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* [Bug dynamic-link/29740] New: Race condition between pthread_exit and fork
@ 2022-11-01 14:12 prekgeo at yahoo dot com
2022-11-01 14:14 ` [Bug dynamic-link/29740] " prekgeo at yahoo dot com
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: prekgeo at yahoo dot com @ 2022-11-01 14:12 UTC (permalink / raw)
To: glibc-bugs
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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* [Bug dynamic-link/29740] Race condition between pthread_exit and fork
2022-11-01 14:12 [Bug dynamic-link/29740] New: Race condition between pthread_exit and fork prekgeo at yahoo dot com
@ 2022-11-01 14:14 ` prekgeo at yahoo dot com
2022-11-01 14:15 ` prekgeo at yahoo dot com
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: prekgeo at yahoo dot com @ 2022-11-01 14:14 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29740
--- Comment #1 from George Prekas <prekgeo at yahoo dot com> ---
Created attachment 14428
--> https://sourceware.org/bugzilla/attachment.cgi?id=14428&action=edit
Reproduction
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* [Bug dynamic-link/29740] Race condition between pthread_exit and fork
2022-11-01 14:12 [Bug dynamic-link/29740] New: Race condition between pthread_exit and fork prekgeo at yahoo dot com
2022-11-01 14:14 ` [Bug dynamic-link/29740] " prekgeo at yahoo dot com
@ 2022-11-01 14:15 ` prekgeo at yahoo dot com
2022-11-01 15:03 ` schwab@linux-m68k.org
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: prekgeo at yahoo dot com @ 2022-11-01 14:15 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29740
George Prekas <prekgeo at yahoo dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |prekgeo at yahoo dot com
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* [Bug dynamic-link/29740] Race condition between pthread_exit and fork
2022-11-01 14:12 [Bug dynamic-link/29740] New: Race condition between pthread_exit and fork prekgeo at yahoo dot com
2022-11-01 14:14 ` [Bug dynamic-link/29740] " prekgeo at yahoo dot com
2022-11-01 14:15 ` prekgeo at yahoo dot com
@ 2022-11-01 15:03 ` schwab@linux-m68k.org
2022-11-01 16:38 ` fw at deneb dot enyo.de
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: schwab@linux-m68k.org @ 2022-11-01 15:03 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29740
Andreas Schwab <schwab@linux-m68k.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
The child may only call async-signal-safe functions, which pthread_create
isn't.
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* [Bug dynamic-link/29740] Race condition between pthread_exit and fork
2022-11-01 14:12 [Bug dynamic-link/29740] New: Race condition between pthread_exit and fork prekgeo at yahoo dot com
` (2 preceding siblings ...)
2022-11-01 15:03 ` schwab@linux-m68k.org
@ 2022-11-01 16:38 ` fw at deneb dot enyo.de
2022-11-01 16:50 ` prekgeo at yahoo dot com
2022-11-01 17:01 ` fw at deneb dot enyo.de
5 siblings, 0 replies; 7+ messages in thread
From: fw at deneb dot enyo.de @ 2022-11-01 16:38 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29740
Florian Weimer <fw at deneb dot enyo.de> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |fw at deneb dot enyo.de
--- Comment #3 from Florian Weimer <fw at deneb dot enyo.de> ---
What's your glibc version? I don't expect unwind-link to have this problem. See
__libc_unwind_link_after_fork. The unwind-link facility was added in glibc
2.34.
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* [Bug dynamic-link/29740] Race condition between pthread_exit and fork
2022-11-01 14:12 [Bug dynamic-link/29740] New: Race condition between pthread_exit and fork prekgeo at yahoo dot com
` (3 preceding siblings ...)
2022-11-01 16:38 ` fw at deneb dot enyo.de
@ 2022-11-01 16:50 ` prekgeo at yahoo dot com
2022-11-01 17:01 ` fw at deneb dot enyo.de
5 siblings, 0 replies; 7+ messages in thread
From: prekgeo at yahoo dot com @ 2022-11-01 16:50 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29740
--- Comment #4 from George Prekas <prekgeo at yahoo dot com> ---
This was tested and reproduces on the latest master (commit
b4174c28d21e1672ef3cc15a058558e97b8471c6).
After reading Andreas' response, I found this snippet in fork's man page which
renders the reproduction code invalid:
* After a fork() in a multithreaded program, the child can
safely call only async-signal-safe functions (see
signal-safety(7)) until such time as it calls execve(2).
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* [Bug dynamic-link/29740] Race condition between pthread_exit and fork
2022-11-01 14:12 [Bug dynamic-link/29740] New: Race condition between pthread_exit and fork prekgeo at yahoo dot com
` (4 preceding siblings ...)
2022-11-01 16:50 ` prekgeo at yahoo dot com
@ 2022-11-01 17:01 ` fw at deneb dot enyo.de
5 siblings, 0 replies; 7+ messages in thread
From: fw at deneb dot enyo.de @ 2022-11-01 17:01 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29740
--- Comment #5 from Florian Weimer <fw at deneb dot enyo.de> ---
I see. So it's probably the half-completed dlopen that confuses the subprocess.
We do this after fork:
/* Reset the lock the dynamic loader uses to protect its data. */
__rtld_lock_initialize (GL(dl_load_lock));
/* Reset the lock protecting dynamic TLS related data. */
__rtld_lock_initialize (GL(dl_load_tls_lock));
That's unfortunately not particularly safe.
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2022-11-01 14:12 [Bug dynamic-link/29740] New: Race condition between pthread_exit and fork prekgeo at yahoo dot com
2022-11-01 14:14 ` [Bug dynamic-link/29740] " prekgeo at yahoo dot com
2022-11-01 14:15 ` prekgeo at yahoo dot com
2022-11-01 15:03 ` schwab@linux-m68k.org
2022-11-01 16:38 ` fw at deneb dot enyo.de
2022-11-01 16:50 ` prekgeo at yahoo dot com
2022-11-01 17:01 ` fw at deneb dot enyo.de
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