Re: core_pattern pipe documentation

Neil Horman
2008-04-25T16:22:25Z

On Fri, Apr 25, 2008 at 03:18:46PM +0200, Michael Kerrisk wrote:
> Hi Neil,
>
> On Wed, Apr 23, 2008 at 4:59 PM, Neil Horman <nhorman-2XuSBdqkA4R54TAoqtyWWQ@public.gmane.org> 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-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org> 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?
>
Oops, you're right. I had initially implemented my core pattern updates this
way, but in the end wound up just ignoring the limit in do_coredump, rather than
re-writing it. Thanks for that. You can scratch this.

> > 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.)
>
Hmm, are you sure, I was under the impression that we fork the usermodehelper in
do_coredump as a parent of current, which has the dumping processes uid/gid. I
do see that in do_coredump we call get_dumpable(mm) and if it returns with the
appropriate value we switch current->fsuid to 0. I wonder if thats what you're
seeing?

Thanks & Regards
Neil

> 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

--
/****************************************************
* Neil Horman <nhorman-2XuSBdqkA4R54TAoqtyWWQ@public.gmane.org>
* Software Engineer, Red Hat
****************************************************/