[Bug stdio/14622] New: exit handlers not thread safe

["law at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14622

             Bug #: 14622
           Summary: exit handlers not thread safe
           Product: glibc
           Version: 2.17
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
        AssignedTo: unassigned@sourceware.org
        ReportedBy: law@redhat.com
    Classification: Unclassified


The exit handlers which flush & close streams are not thread safe.  

For example, given this code:

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

void *test(void *arg)
{
    return NULL;
}
void *writer(void *arg)
{
    for(;;) {
        char a[100];
        FILE *f = fopen("out", "w");

        if(f == NULL)
           abort();

        fputs("Test", f);

        if(fgets(a, 100, stdin))
            fputs(a, f);
        fclose(f);
    }

    return NULL;
}

int main(int argc, char *argv[])
{
    pthread_t tid1,tid2;


    pthread_create(&tid1, NULL, writer, NULL);
    pthread_create(&tid2, NULL, test, NULL);
    pthread_join(tid2, NULL);
    return 0;
}


while [ true ]; do 
  echo test | valgrind --error-exitcode=2 ./a.out  || break  
done  


If run under valgrind enough times you'll eventually see the IO cleanup
handlers referencing free'd memory:

==7234== Invalid read of size 4
==7234==    at 0x34A7275FC8: _IO_file_write@@GLIBC_2.2.5 (in
/usr/lib64/libc-2.15.so)
==7234==    by 0x34A7275EA1: new_do_write (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7276D44: _IO_do_write@@GLIBC_2.2.5 (in
/usr/lib64/libc-2.15.so)
==7234==    by 0x34A7278DB6: _IO_flush_all_lockp (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7278F07: _IO_cleanup (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7238BBF: __run_exit_handlers (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A7238BF4: exit (in /usr/lib64/libc-2.15.so)
==7234==    by 0x34A722173B: (below main) (in /usr/lib64/libc-2.15.so)
==7234==  Address 0x542f2e0 is 0 bytes inside a block of size 568 free'd
==7234==    at 0x4A079AE: free (vg_replace_malloc.c:427)
==7234==    by 0x34A726B11C: fclose@@GLIBC_2.2.5 (in /usr/lib64/libc-2.15.so)
==7234==    by 0x40087C: writer (t.c:22)
==7234==    by 0x34A7607D13: start_thread (in /usr/lib64/libpthread-2.15.so)
==7234==    by 0x34A72F167C: clone (in /usr/lib64/libc-2.15.so)



What's happening, is the thread in "writer" and the main program's thread are
racing.  If the "main" thread starts processing its exit IO handlers, then the
"writer" thread fcloses the stream (which deallocates the associated memory),
then the "main" thread continues processing its exit IO handlers and starts
dereferencing free'd memory.

The exit IO handlers explicitly avoid locking on the stream and the
list_all_lock.  Presumably to avoid having exit hang on an event that's never
going to happen.


At first I thought we could continue to avoid taking the stream lock, but
always honour the list_all_lock in IO_flush_all_lockp when called from the exit
IO handler.  However, that can block in a 3 thread case.  Thread 1 is blocked
waiting on an event that will never happen with its stream locked.  Thread 2
tries to fclose the same stream, it'll acquire the list_all_lock, then block on
the stream lock.  Thread 3 calls exit and blocks because it can't acquire the
list_all_lock.

I don't offhand see a good solution.  Even playing games with last_stamp
doesn't seem like it's work to me.

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