Re: core_pattern pipe documentation

Michael Kerrisk
2008-04-25T18:13:21Z

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