Patchwork [v2,2/5] Don't return stale data from fbsd_pid_to_exec_file for kernel processes.

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Submitter John Baldwin
Date Jan. 4, 2018, 1:49 a.m.
Message ID <20180104014923.11899-3-jhb@FreeBSD.org>
Download mbox | patch
Permalink /patch/25199/
State New
Headers show

Comments

John Baldwin - Jan. 4, 2018, 1:49 a.m.
For processes without an associated executable (such as kernel processes),
the kern.proc.pathname.<pid> system control node returns a length of zero
without modifying the user's buffer.  Detect this case and return NULL
rather than the previous contents of the static buffer 'buf'.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_pid_to_exec_file) [KERN_PROC_PATHNAME]: Return
	NULL for an empty pathname.
---
 gdb/ChangeLog  | 5 +++++
 gdb/fbsd-nat.c | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)
Simon Marchi - Jan. 5, 2018, 2:57 a.m.
On 2018-01-03 08:49 PM, John Baldwin wrote:
> For processes without an associated executable (such as kernel processes),
> the kern.proc.pathname.<pid> system control node returns a length of zero
> without modifying the user's buffer.  Detect this case and return NULL
> rather than the previous contents of the static buffer 'buf'.
> 
> gdb/ChangeLog:
> 
> 	* fbsd-nat.c (fbsd_pid_to_exec_file) [KERN_PROC_PATHNAME]: Return
> 	NULL for an empty pathname.
> ---
>  gdb/ChangeLog  | 5 +++++
>  gdb/fbsd-nat.c | 2 +-
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/ChangeLog b/gdb/ChangeLog
> index 29cfbb287b..804dd4f402 100644
> --- a/gdb/ChangeLog
> +++ b/gdb/ChangeLog
> @@ -1,3 +1,8 @@
> +2018-01-03  John Baldwin  <jhb@FreeBSD.org>
> +
> +	* fbsd-nat.c (fbsd_pid_to_exec_file) [KERN_PROC_PATHNAME]: Return
> +	NULL for an empty pathname.
> +
>  2018-01-03  John Baldwin  <jhb@FreeBSD.org>
>  
>  	* fbsd-tdep.c (KVE_STRUCTSIZE, KVE_START, KVE_END, KVE_OFFSET)
> diff --git a/gdb/fbsd-nat.c b/gdb/fbsd-nat.c
> index ec4eed9abe..7b1d1bf148 100644
> --- a/gdb/fbsd-nat.c
> +++ b/gdb/fbsd-nat.c
> @@ -63,7 +63,7 @@ fbsd_pid_to_exec_file (struct target_ops *self, int pid)
>    mib[3] = pid;
>    buflen = sizeof buf;
>    if (sysctl (mib, 4, buf, &buflen, NULL, 0) == 0)
> -    return buf;
> +    return buflen == 0 ? NULL : buf;
>  #endif
>  
>    xsnprintf (name, PATH_MAX, "/proc/%d/exe", pid);
> 

LGTM, but maybe add a comment saying in what situation the sysctl can
return a buflen of 0.

Does the alternative method that reads from /proc/<pid>/exe work in that
case too?

Simon
John Baldwin - Jan. 5, 2018, 6:49 p.m.
On Thursday, January 04, 2018 09:57:00 PM Simon Marchi wrote:
> On 2018-01-03 08:49 PM, John Baldwin wrote:
> > For processes without an associated executable (such as kernel processes),
> > the kern.proc.pathname.<pid> system control node returns a length of zero
> > without modifying the user's buffer.  Detect this case and return NULL
> > rather than the previous contents of the static buffer 'buf'.
> > 
> > gdb/ChangeLog:
> > 
> > 	* fbsd-nat.c (fbsd_pid_to_exec_file) [KERN_PROC_PATHNAME]: Return
> > 	NULL for an empty pathname.
> > ---
> >  gdb/ChangeLog  | 5 +++++
> >  gdb/fbsd-nat.c | 2 +-
> >  2 files changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/gdb/ChangeLog b/gdb/ChangeLog
> > index 29cfbb287b..804dd4f402 100644
> > --- a/gdb/ChangeLog
> > +++ b/gdb/ChangeLog
> > @@ -1,3 +1,8 @@
> > +2018-01-03  John Baldwin  <jhb@FreeBSD.org>
> > +
> > +	* fbsd-nat.c (fbsd_pid_to_exec_file) [KERN_PROC_PATHNAME]: Return
> > +	NULL for an empty pathname.
> > +
> >  2018-01-03  John Baldwin  <jhb@FreeBSD.org>
> >  
> >  	* fbsd-tdep.c (KVE_STRUCTSIZE, KVE_START, KVE_END, KVE_OFFSET)
> > diff --git a/gdb/fbsd-nat.c b/gdb/fbsd-nat.c
> > index ec4eed9abe..7b1d1bf148 100644
> > --- a/gdb/fbsd-nat.c
> > +++ b/gdb/fbsd-nat.c
> > @@ -63,7 +63,7 @@ fbsd_pid_to_exec_file (struct target_ops *self, int pid)
> >    mib[3] = pid;
> >    buflen = sizeof buf;
> >    if (sysctl (mib, 4, buf, &buflen, NULL, 0) == 0)
> > -    return buf;
> > +    return buflen == 0 ? NULL : buf;
> >  #endif
> >  
> >    xsnprintf (name, PATH_MAX, "/proc/%d/exe", pid);
> > 
> 
> LGTM, but maybe add a comment saying in what situation the sysctl can
> return a buflen of 0.

Ok.

> Does the alternative method that reads from /proc/<pid>/exe work in that
> case too?

No.  FreeBSD systems don't generally mount /proc (it's not enabled in the
default install), but in the case that 0 is returned, the relevant pointer
in the process structure (p_textvp) that '/proc/%d/exe' returns a path for
is NULL, so the procfs method will also fail.

Actually, I looked at FreeBSD's procfs and the node is named /proc/%d/file,
not /proc/%d/exe.  I did confirm that FreeBSD's procfs does not create
'file' nodes for kernel processes (but also not for init (pid 1)).

I should in fact probably refine this function further to only use procfs
in the #else case if not remove it all together: the last release to not
include the pathname sysctl was FreeBSD 5.5 released in May 2006.

Patch

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 29cfbb287b..804dd4f402 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@ 
+2018-01-03  John Baldwin  <jhb@FreeBSD.org>
+
+	* fbsd-nat.c (fbsd_pid_to_exec_file) [KERN_PROC_PATHNAME]: Return
+	NULL for an empty pathname.
+
 2018-01-03  John Baldwin  <jhb@FreeBSD.org>
 
 	* fbsd-tdep.c (KVE_STRUCTSIZE, KVE_START, KVE_END, KVE_OFFSET)
diff --git a/gdb/fbsd-nat.c b/gdb/fbsd-nat.c
index ec4eed9abe..7b1d1bf148 100644
--- a/gdb/fbsd-nat.c
+++ b/gdb/fbsd-nat.c
@@ -63,7 +63,7 @@  fbsd_pid_to_exec_file (struct target_ops *self, int pid)
   mib[3] = pid;
   buflen = sizeof buf;
   if (sysctl (mib, 4, buf, &buflen, NULL, 0) == 0)
-    return buf;
+    return buflen == 0 ? NULL : buf;
 #endif
 
   xsnprintf (name, PATH_MAX, "/proc/%d/exe", pid);