[Bug nptl/14697] New: Behavior of exit is nonconformant with respect to threads and stdio

[Bug nptl/14697] New: Behavior of exit is nonconformant with respect to threads and stdio

[bugdal at aerifal dot cx]

http://sourceware.org/bugzilla/show_bug.cgi?id=14697

             Bug #: 14697
           Summary: Behavior of exit is nonconformant with respect to
                    threads and stdio
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
        AssignedTo: unassigned@sourceware.org
        ReportedBy: bugdal@aerifal.cx
                CC: drepper.fsp@gmail.com
    Classification: Unclassified


Consider the following program:

#include <pthread.h>
#include <stdio.h>
#include <semaphore.h>
#include <stdlib.h>

void *f(void *p) { flockfile(stdin); sem_post(p); for (;;) pause(); }

int main()
{
    sem_t sem;
    sem_init(&sem, 0, 0);
    pthread_create(&(pthread_t){0}, 0, f, &sem);
    while (sem_wait(&sem));
    exit(0);
}

Per Austin Group interpretation for issue #611
(http://austingroupbugs.net/view.php?id=611), this program should deadlock in
exit waiting for the lock it will never obtain. Under glibc/NPTL, it exits
immediately.

If you'd like to make the example more interesting, you could have the thread
wake up after 5 seconds and unlock stdin; in that case, the program should run
for at least 5 seconds, rather than exiting immediately.

To make it even more interesting, have the thread performing a long-running
write operation that's intended to be atomic with respect to other threads and
also with respect to program termination (such that on normal program
termination, either the entire write happened, or no write happened at all).

This bug is due to intentional hackery in glibc to avoid hanging on exit() due
to locks being held by other threads, under the wrong assumption that exit()
"should" immediately exit in this case. There is no language in the standards
to support what glibc is doing.

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[Bug nptl/14697] Behavior of exit is nonconformant with respect to threads and stdio

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14697

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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[Bug nptl/14697] Behavior of exit is nonconformant with respect to threads and stdio

[bugdal at aerifal dot cx]

https://sourceware.org/bugzilla/show_bug.cgi?id=14697

--- Comment #1 from Rich Felker <bugdal at aerifal dot cx> ---
It seems that this bug can also result in more serious corruption such as
duplicate output, even without any explicit file locking. See the example in
this question on Stack Overflow, which is a perfectly valid program producing
incorrect output:

http://stackoverflow.com/questions/26211423/unexpected-output-in-a-multithreaded-program

Of course the program has unpredictable output, but there is a finite set of
outputs it can produce on a correct implementation: different interleavings of
the lines, and different cutoffs for the number of lines produced by
sample_thread. A real-world example where this could easily happen is writing a
log file using stdio.

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[Bug nptl/14697] Behavior of exit is nonconformant with respect to threads and stdio

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14697

--- Comment #4 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Andreas Schwab from comment #2)
> Note that nptl/tst-stdio1 expects this behaviour.

I posted patches to fix the test:

[PATCH 1/2] support: Add delayed__exit (with two underscores)
<https://sourceware.org/pipermail/libc-alpha/2023-June/148829.html>

[PATCH 2/2] pthreads: Use _exit to terminate the tst-stdio1 test
<https://sourceware.org/pipermail/libc-alpha/2023-June/148830.html>

I wonder if we should add a new symbol version for exit, so that the behavior
only changes for new applications. We did that for quick_exit. (Otherwise we
may have to add a tunable to bring back the old behavior—or we may have to do
that anyway.)

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[Bug nptl/14697] Behavior of exit is nonconformant with respect to threads and stdio

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=14697

--- Comment #5 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Andreas Schwab <schwab@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=af130d27099651e0d27b2cf2cfb44dafd6fe8a26

commit af130d27099651e0d27b2cf2cfb44dafd6fe8a26
Author: Andreas Schwab <schwab@suse.de>
Date:   Tue Jan 30 10:16:00 2018 +0100

    Always do locking when accessing streams (bug 15142, bug 14697)

    Now that abort no longer calls fflush there is no reason to avoid locking
    the stdio streams anywhere.  This fixes a conformance issue and potential
    heap corruption during exit.

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[Bug nptl/14697] Behavior of exit is nonconformant with respect to threads and stdio

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14697

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |2.38
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #6 from Florian Weimer <fweimer at redhat dot com> ---
Fixed for 2.38.

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[Bug nptl/14697] Behavior of exit is nonconformant with respect to threads and stdio

[gabravier at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14697

Gabriel Ravier <gabravier at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gabravier at gmail dot com

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