node Linux kernel ptrace restrictions
>> +@appendix Linux kernel @code{ptrace} restrictions
>> +@cindex linux kernel ptrace restrictions, attach
>> +
>> +The @code{ptrace} system call is used by @value{GDBN} on GNU/Linux to,
>> +among other things, attach to a new or existing inferior in order to
>> +start debugging it.  Due to security concerns, some distributions and
>> +vendors disable or severily restrict the ability to perform these
>
> typo: severily -> severely

Fixed.

>
>
>> +operations, which can make @value{GDBN} malfunction.  In this section,
>> +we will expand on how this malfunction can manifest itself, and how to
>> +modify the system's settings in order to be able to use @value{GDBN}
>> +properly.
>> +
>> +@menu
>> +* The GDB error message::               The error message displayed when the
>> +                                        system prevents @value{GDBN} from using
>> +                                        @code{ptrace}
>> +* SELinux's @code{deny_ptrace}::        SELinux and the @code{deny_ptrace} option
>> +* Yama's @code{ptrace_scope}::          Yama and the @code{ptrace_scope} setting
>> +* Docker and @code{seccomp}::           Docker and the @code{seccomp}
>> +                                        infrastructure
>> +@end menu
>> +
>> +@node The GDB error message
>> +@appendixsection The @value{GDBN} error message
>> +
>> +When the system prevents @value{GDBN} from using the @code{ptrace}
>> +system call, you will likely see a descriptive error message
>> +explaining what is wrong and how to attempt to fix the problem.  For
>> +example, when SELinux's @code{deny_ptrace} option is enabled, you can
>> +see:
>> +
>> +@smallexample
>> +$ gdb program
>> +...
>> +(@value{GDBP}) run
>> +Starting program: program
>> +warning: Could not trace the inferior process.
>> +Error:
>> +warning: ptrace: Permission denied
>> +The SELinux 'deny_ptrace' option is enabled and preventing GDB
>> +from using 'ptrace'.  Please, disable it by executing (as root):
>> +
>> +  setsebool deny_ptrace off
>> +
>> +During startup program exited with code 127.
>> +(@value{GDBP})
>> +@end smallexample
>> +
>> +Sometimes, it may not be possible to acquire the necessary data to
>> +determine the root cause of the failure.  In this case, you will see a
>> +generic error message pointing you to this section:
>> +
>> +@smallexample
>> +$ gdb program
>> +...
>> +Starting program: program
>> +warning: Could not trace the inferior process.
>> +Error:
>> +warning: ptrace: Permission denied
>> +There might be restrictions preventing ptrace from working.  Please see
>> +the appendix "Linux kernel ptrace restrictions" in the GDB documentation
>> +for more details.
>> +During startup program exited with code 127.
>> +(@value{GDBP})
>> +@end smallexample
>> +
>> +@node SELinux's deny_ptrace
>> +@appendixsection SELinux's @code{deny_ptrace}
>> +@cindex SELinux
>> +@cindex deny_ptrace
>> +
>> +If you are using SELinux, you might want to check whether the
>> +@code{deny_ptrace} option is enabled by doing:
>> +
>> +@smallexample
>> +$ getsebool deny_ptrace
>> +deny_ptrace --> on
>> +@end smallexample
>> +
>> +If the option is enabled, you can disable it by doing, as root:
>> +
>> +@smallexample
>> +# setsebool deny_ptrace off
>> +@end smallexample
>> +
>> +The option will be disabled until the next reboot.  If you would like
>> +to disable it permanently, you can do (as root):
>> +
>> +@smallexample
>> +# setsebool -P deny_ptrace off
>> +@end smallexample
>> +
>> +@node Yama's ptrace_scope
>> +@appendixsection Yama's @code{ptrace_scope}
>> +@cindex yama, ptrace_scope
>> +
>> +If your system has Yama enabled, you might want to check whether the
>> +@code{ptrace_scope} setting is enabled by checking the value of
>> +@file{/proc/sys/kernel/yama/ptrace_scope}:
>> +
>> +@smallexample
>> +$ cat /proc/sys/kernel/yama/ptrace_scope
>> +0
>> +@end smallexample
>> +
>> +If you see anything other than @code{0}, @value{GDBN} can be affected
>> +by it.  You can temporarily disable the feature by doing, as root:
>> +
>> +@smallexample
>> +# sysctl kernel.yama.ptrace_scope=0
>> +kernel.yama.ptrace_scope = 0
>> +@end smallexample
>> +
>> +You can make this permanent by doing, as root:
>> +
>> +@smallexample
>> +# sysctl -w kernel.yama.ptrace_scope=0
>> +kernel.yama.ptrace_scope = 0
>> +@end smallexample
>> +
>> +@node Docker and seccomp
>> +@appendixsection Docker and @code{seccomp}
>> +@cindex docker, seccomp
>> +
>> +If you are using Docker (@uref{https://www.docker.com/}) containers,
>> +you will probably have to disable its @code{seccomp} protections in
>> +order to be able to use @value{GDBN}.  To do that, you can use the
>> +options @code{--cap-add=SYS_PTRACE --security-opt seccomp=unconfined}
>> +when invoking Docker:
>> +
>> +@smallexample
>> +$ docker run --cap-add=SYS_PTRACE --security-opt seccomp=unconfined
>> +@end smallexample
>> +
>>  @node Trace File Format
>>  @appendix Trace File Format
>>  @cindex trace file format
>> diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
>> index 3113017ae6..1e0a5cbf54 100644
>> --- a/gdb/gdbserver/linux-low.c
>> +++ b/gdb/gdbserver/linux-low.c
>> @@ -971,7 +971,8 @@ linux_ptrace_fun ()
>>  {
>>    if (ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) 0,
>>  	      (PTRACE_TYPE_ARG4) 0) < 0)
>> -    trace_start_error_with_name ("ptrace");
>> +    trace_start_error_with_name ("ptrace",
>> +				 linux_ptrace_me_fail_reason (errno).c_str ());
>>  
>>    if (setpgid (0, 0) < 0)
>>      trace_start_error_with_name ("setpgid");
>> diff --git a/gdb/gdbsupport/gdb-dlfcn.c b/gdb/gdbsupport/gdb-dlfcn.c
>> index 921f10f3d8..9e5a992c17 100644
>> --- a/gdb/gdbsupport/gdb-dlfcn.c
>> +++ b/gdb/gdbsupport/gdb-dlfcn.c
>> @@ -58,7 +58,7 @@ is_dl_available (void)
>>  #else /* NO_SHARED_LIB */
>>  
>>  gdb_dlhandle_up
>> -gdb_dlopen (const char *filename)
>> +gdb_dlopen (const char *filename, bool dont_throw)
>>  {
>>    void *result;
>>  #ifdef HAVE_DLFCN_H
>> @@ -66,7 +66,7 @@ gdb_dlopen (const char *filename)
>>  #elif __MINGW32__
>>    result = (void *) LoadLibrary (filename);
>>  #endif
>> -  if (result != NULL)
>> +  if (dont_throw || result != NULL)
>>      return gdb_dlhandle_up (result);
>>  
>>  #ifdef HAVE_DLFCN_H
>> diff --git a/gdb/gdbsupport/gdb-dlfcn.h b/gdb/gdbsupport/gdb-dlfcn.h
>> index 6a39d38941..a8ddbc03da 100644
>> --- a/gdb/gdbsupport/gdb-dlfcn.h
>> +++ b/gdb/gdbsupport/gdb-dlfcn.h
>> @@ -32,10 +32,11 @@ struct dlclose_deleter
>>  typedef std::unique_ptr<void, dlclose_deleter> gdb_dlhandle_up;
>>  
>>  /* Load the dynamic library file named FILENAME, and return a handle
>> -   for that dynamic library.  Return NULL if the loading fails for any
>> -   reason.  */
>> +   for that dynamic library.  If the loading fails, return NULL if
>> +   DONT_THROW is true, or throw an exception otherwise (default
>> +   behaviour).  */
>>  
>> -gdb_dlhandle_up gdb_dlopen (const char *filename);
>> +gdb_dlhandle_up gdb_dlopen (const char *filename, bool dont_throw = false);
>>  
>>  /* Return the address of the symbol named SYMBOL inside the shared
>>     library whose handle is HANDLE.  Return NULL when the symbol could
>> diff --git a/gdb/inf-ptrace.c b/gdb/inf-ptrace.c
>> index 4a8e732373..b6cfd803cf 100644
>> --- a/gdb/inf-ptrace.c
>> +++ b/gdb/inf-ptrace.c
>> @@ -94,6 +94,22 @@ inf_ptrace_target::remove_fork_catchpoint (int pid)
>>  #endif /* PT_GET_PROCESS_STATE */
>>  
>>  
>> +/* Default method for "inf_ptrace_me_fail_reason", which returns an
>> +   empty string.  */
>> +
>> +static std::string
>> +default_inf_ptrace_me_fail_reason (int err)
>> +{
>> +  return {};
>> +}
>> +
>> +/* Point to "inf_ptrace_me_fail_reason", 
>
> Did you mean "Pointer"?  This reads strange because 
> "inf_ptrace_me_fail_reason" is the name of the pointer itself,
> so how to point _to_ it?

Yes, I meant "Pointer", thanks.  Fixed.

> which implements a function
>> +   that can be called by "inf_ptrace_me" in order to obtain the reason
>> +   for failure.  */
>> +
>> +std::string (*inf_ptrace_me_fail_reason) (int err)
>> +  = default_inf_ptrace_me_fail_reason;
>> +
>>  /* Prepare to be traced.  */
>>  
>>  static void
>> @@ -101,7 +117,8 @@ inf_ptrace_me (void)
>>  {
>>    /* "Trace me, Dr. Memory!"  */
>>    if (ptrace (PT_TRACE_ME, 0, (PTRACE_TYPE_ARG3) 0, 0) < 0)
>> -    trace_start_error_with_name ("ptrace");
>> +    trace_start_error_with_name ("ptrace",
>> +				 inf_ptrace_me_fail_reason (errno).c_str ());
>>  }
>>  
>>  /* Start a new inferior Unix child process.  EXEC_FILE is the file to
>> diff --git a/gdb/inf-ptrace.h b/gdb/inf-ptrace.h
>> index 98b5d2e09e..7cdab9af89 100644
>> --- a/gdb/inf-ptrace.h
>> +++ b/gdb/inf-ptrace.h
>> @@ -83,4 +83,14 @@ protected:
>>  
>>  extern pid_t get_ptrace_pid (ptid_t);
>>  
>> +/* Pointer to "inf_ptrace_me_fail_reason", which implements a function
>
> Ah, here it's "pointer".  But still reads strange.  I think the comment
> in the .c file should just say the usual "See inf-ptrace.h.".

Indeed.  I'll update the comment there.

>
>> +   that can be called by "inf_ptrace_me" in order to obtain the reason
>> +   for a ptrace failure.  ERR is the ERRNO value set by the failing
>> +   ptrace call.
>> +
>> +   This pointer can be overriden by targets that want to personalize
>> +   the error message printed when ptrace fails (see linux-nat.c, for
>> +   example).  */
>> +extern std::string (*inf_ptrace_me_fail_reason) (int err);
>> +
>>  #endif
>> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
>> index 945c19f666..b5a9eaf72e 100644
>> --- a/gdb/linux-nat.c
>> +++ b/gdb/linux-nat.c
>> @@ -1191,8 +1191,9 @@ linux_nat_target::attach (const char *args, int from_tty)
>>      }
>>    catch (const gdb_exception_error &ex)
>>      {
>> +      int saved_errno = errno;
>>        pid_t pid = parse_pid_to_attach (args);
>> -      std::string reason = linux_ptrace_attach_fail_reason (pid);
>> +      std::string reason = linux_ptrace_attach_fail_reason (pid, saved_errno);
>>  
>>        if (!reason.empty ())
>>  	throw_error (ex.error, "warning: %s\n%s", reason.c_str (),
>> @@ -4696,6 +4697,10 @@ Enables printf debugging output."),
>>    sigemptyset (&blocked_mask);
>>  
>>    lwp_lwpid_htab_create ();
>> +
>> +  /* Set the proper function to generate a message when ptrace
>> +     fails.  */
>> +  inf_ptrace_me_fail_reason = linux_ptrace_me_fail_reason;
>>  }
>>  
>>  
>> diff --git a/gdb/nat/fork-inferior.c b/gdb/nat/fork-inferior.c
>> index 68b51aa814..72ac623e20 100644
>> --- a/gdb/nat/fork-inferior.c
>> +++ b/gdb/nat/fork-inferior.c
>> @@ -591,7 +591,7 @@ trace_start_error (const char *fmt, ...)
>>  /* See nat/fork-inferior.h.  */
>>  
>>  void
>> -trace_start_error_with_name (const char *string)
>> +trace_start_error_with_name (const char *string, const char *append)
>>  {
>> -  trace_start_error ("%s: %s", string, safe_strerror (errno));
>> +  trace_start_error ("%s: %s%s", string, safe_strerror (errno), append);
>>  }
>> diff --git a/gdb/nat/fork-inferior.h b/gdb/nat/fork-inferior.h
>> index 1d0519fb26..7e6b889210 100644
>> --- a/gdb/nat/fork-inferior.h
>> +++ b/gdb/nat/fork-inferior.h
>> @@ -98,9 +98,10 @@ extern void trace_start_error (const char *fmt, ...)
>>    ATTRIBUTE_NORETURN ATTRIBUTE_PRINTF (1, 2);
>>  
>>  /* Like "trace_start_error", but the error message is constructed by
>> -   combining STRING with the system error message for errno.  This
>> -   function does not return.  */
>> -extern void trace_start_error_with_name (const char *string)
>> +   combining STRING with the system error message for errno, and
>> +   (optionally) with APPEND.  This function does not return.  */
>> +extern void trace_start_error_with_name (const char *string,
>> +					 const char *append = "")
>>    ATTRIBUTE_NORETURN;
>>  
>>  #endif /* NAT_FORK_INFERIOR_H */
>> diff --git a/gdb/nat/linux-ptrace.c b/gdb/nat/linux-ptrace.c
>> index c1ebc0a032..599d9cfb55 100644
>> --- a/gdb/nat/linux-ptrace.c
>> +++ b/gdb/nat/linux-ptrace.c
>> @@ -21,6 +21,8 @@
>>  #include "linux-procfs.h"
>>  #include "linux-waitpid.h"
>>  #include "gdbsupport/buffer.h"
>> +#include "gdbsupport/gdb-dlfcn.h"
>> +#include "nat/fork-inferior.h"
>>  #ifdef HAVE_SYS_PROCFS_H
>>  #include <sys/procfs.h>
>>  #endif
>> @@ -30,11 +32,70 @@
>>     of 0 means there are no supported features.  */
>>  static int supported_ptrace_options = -1;
>>  
>> -/* Find all possible reasons we could fail to attach PID and return these
>> -   as a string.  An empty string is returned if we didn't find any reason.  */
>> +typedef int (*selinux_ftype) (const char *);
>> +
>> +/* Helper function which checks if ptrace is probably restricted
>> +   (i.e., if ERR is either EACCES or EPERM), and returns a string with
>> +   possible workarounds.  */
>> +
>> +static std::string
>> +linux_ptrace_restricted_fail_reason (int err)
>> +{
>> +  if (err != EACCES && err != EPERM)
>> +    {
>> +      /* It just makes sense to perform the checks below if errno was
>> +	 either EACCES or EPERM.  */
>> +      return {};
>> +    }
>> +
>> +  std::string ret;
>> +  gdb_dlhandle_up handle = gdb_dlopen ("libselinux.so", true);
>> +
>> +  if (handle.get () != NULL)
>
> No need for .get() here.  This:
>
>   if (handle != NULL)
>
> works the same.

While writing this code I thought I remembered something like that, but
I wasn't sure.  Thanks for clarifying.  Updated.

> (I'd write nullptr throughout instead of NULL in new code.)

OK.  Updated.

>> +    {
>> +      selinux_ftype selinux_get_bool
>> +	= (selinux_ftype) gdb_dlsym (handle, "security_get_boolean_active");
>> +
>> +      if (selinux_get_bool != NULL
>> +	  && (*selinux_get_bool) ("deny_ptrace") == 1)
>> +	string_appendf (ret,
>> +			_("\n\
>> +The SELinux 'deny_ptrace' option is enabled and preventing GDB\n\
>> +from using 'ptrace'.  Please, disable it by executing (as root):\n\
>> +\n\
>> +  setsebool deny_ptrace off\n"));
>> +    }
>> +
>> +  FILE *f = fopen ("/proc/sys/kernel/yama/ptrace_scope", "r");
>> +
>
> Use gdb_fopen_cloexec ?

Ah, I hadn't realized that this existed.  Much better, thanks!

>> +  if (f != NULL)
>> +    {
>> +      char yama_scope = fgetc (f);
>> +
>> +      fclose (f);
>> +
>> +      if (yama_scope != '0')
>> +	string_appendf (ret,
>> +			_("\n\
>> +The Linux kernel's Yama ptrace scope is in effect, which can prevent\n\
>> +GDB from using 'ptrace'.  Please, disable it by executing (as root):\n\
>> +\n\
>> +  echo 0 > /proc/sys/kernel/yama/ptrace_scope\n"));
>> +    }
>> +
>> +  if (ret.empty ())
>> +    ret = _("\n\
>> +There might be restrictions preventing ptrace from working.  Please see\n\
>> +the appendix \"Linux kernel ptrace restrictions\" in the GDB documentation\n\
>> +for more details.\n");
>> +
>> +  return ret;
>> +}
>> +
>> +/* See declaration in linux-ptrace.h.  */
>>  
>>  std::string
>> -linux_ptrace_attach_fail_reason (pid_t pid)
>> +linux_ptrace_attach_fail_reason (pid_t pid, int err)
>>  {
>>    pid_t tracerpid = linux_proc_get_tracerpid_nowarn (pid);
>>    std::string result;
>> @@ -50,6 +111,11 @@ linux_ptrace_attach_fail_reason (pid_t pid)
>>  		      "terminated"),
>>  		    (int) pid);
>>  
>> +  std::string ptrace_restrict = linux_ptrace_restricted_fail_reason (err);
>> +
>> +  if (!ptrace_restrict.empty ())
>> +    result += "\n" + ptrace_restrict;
>> +
>>    return result;
>>  }
>>  
>> @@ -68,6 +134,14 @@ linux_ptrace_attach_fail_reason_string (ptid_t ptid, int err)
>>      return string_printf ("%s (%d)", safe_strerror (err), err);
>>  }
>>  
>> +/* See linux-ptrace.h.  */
>> +
>> +std::string
>> +linux_ptrace_me_fail_reason (int err)
>> +{
>> +  return linux_ptrace_restricted_fail_reason (err);
>> +}
>> +
>>  #if defined __i386__ || defined __x86_64__
>>  
>>  /* Address of the 'ret' instruction in asm code block below.  */
>> @@ -321,7 +395,11 @@ linux_grandchild_function (void *child_stack)
>>  static int
>>  linux_child_function (void *child_stack)
>>  {
>> -  ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) 0, (PTRACE_TYPE_ARG4) 0);
>> +  if (ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) 0,
>> +	      (PTRACE_TYPE_ARG4) 0) != 0)
>> +    trace_start_error_with_name ("ptrace",
>> +				 linux_ptrace_me_fail_reason (errno).c_str ());
>> +
>>    kill (getpid (), SIGSTOP);
>>  
>>    /* Fork a grandchild.  */
>> diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
>> index fd2f12a342..04ada53bf6 100644
>> --- a/gdb/nat/linux-ptrace.h
>> +++ b/gdb/nat/linux-ptrace.h
>> @@ -176,13 +176,26 @@ struct buffer;
>>  # define TRAP_HWBKPT 4
>>  #endif
>>  
>> -extern std::string linux_ptrace_attach_fail_reason (pid_t pid);
>> +/* Find all possible reasons we could fail to attach PID and return
>> +   these as a string.  An empty string is returned if we didn't find
>> +   any reason.  If ERR is EACCES or EPERM, we also add a warning about
>> +   possible restrictions to use ptrace.  */
>> +extern std::string linux_ptrace_attach_fail_reason (pid_t pid, int err = -1);
>
> If ERR is an errno number, then it's a bit odd to use -1 for default,
> since errno == 0 is the traditional "no error" number.  Pedantically, I believe
> there's no garantee that a valid error number must be a positive integer.
>
> But, why the default argument in the first place?  What calls this
> without passing an error?

So, the only place that calls linux_ptrace_attach_fail_reason without
passing the ERR argument is linux_ptrace_attach_fail_reason_string:

  std::string
  linux_ptrace_attach_fail_reason_string (ptid_t ptid, int err)
  {
    long lwpid = ptid.lwp ();
    std::string reason = linux_ptrace_attach_fail_reason (lwpid);

    if (!reason.empty ())
      return string_printf ("%s (%d), %s", safe_strerror (err), err,
                            reason.c_str ());
    else
      return string_printf ("%s (%d)", safe_strerror (err), err);
  }

In this case, I opted to keep it as is because the function will compose
a string contaning like:

  A (B)[: C]

Where:

 A = safe_strerror
 B = errno
 C = fail reason (optional)

This function (linux_ptrace_attach_fail_reason_string) is called in
three places:

gdb/linux-nat.c:

	      std::string reason
		= linux_ptrace_attach_fail_reason_string (ptid, err);

	      warning (_("Cannot attach to lwp %d: %s"),
		       lwpid, reason.c_str ());

gdb/gdbserver/linux-low.c:

	  std::string reason
	    = linux_ptrace_attach_fail_reason_string (ptid, err);

	  warning (_("Cannot attach to lwp %d: %s"), lwpid, reason.c_str ());

and

      std::string reason = linux_ptrace_attach_fail_reason_string (ptid, err);
      error ("Cannot attach to process %ld: %s", pid, reason.c_str ());


It seems to me like these error messages are expecting a short string to
just append to their existing strings, so I didn't think it made much
sense to extend the ptrace error checking here as well.  That's why I
didn't extend linux_ptrace_attach_fail_reason_string to pass ERR down to
linux_ptrace_attach_fail_reason.
  

On Thursday, August 29 2019, I wrote:

> On Thursday, August 29 2019, Pedro Alves wrote:
>
>>>   - It reads the contents of "/proc/sys/kernel/yama/ptrace_scope" and
>>>     checks if it's different than 0.
>>> 
>>> For each of these checks, if it succeeds, the user will see a message
>>> informing about the restriction in place, and how it can be disabled.
>>> For example, if "deny_ptrace" is enabled, the user will see:
>>> 
>>>   # gdb /bin/true
>>>   ...
>>>   Starting program: /usr/bin/true
>>>   warning: Could not trace the inferior process.
>>>   Error:
>>>   warning: ptrace: Permission denied
>>
>> Curious, that:
>>
>>  warning:
>>  Error:
>>  warning:
>>
>> looks a bit odd, specifically the "Error:" line.  Do you know where is
>> that coming from?
>
> Not offhand; I'll investigate and come back with the results.

So, the problem is with nat/fork-inferior.c:trace_start_error:

  void
  trace_start_error (const char *fmt, ...)
  {
    va_list ap;

    va_start (ap, fmt);
    warning ("Could not trace the inferior process.\nError: ");
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    vwarning (fmt, ap);
    va_end (ap);

    gdb_flush_out_err ();
    _exit (0177);
  }

  /* See nat/fork-inferior.h.  */

  void
  trace_start_error_with_name (const char *string, const char *append)
  {
    trace_start_error ("%s: %s%s", string, safe_strerror (errno), append);
  }


You can see that we're calling "warning", which puts a newline at the
end of the string.  I think it'd be best to just get rid of the
"\nError: " suffix.  We'd then have something like:

   Starting program: /usr/bin/true
   warning: Could not trace the inferior process.
   warning: ptrace: Permission denied
   ....

WDYT?
  

FYI, I'm seeing these warnings with your patch:

$ make
makeinfo --split-size=5000000  -DHAVE_MAKEINFO_CLICK -I /home/pedro/gdb/binutils-gdb/src/gdb/doc/../../readline/doc -I /home/pedro/gdb/binutils-gdb/src/gdb/doc/../mi -I /home/pedro/gdb/binutils-gdb/src/gdb/doc \
        -o gdb.info /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo
/home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:185: warning: @menu entry node name `Linux kernel @code{ptrace} restrictions' different from node name `Linux kernel ptrace restrictions'
/home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:44712: warning: @menu entry node name `SELinux's @code{deny_ptrace}' different from node name `SELinux's deny_ptrace'
/home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:44713: warning: @menu entry node name `Yama's @code{ptrace_scope}' different from node name `Yama's ptrace_scope'
/home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:44714: warning: @menu entry node name `Docker and @code{seccomp}' different from node name `Docker and seccomp'

Thanks,
Pedro Alves
  

> Cc: Eli Zaretskii <eliz@gnu.org>, Ruslan Kabatsayev <b7.10110111@gmail.com>
> From: Pedro Alves <palves@redhat.com>
> Date: Fri, 30 Aug 2019 13:47:06 +0100
> 
> FYI, I'm seeing these warnings with your patch:
> 
> $ make
> makeinfo --split-size=5000000  -DHAVE_MAKEINFO_CLICK -I /home/pedro/gdb/binutils-gdb/src/gdb/doc/../../readline/doc -I /home/pedro/gdb/binutils-gdb/src/gdb/doc/../mi -I /home/pedro/gdb/binutils-gdb/src/gdb/doc \
>         -o gdb.info /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo
> /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:185: warning: @menu entry node name `Linux kernel @code{ptrace} restrictions' different from node name `Linux kernel ptrace restrictions'
> /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:44712: warning: @menu entry node name `SELinux's @code{deny_ptrace}' different from node name `SELinux's deny_ptrace'
> /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:44713: warning: @menu entry node name `Yama's @code{ptrace_scope}' different from node name `Yama's ptrace_scope'
> /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:44714: warning: @menu entry node name `Docker and @code{seccomp}' different from node name `Docker and seccomp'

We shouldn't use @-commands in node names.
  

On 8/29/19 8:48 PM, Sergio Durigan Junior wrote:

> You can see that we're calling "warning", which puts a newline at the
> end of the string.  I think it'd be best to just get rid of the
> "\nError: " suffix.  We'd then have something like:
> 
>    Starting program: /usr/bin/true
>    warning: Could not trace the inferior process.
>    warning: ptrace: Permission denied
>    ....
> 
> WDYT?

Sounds fine to me.

Thanks,
Pedro Alves
  

On Friday, August 30 2019, Eli Zaretskii wrote:

>> Cc: Eli Zaretskii <eliz@gnu.org>, Ruslan Kabatsayev <b7.10110111@gmail.com>
>> From: Pedro Alves <palves@redhat.com>
>> Date: Fri, 30 Aug 2019 13:47:06 +0100
>> 
>> FYI, I'm seeing these warnings with your patch:
>> 
>> $ make
>> makeinfo --split-size=5000000 -DHAVE_MAKEINFO_CLICK -I
>> /home/pedro/gdb/binutils-gdb/src/gdb/doc/../../readline/doc -I
>> /home/pedro/gdb/binutils-gdb/src/gdb/doc/../mi -I
>> /home/pedro/gdb/binutils-gdb/src/gdb/doc \
>>         -o gdb.info /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo
>> /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:185: warning:
>> @menu entry node name `Linux kernel @code{ptrace} restrictions'
>> different from node name `Linux kernel ptrace restrictions'
>> /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:44712: warning: @menu entry node name `SELinux's @code{deny_ptrace}' different from node name `SELinux's deny_ptrace'
>> /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:44713: warning: @menu entry node name `Yama's @code{ptrace_scope}' different from node name `Yama's ptrace_scope'
>> /home/pedro/gdb/binutils-gdb/src/gdb/doc/gdb.texinfo:44714: warning: @menu entry node name `Docker and @code{seccomp}' different from node name `Docker and seccomp'
>
> We shouldn't use @-commands in node names.

But I fixed my patch, and v2 doesn't use @-commands in node names
anymore.

Hm, I think the problem is that I'm using @-commands inside @menu.  OK,
I'll fix that, and I'll also remove @-commands from @appendix.  Thanks
for pointing it out.