Bug 14697 - Behavior of exit is nonconformant with respect to threads and stdio
Summary: Behavior of exit is nonconformant with respect to threads and stdio
Status: NEW
Alias: None
Product: glibc
Classification: Unclassified
Component: nptl (show other bugs)
Version: unspecified
: P2 normal
Target Milestone: ---
Assignee: Not yet assigned to anyone
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2012-10-10 18:04 UTC by Rich Felker
Modified: 2014-10-06 17:51 UTC (History)
1 user (show)

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fweimer: security-


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Description Rich Felker 2012-10-10 18:04:29 UTC
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.
Comment 1 Rich Felker 2014-10-06 17:51:29 UTC
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.