Re: core_pattern pipe documentation

Michael Kerrisk
2008-04-25T19:50:24Z

Andi, Can you offer any input on the last topic in the message below?

On Fri, Apr 25, 2008 at 8:54 PM, Neil Horman <nhorman-2XuSBdqkA4R54TAoqtyWWQ@public.gmane.org> wrote:
>
> On Fri, Apr 25, 2008 at 08:13:21PM +0200, Michael Kerrisk wrote:
> > On Fri, Apr 25, 2008 at 6:22 PM, Neil Horman <nhorman-2XuSBdqkA4R54TAoqtyWWQ@public.gmane.org> wrote:
> > >
> > > 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?
> >
> > Have a look at the following. It demonstrates what I'm seeing (that
> > the coredump program is run as root/root).
> >
> > ===
> > $ cat core_pattern_test.c
> > /* core_pattern_test.c */
> >
> > #define _GNU_SOURCE
> > #include <sys/stat.h>
> > #include <fcntl.h>
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <unistd.h>
> >
> > #define BUF_SIZE 1024
> >
> > int
> > main(int argc, char *argv[])
> > {
> > int fd, tot, j;
> > ssize_t nread;
> > char buf[BUF_SIZE];
> > FILE *fp;
> >
> > fd = open(argv[1], O_CREAT | O_WRONLY | O_TRUNC, 0666);
> > if (fd == -1)
> > exit(EXIT_FAILURE);
> >
> > fp = fdopen(fd, "a");
> >
> > fprintf(fp, "PID=%ld\n", (long) getpid());
> > fprintf(fp, "cwd=%s\n", get_current_dir_name());
> > fprintf(fp, "UID=%ld; EUID=%ld\n", (long) getuid(), (long) geteuid());
> > fprintf(fp, "GID=%ld; EGID=%ld\n", (long) getgid(), (long) getegid());
> >
> > fprintf(fp, "argc=%d\n", argc);
> > for (j = 0; j < argc; j++)
> > fprintf(fp, "argc[%d]=<%s>\n", j, argv[j]);
> >
> > /* Count bytes in standard input */
> >
> > tot = 0;
> > while ((nread = read(STDIN_FILENO, buf, BUF_SIZE)) > 0)
> > tot += nread;
> > fprintf(fp, "Total bytes in core dump: %d\n", tot);
> >
> > exit(EXIT_SUCCESS);
> > }
> > $ cc core_pattern_test.c
> > $ sudo sh -c 'echo "|$PWD/core_pattern_test $PWD/c p_%p u_%u g_%g t_%t
> > c_%c" > /proc/sys/kernel/core_pattern'
> > root's password:
> > $ id
> > uid=1000(mtk) gid=100(users) groups=16(dialout),33(video),100(users)
> > $ sleep 100
> > [type ^\]
> > Quit (core dumped)
> > $ cat c
> > PID=6743
> > cwd=/
> > UID=0; EUID=0
> > GID=0; EGID=0
> > argc=7
> > argc[0]=</home/mtk/man-pages/man5/core_pattern_test>
> > argc[1]=</home/mtk/man-pages/man5/c>
> > argc[2]=<p_6742>
> > argc[3]=<u_1000>
> > argc[4]=<g_100>
> > argc[5]=<t_1209146940>
> > argc[6]=<c_4294967295>
> > Total bytes in core dump: 282624
> > $
> > ===
> >
> > Your thoughts?
> >
> > Cheers,
> >
> > Michael
>
> I certainly don't doubt your results. The uid/gid options IIRC were there when
> I made my updates and so I left them alone, wokring under the asumption that
> thats what they were there for. Clearly you have evidence to the contrary here.
> It seems like we should run the core collector as the uid of the dumping
> process, simply because it doesn't need to be run as root (and one could force
> the running of the commadn as root using the suid bit on the executable file).
>
> I say documented like your experiment shows it to work, and I'll try to find
> some time to investigate where we switch uids and why.
>
> Thanks for the info!
> Neil
>
> --
> /****************************************************
> * Neil Horman <nhorman-2XuSBdqkA4R54TAoqtyWWQ@public.gmane.org>
> * Software Engineer, Red Hat
> ****************************************************/
>

--
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