Patchwork [v4] Improve ptrace-error detection on Linux targets

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Submitter Sergio Durigan Junior
Date Sept. 11, 2019, 1:11 a.m.
Message ID <20190911011103.12774-1-sergiodj@redhat.com>
Download mbox | patch
Permalink /patch/34496/
State New
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Comments

Sergio Durigan Junior - Sept. 11, 2019, 1:11 a.m.
Changes from v3:

- Implement a better way to check for the ptrace errno inside
  linux_fork_to_function et al.

- Implement error message passing (gdbserver -> GDB) when attach fails
  on gdbserver.

- Extend docs a little bit to mention that the ptrace restrictions
  apply also to gdbserver (very trivial).

- Improve error message to instruct the user to perform the necessary
  actions in the target side when remote debugging is in play.

- Very minor nits.


In Fedora GDB, we carry the following patch:

  https://src.fedoraproject.org/rpms/gdb/blob/8ac06474ff1e2aa4920d14e0666b083eeaca8952/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_TRACEME 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.1" 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 /usr/bin/true
  ...
  Starting program: /usr/bin/true
  warning: Could not trace the inferior process.
  warning: ptrace: Permission denied
  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

  If you are debugging the inferior remotely, the ptrace restriction(s) need
  to be disabled in the target system (e.g., where GDBserver is running).
  During startup program exited with code 127.
  (gdb)

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

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

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

  If you are debugging the inferior remotely, the ptrace restriction(s) need
  to be disabled in the target system (e.g., where GDBserver is running).
  During startup program exited with code 127.
  (gdb)

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 /usr/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 /usr/bin/true
  gdbserver: linux_ptrace_test_ret_to_nx: Cannot PTRACE_TRACEME: Permission denied
  gdbserver: linux_ptrace_test_ret_to_nx: status 256 is not WIFSTOPPED!
  gdbserver: linux_ptrace_test_ret_to_nx: failed to kill child pid 2766868 No such process
  gdbserver: Could not trace the inferior process.
  gdbserver: ptrace: Permission denied
  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

  If you are debugging the inferior remotely, the ptrace restriction(s) need
  to be disabled in the target system (e.g., where GDBserver is running).
  #

(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.
  If you are debugging the inferior remotely, the ptrace restriction(s) need
  to be disabled in the target system (e.g., where GDBserver is running).

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.

Notice how, on every message, we instruct the user to "do the right
thing" if gdbserver is being used.  This is because if the user
started gdbserver *before* any ptrace restriction was in place, and
then, for some reason, one or more restrictions get enabled, then the
error message will be displayed both on gdbserver *and* on the
connected GDB.  Since the user will be piloting GDB, it's important to
explicitly say that the ptrace restrictions are enabled in the target,
where gdbserver is running.

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>
	    Pedro Alves  <palves@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 (attach_proc_task_lwp_callback): Call
	'linux_ptrace_attach_fail_reason_lwp'.
	(linux_nat_target::attach): Update call to
	'linux_ptrace_attach_fail_reason'.
	(_initialize_linux_nat): Set 'inf_ptrace_me_fail_reason'.
	* 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",
	"gdbsupport/filestuff.h" and "nat/fork-inferior.h".
	(selinux_ftype): New typedef.
	(linux_ptrace_restricted_fail_reason): New function.
	(linux_ptrace_attach_fail_reason_1): New function.
	(linux_ptrace_attach_fail_reason): Change first argument type
	from 'ptid_t' to 'pid_t'.  Call
	'linux_ptrace_attach_fail_reason_1' and
	'linux_ptrace_restricted_fail_reason'.
	(linux_ptrace_attach_fail_reason_lwp): New function.
	(linux_ptrace_me_fail_reason): New function.
	(errno_pipe): New variable.
	(linux_fork_to_function): Initialize pipe before forking.
	(linux_child_function): Deal with errno-passing from child.
	Handle ptrace error.
	(linux_check_child_ptrace_errno): New function.
	(linux_check_child_ptrace_errno): Call
	'linux_check_child_ptrace_errno'.
	* nat/linux-ptrace.h (linux_ptrace_attach_fail_reason): Update
	prototype.
	(linux_ptrace_attach_fail_reason_lwp): New prototype.
	(linux_ptrace_me_fail_reason): New prototype.
	* remote.c (extended_remote_target::attach): Handle error
	message passed by the server when attach fails.

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

	* linux-low.c (linux_ptrace_fun): Call
	'linux_ptrace_me_fail_reason'.
	(attach_proc_task_lwp_callback): Call
	'linux_ptrace_attach_fail_reason_lwp'.
	(linux_attach): Call 'linux_ptrace_attach_fail_reason'.
	* server.c (handle_v_attach): Use try..catch when calling
	'attach_inferior', and send an error message to the client
	when needed.
	* thread-db.c (attach_thread): Call
	'linux_ptrace_attach_fail_reason_lwp'.
---
 gdb/doc/gdb.texinfo        | 143 +++++++++++++++++++++++++++++
 gdb/gdbserver/linux-low.c  |   9 +-
 gdb/gdbserver/server.c     |  14 ++-
 gdb/gdbserver/thread-db.c  |   2 +-
 gdb/gdbsupport/gdb-dlfcn.c |   4 +-
 gdb/gdbsupport/gdb-dlfcn.h |   7 +-
 gdb/inf-ptrace.c           |  17 +++-
 gdb/inf-ptrace.h           |  10 ++
 gdb/linux-nat.c            |   9 +-
 gdb/nat/fork-inferior.c    |   4 +-
 gdb/nat/fork-inferior.h    |   7 +-
 gdb/nat/linux-ptrace.c     | 183 +++++++++++++++++++++++++++++++++++--
 gdb/nat/linux-ptrace.h     |  27 ++++--
 gdb/remote.c               |  16 +++-
 14 files changed, 418 insertions(+), 34 deletions(-)
Eli Zaretskii - Sept. 12, 2019, 12:39 p.m.
> 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: Tue, 10 Sep 2019 21:11:03 -0400
> 
> +The SELinux 'deny_ptrace' option is enabled and preventing GDB
                                                              ^^^
@value{GDBN}

Other than that, the documentation part is OK.

Thanks.
Sergio Durigan Junior - Sept. 12, 2019, 6:29 p.m.
On Thursday, September 12 2019, Eli Zaretskii wrote:

>> 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: Tue, 10 Sep 2019 21:11:03 -0400
>> 
>> +The SELinux 'deny_ptrace' option is enabled and preventing GDB
>                                                               ^^^
> @value{GDBN}
>
> Other than that, the documentation part is OK.

Thanks, fixed.

Patch

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 53b7de91e4..ffaedc4ba8 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 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
@@ -44689,6 +44692,146 @@  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} and
+@code{gdbserver} 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 severely restrict
+the ability to perform these operations, which can make @value{GDBN}
+or @code{gdbserver} 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} and
+@code{gdbserver} properly.
+
+@menu
+* The error message::                   The error message displayed when the
+                                        system prevents @value{GDBN}
+                                        or @code{gdbserver} from using
+                                        @code{ptrace}
+* SELinux's deny_ptrace::               SELinux and the @code{deny_ptrace} option
+* Yama's ptrace_scope::                 Yama and the @code{ptrace_scope} setting
+* Docker and seccomp::                  Docker and the @code{seccomp}
+                                        infrastructure
+@end menu
+
+@node The error message
+@appendixsection The error message
+
+When the system prevents @value{GDBN} or @code{gdbserver} 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'.  You can disable it by executing (as root):
+
+  setsebool deny_ptrace off
+
+If you are debugging the inferior remotely, the instruction(s) above must
+be performed in the target system (e.g., where GDBserver is running).
+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} or
+@code{gdbserver} 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} or @code{gdbserver}.  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..2b3c586125 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");
@@ -1169,7 +1170,7 @@  attach_proc_task_lwp_callback (ptid_t ptid)
       else if (err != 0)
 	{
 	  std::string reason
-	    = linux_ptrace_attach_fail_reason_string (ptid, err);
+	    = linux_ptrace_attach_fail_reason_lwp (ptid, err);
 
 	  warning (_("Cannot attach to lwp %d: %s"), lwpid, reason.c_str ());
 	}
@@ -1201,8 +1202,8 @@  linux_attach (unsigned long pid)
     {
       remove_process (proc);
 
-      std::string reason = linux_ptrace_attach_fail_reason_string (ptid, err);
-      error ("Cannot attach to process %ld: %s", pid, reason.c_str ());
+      std::string reason = linux_ptrace_attach_fail_reason (pid, err);
+      error (_("Cannot attach to process %ld: %s"), pid, reason.c_str ());
     }
 
   /* Don't ignore the initial SIGSTOP if we just attached to this
diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index 127cd3840b..51a7066594 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -2913,9 +2913,21 @@  handle_v_attach (char *own_buf)
 {
   client_state &cs = get_client_state ();
   int pid;
+  int ret;
 
   pid = strtol (own_buf + 8, NULL, 16);
-  if (pid != 0 && attach_inferior (pid) == 0)
+
+  try
+    {
+      ret = attach_inferior (pid);
+    }
+  catch (const gdb_exception_error &e)
+    {
+      sprintf (own_buf, "E.%s", e.what ());
+      return 0;
+    }
+
+  if (pid != 0 && ret == 0)
     {
       /* Don't report shared library events after attaching, even if
 	 some libraries are preloaded.  GDB will always poll the
diff --git a/gdb/gdbserver/thread-db.c b/gdb/gdbserver/thread-db.c
index b2791b0513..cfba05977e 100644
--- a/gdb/gdbserver/thread-db.c
+++ b/gdb/gdbserver/thread-db.c
@@ -225,7 +225,7 @@  attach_thread (const td_thrhandle_t *th_p, td_thrinfo_t *ti_p)
   err = linux_attach_lwp (ptid);
   if (err != 0)
     {
-      std::string reason = linux_ptrace_attach_fail_reason_string (ptid, err);
+      std::string reason = linux_ptrace_attach_fail_reason_lwp (ptid, err);
 
       warning ("Could not attach to thread %ld (LWP %d): %s",
 	       (unsigned long) ti_p->ti_tid, ti_p->ti_lid, reason.c_str ());
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..b792af00d1 100644
--- a/gdb/inf-ptrace.c
+++ b/gdb/inf-ptrace.c
@@ -94,6 +94,20 @@  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 {};
+}
+
+/* See inf-ptrace.h.  */
+
+std::string (*inf_ptrace_me_fail_reason) (int err)
+  = default_inf_ptrace_me_fail_reason;
+
 /* Prepare to be traced.  */
 
 static void
@@ -101,7 +115,8 @@  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
+   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..22cb55e6dc 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -1136,7 +1136,7 @@  attach_proc_task_lwp_callback (ptid_t ptid)
 	  else
 	    {
 	      std::string reason
-		= linux_ptrace_attach_fail_reason_string (ptid, err);
+		= linux_ptrace_attach_fail_reason_lwp (ptid, err);
 
 	      warning (_("Cannot attach to lwp %d: %s"),
 		       lwpid, reason.c_str ());
@@ -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 355e9bef43..2ead4a4858 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..694f390de0 100644
--- a/gdb/nat/linux-ptrace.c
+++ b/gdb/nat/linux-ptrace.c
@@ -21,6 +21,9 @@ 
 #include "linux-procfs.h"
 #include "linux-waitpid.h"
 #include "gdbsupport/buffer.h"
+#include "gdbsupport/gdb-dlfcn.h"
+#include "nat/fork-inferior.h"
+#include "gdbsupport/filestuff.h"
 #ifdef HAVE_SYS_PROCFS_H
 #include <sys/procfs.h>
 #endif
@@ -30,11 +33,85 @@ 
    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 *);
 
-std::string
-linux_ptrace_attach_fail_reason (pid_t pid)
+/* 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.1", true);
+
+  if (handle != nullptr)
+    {
+      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'.  You can disable it by executing (as root):\n\
+\n\
+  setsebool deny_ptrace off\n"));
+    }
+
+  gdb_file_up yama_ptrace_scope
+    = gdb_fopen_cloexec ("/proc/sys/kernel/yama/ptrace_scope", "r");
+
+  if (yama_ptrace_scope != nullptr)
+    {
+      char yama_scope = fgetc (yama_ptrace_scope.get ());
+
+      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'.  You can disable it by executing (as root):\n\
+\n\
+  echo 0 > /proc/sys/kernel/yama/ptrace_scope\n"));
+    }
+
+  if (ret.empty ())
+    {
+      /* It wasn't possible to determine the exact reason for the
+	 ptrace error.  Let's just emit a generic error message
+	 pointing the user to our documentation, where she can find
+	 instructions on how to try to diagnose the problem.  */
+      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.");
+    }
+
+  /* The user may be debugging remotely, so we have to warn that
+     the instructions above should be performed in the target.  */
+  string_appendf (ret,
+		  _("\n\
+If you are debugging the inferior remotely, the ptrace restriction(s) must\n\
+be disabled in the target system (e.g., where GDBserver is running)."));
+
+  return ret;
+}
+
+/* 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.  Helper for linux_ptrace_attach_fail_reason and
+   linux_ptrace_attach_fail_reason_lwp.  */
+
+static std::string
+linux_ptrace_attach_fail_reason_1 (pid_t pid)
 {
   pid_t tracerpid = linux_proc_get_tracerpid_nowarn (pid);
   std::string result;
@@ -56,10 +133,24 @@  linux_ptrace_attach_fail_reason (pid_t pid)
 /* See linux-ptrace.h.  */
 
 std::string
-linux_ptrace_attach_fail_reason_string (ptid_t ptid, int err)
+linux_ptrace_attach_fail_reason (pid_t pid, int err)
+{
+  std::string result = linux_ptrace_attach_fail_reason_1 (pid);
+  std::string ptrace_restrict = linux_ptrace_restricted_fail_reason (err);
+
+  if (!ptrace_restrict.empty ())
+    result += "\n" + ptrace_restrict;
+
+  return result;
+}
+
+/* See linux-ptrace.h.  */
+
+std::string
+linux_ptrace_attach_fail_reason_lwp (ptid_t ptid, int err)
 {
   long lwpid = ptid.lwp ();
-  std::string reason = linux_ptrace_attach_fail_reason (lwpid);
+  std::string reason = linux_ptrace_attach_fail_reason_1 (lwpid);
 
   if (!reason.empty ())
     return string_printf ("%s (%d), %s", safe_strerror (err), err,
@@ -68,6 +159,14 @@  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.  */
@@ -257,6 +356,12 @@  linux_ptrace_test_ret_to_nx (void)
 #endif /* defined __i386__ || defined __x86_64__ */
 }
 
+/* If the PTRACE_TRACEME call on linux_child_function errors, we need
+   to be able to send ERRNO back to the parent so that it can check
+   whether there are restrictions in place preventing ptrace from
+   working.  We do that with a pipe.  */
+static int errno_pipe[2];
+
 /* Helper function to fork a process and make the child process call
    the function FUNCTION, passing CHILD_STACK as parameter.
 
@@ -273,6 +378,11 @@  linux_fork_to_function (gdb_byte *child_stack, int (*function) (void *))
   /* Sanity check the function pointer.  */
   gdb_assert (function != NULL);
 
+  /* Create the pipe that will be used by the child to pass ERRNO
+     after the PTRACE_TRACEME call.  */
+  if (pipe (errno_pipe) != 0)
+    trace_start_error_with_name ("pipe");
+
 #if defined(__UCLIBC__) && defined(HAS_NOMMU)
 #define STACK_SIZE 4096
 
@@ -321,7 +431,21 @@  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);
+  /* Close read end.  */
+  close (errno_pipe[0]);
+
+  int ret = ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) 0,
+		    (PTRACE_TYPE_ARG4) 0);
+  int ptrace_errno = errno;
+
+  /* Write ERRNO to the pipe, even if it's zero, and close the writing
+     end of the pipe.  */
+  write (errno_pipe[1], &ptrace_errno, sizeof (ptrace_errno));
+  close (errno_pipe[1]);
+
+  if (ret != 0)
+    _exit (0);
+
   kill (getpid (), SIGSTOP);
 
   /* Fork a grandchild.  */
@@ -336,6 +460,48 @@  static void linux_test_for_tracesysgood (int child_pid);
 static void linux_test_for_tracefork (int child_pid);
 static void linux_test_for_exitkill (int child_pid);
 
+/* Helper function to wait for the child to send us the ptrace ERRNO,
+   and check if it's OK.  */
+
+static void
+linux_check_child_ptrace_errno ()
+{
+  int child_errno;
+  fd_set rset;
+  struct timeval timeout;
+
+  /* Close the writing end of the pipe.  */
+  close (errno_pipe[1]);
+
+  FD_ZERO (&rset);
+  FD_SET (errno_pipe[0], &rset);
+
+  /* One second should be plenty of time to wait for the child's
+     reply.  */
+  timeout.tv_sec = 1;
+  timeout.tv_usec = 0;
+
+  int ret = select (errno_pipe[0] + 1, &rset, NULL, NULL, &timeout);
+
+  if (ret < 0)
+    trace_start_error_with_name ("select");
+  else if (ret == 0)
+    error (_("Timeout while waiting for child's ptrace errno"));
+  else
+    read (errno_pipe[0], &child_errno, sizeof (child_errno));
+
+  if (child_errno != 0)
+    {
+      /* The child can't use PTRACE_TRACEME.  We just bail out.  */
+      std::string reason = linux_ptrace_restricted_fail_reason (child_errno);
+
+      errno = child_errno;
+      trace_start_error_with_name ("ptrace", reason.c_str ());
+    }
+
+  close (errno_pipe[0]);
+}
+
 /* Determine ptrace features available on this target.  */
 
 void
@@ -352,6 +518,9 @@  linux_check_ptrace_features (void)
      reporting.  */
   child_pid = linux_fork_to_function (NULL, linux_child_function);
 
+  /* Check if the child can successfully use ptrace.  */
+  linux_check_child_ptrace_errno ();
+
   ret = my_waitpid (child_pid, &status, 0);
   if (ret == -1)
     perror_with_name (("waitpid"));
diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
index fd2f12a342..90afb60f34 100644
--- a/gdb/nat/linux-ptrace.h
+++ b/gdb/nat/linux-ptrace.h
@@ -176,12 +176,27 @@  struct buffer;
 # define TRAP_HWBKPT 4
 #endif
 
-extern std::string linux_ptrace_attach_fail_reason (pid_t pid);
-
-/* 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);
+/* 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);
+
+/* Find all possible reasons we could have failed to attach to PID's
+   LWPID and return them as a string.  ERR is the error PTRACE_ATTACH
+   failed with (an errno).  Unlike linux_ptrace_attach_fail_reason,
+   this function should be used when attaching to an LWP other than
+   the leader; it does not warn about ptrace restrictions.  */
+extern std::string linux_ptrace_attach_fail_reason_lwp (ptid_t pid, 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);
diff --git a/gdb/remote.c b/gdb/remote.c
index ae06c4ba79..e5780bf586 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -5831,8 +5831,20 @@  extended_remote_target::attach (const char *args, int from_tty)
     case PACKET_UNKNOWN:
       error (_("This target does not support attaching to a process"));
     default:
-      error (_("Attaching to %s failed"),
-	     target_pid_to_str (ptid_t (pid)).c_str ());
+      {
+	std::string errmsg = rs->buf.data ();
+
+	if (!errmsg.empty ())
+	  {
+	    /* Get rid of the "E." prefix.  */
+	    errmsg.erase (0, 2);
+	  }
+
+	error (_("Attaching to %s failed%s%s"),
+	       target_pid_to_str (ptid_t (pid)).c_str (),
+	       !errmsg.empty () ? "\n" : "",
+	       errmsg.c_str ());
+      }
     }
 
   set_current_inferior (remote_add_inferior (false, pid, 1, 0));