Re: Segfault in _Unwind_* code called from pthread_exit

On Thu, 24 Aug 2017 18:42:35 +0300 Konstantin Belousov <kostikbel_at_gmail.com> wrote:
> On Wed, Aug 23, 2017 at 04:37:07PM +0200, Tijl Coosemans wrote:
>> The following program segfaults for me on amd64 when linked like this:
>> 
>> cc -o test test.c -lpthread -L/usr/local/lib/gcc5 -lgcc_s -rpath /usr/local/lib/gcc5
>> 
>> --------------------------------
>> #include <pthread.h>
>> #include <stdio.h>
>> 
>> void *
>> thr( void *arg ) {
>> 	return( NULL );
>> }
>> 
>> int
>> main( void ) {
>> 	pthread_t thread;
>> 
>> 	for( int i = 1; i < 20; i++ ) {
>> 		fprintf( stderr, "%d\n", i );
>> 		pthread_create( &thread, NULL, thr, NULL );
>> 		pthread_join( thread, NULL );
>> 	}
>> 	return( 0 );
>> }
>> --------------------------------
>> 
>> The backtrace looks like this:
>> 
>> Thread 7 received signal SIGSEGV, Segmentation fault.
>> [Switching to LWP 100511 of process 1886]
>> uw_frame_state_for (context=context_at_entry=0x7fffdfffddc0, 
>>     fs=fs_at_entry=0x7fffdfffdb10)
>>     at /usr/ports/lang/gcc5/work/gcc-5.4.0/libgcc/unwind-dw2.c:1249
>> 1249	/usr/ports/lang/gcc5/work/gcc-5.4.0/libgcc/unwind-dw2.c: No such file or directory.
>> (gdb) bt
>> #0  uw_frame_state_for (context=context_at_entry=0x7fffdfffddc0, 
>>     fs=fs_at_entry=0x7fffdfffdb10)
>>     at /usr/ports/lang/gcc5/work/gcc-5.4.0/libgcc/unwind-dw2.c:1249
>> #1  0x0000000800a66ecb in _Unwind_ForcedUnwind_Phase2 (
>>     exc=exc_at_entry=0x800658730, context=context_at_entry=0x7fffdfffddc0)
>>     at /usr/ports/lang/gcc5/work/gcc-5.4.0/libgcc/unwind.inc:155
>> #2  0x0000000800a67200 in _Unwind_ForcedUnwind (exc=0x800658730, 
>>     stop=0x8008428b0 <thread_unwind_stop>, stop_argument=0x0)
>>     at /usr/ports/lang/gcc5/work/gcc-5.4.0/libgcc/unwind.inc:207
>> #3  0x0000000800842224 in _Unwind_ForcedUnwind (ex=0x800658730, 
>>     stop_func=0x8008428b0 <thread_unwind_stop>, stop_arg=0x0)
>>     at /usr/src/lib/libthr/thread/thr_exit.c:106
>> #4  0x000000080084269f in thread_unwind ()
>>     at /usr/src/lib/libthr/thread/thr_exit.c:172
>> #5  0x00000008008424d6 in _pthread_exit_mask (status=0x0, mask=0x0)
>>     at /usr/src/lib/libthr/thread/thr_exit.c:254
>> #6  0x0000000800842359 in _pthread_exit (status=0x0)
>>     at /usr/src/lib/libthr/thread/thr_exit.c:206
>> #7  0x000000080082ccb1 in thread_start (curthread=0x800658500)
>>     at /usr/src/lib/libthr/thread/thr_create.c:289
>> #8  0x00007fffdfdfe000 in ?? ()
>> Backtrace stopped: Cannot access memory at address 0x7fffdfffe000
>> 
>> 
>> It happens with gcc6 as well, but not with base libgcc_s.
>> Can anyone reproduce this?  Have there been any changes to stack
>> unwinding recently (last few months)?  
> 
> I can reproduce this, and there was a change in gcc unwinder, it seems.
> Below is a patch which I did not even compiled.  Still, it should give
> an idea how it might be approached.  The patch is against gcc head.

Currently I'm thinking to patch our cpu_set_upcall in vm_machdep.c to set
the return address for the thread entry point to NULL (#8 in the backtrace
above).  For new stacks this is implicitly NULL, but "Thread 7" (as gdb
calls it) uses a recycled stack and libthr stores a 'struct stack' at the
end of such stacks (to keep them in a linked list).  I'm still looking at
how base libgcc_s which uses LLVM libunwind avoids this problem.