[Bug dynamic-link/29740] New: Race condition between pthread_exit and fork

["prekgeo at yahoo dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29740

            Bug ID: 29740
           Summary: Race condition between pthread_exit and fork
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: prekgeo at yahoo dot com
  Target Milestone: ---

Created attachment 14427
  --> https://sourceware.org/bugzilla/attachment.cgi?id=14427&action=edit
glibc patch to add delay in do_lookup_x

I have observed a crash in a Python program that uses threads and fork. Below,
I attach the following:

* a patch for the latest glibc master (commit
b4174c28d21e1672ef3cc15a058558e97b8471c6) that introduces a delay to force the
race condition.
* a minimal C reproduction.

My understanding of the problem is that the following sequence of events takes
place:

parent: main-thread: starts thread-1
parent: thread-1: exits
parent: thread-1: tries to call _Unwind_Find_FDE
parent: thread-1: ld.so loads libgcc_s.so in memory
parent: main-thread: fork()
parent: thread-1: resolves _Unwind_Find_FDE@got.plt
child: main-thread: starts thread-1
child: thread-1: exits
child: thread-1: calls _Unwind_Find_FDE using the unresolved
_Unwind_Find_FDE@got.plt

The child process crashes with a SIGSEGV with the following backtrace:

#0  0x0000000000003230 in ?? ()
#1  0x00007ffff75c2868 in uw_frame_state_for (context=0x7ffff7dcdc70,
fs=0x7ffff7dcdab0) at ../../../src/libgcc/unwind-dw2.c:1263
#2  0x00007ffff75c3a20 in uw_init_context_1 (context=0x7ffff7dcdc70,
outer_cfa=0x7ffff7dcdea0,
    outer_ra=0x7ffff7e64e36 <__GI___pthread_unwind+70>) at
../../../src/libgcc/unwind-dw2.c:1592
#3  0x00007ffff75c431a in _Unwind_ForcedUnwind (exc=0x7ffff7dced30,
stop=stop@entry=0x7ffff7e64ca0 <unwind_stop>,
    stop_argument=0x7ffff7dcdef0) at ../../../src/libgcc/unwind.inc:211
#4  0x00007ffff7e64e36 in __GI___pthread_unwind (buf=<optimized out>) at
unwind.c:130
#5  0x00007ffff7e5ddba in __do_cancel () at ../sysdeps/nptl/pthreadP.h:276
#6  __GI___pthread_exit (value=0x0) at pthread_exit.c:36
#7  0x00007ffff7fbf283 in ?? ()
#8  0x0000000000000000 in ?? ()

0x3230 is the value of _Unwind_Find_FDE@got.plt in the binary:

$ gdb -q /usr/lib/x86_64-linux-gnu/libgcc_s.so.1
(gdb) p '_Unwind_Find_FDE@got.plt'
$1 = (<text from jump slot in .got.plt, no debug info>) 0x3230

I have also a Python reproduction that fails without the forced delay and by
just enabling LD_DEBUG=all (which seems to add the needed delays). Let me know
if it's valuable to attach in this bug.

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