Re: [PATCH v2 2/5] Don't return stale data from fbsd_pid_to_exec_file for kernel processes.

[John Baldwin <jhb@freebsd.org>]

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.

-- 
John Baldwin