Re: core_pattern pipe documentation

[Michael Kerrisk]

Hi Neil,

On Wed, Apr 23, 2008 at 4:59 PM, Neil Horman <nhorman@xxxxxxxxxxxxx> wrote:
>
> On Wed, Apr 23, 2008 at 02:09:14PM +0200, Michael Kerrisk wrote:
> > Andi -- ping!
> >
> > Adding Neil to CC, since it looks like he also did some work here, and
> > so can perhaps comment.
> >
> > On Fri, Apr 18, 2008 at 6:53 PM, Michael Kerrisk
> > <mtk.manpages@xxxxxxxxxxxxxx> wrote:
> > > Andi,
> > >
> > > I wrote the following description of the core_pattern pipe feature.  Does this
> > > seem okay?
> > >
> > >   Piping core dumps to a program
> > >       Since kernel 2.6.19, Linux supports an  alternate  syntax
> > >       for the /proc/sys/kernel/core_pattern file.  If the first
> > >       character of this file is a pipe  symbol  (|),  then  the
> > >       remainder  of  the line is interpreted as a program to be
> > >       executed.  Instead of being written to a disk  file,  the
> > >       core  dump  is  given  as  standard input to the program.
> > >       Note the following points:
> > >
> > >       *  The program must be specified using an absolute  path-
> > >          name  (or  a  pathname relative to the root directory,
> > >          /), and must immediately follow the '|' character.
> > >
> > >       *  The process created to run the program  runs  as  user
> > >          and group root.
> > >
> > >       *  Arguments can be supplied to the program, delimited by
> > >          white space (up to a total line length of 128  bytes).
> > >
> > > Cheers,
> > >
> > > Michael
> > >
> Thanks for CC'ing me.  The above all looks good.  I would add documentation
> however, about the available macros that can be used when core_pattern is
> specified as a pipe.  Adding something like the following would be good:
>
>        * Arguments can be statically declared or implied via the use of macros,
>          denoted by the use of the %sign.  The following macros are supported:
>                * %% - output a literal % sign on the command line
>                * %p - the pid of the crashing process
>                * %u - the uid of the crashing process
>                * %g - the gid of the crashing process
>                * %s - the signal that caused the crashing process to crash
>                * %t - the time the crashing process dumped
>                * %h - the hostname of the system
>                * %e - the executable name of the crashing process
>                * %c - the core limit size of the crashing process

Thanks for pointing that out!  I'll note it in the page.

>          Note that the core limit size macro may be a different value than what
>          is returned by getrlimit(RLIMIT_CORE,...).  This is due to the fact
>          that the core_pattern specified executible will be run as the same uid
>          as the crashing process, and to facilitate reception of the entire
>          core, the kernel will temporarily set RLIMIT_CORE to unlimited while
>          the dump is in progress.

Actually, I can't seem to get an example of this behavior.  In my
experiments, %c always seems to give the "right" info (i.e., I don't
ever see %c showing 2^32 (unlimited) when I set a soft limit).  Can
you show a specific case where it doesn't give the "right" value?

>          Note also %u and %g may be different values
>          than getuid/getgid in the event that the core_pattern executable is
>          set[u|g]id root

I'm slightly confused by that last point.  According to my
experiments, the core_pattern executable is always run as user and
group root, so making it set[ug]id root would seem to be a no-op.
(But anyway, %u and %g do give the "right" values -- the UID and GID
of the dumping process.)

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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