Re: [Qemu-devel] [6095] User-mode GDB stub improvements - handle fork

[Martin Mohring <martin.mohring@opensuse.org>]

Martin Mohring wrote:
> I seem to have found the cause, why ARM user mode with nptl does not
> work as before. see below.
>
> Martin
>
> Aurelien Jarno wrote:
>   
>> Revision: 6095
>>           http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=6095
>> Author:   aurel32
>> Date:     2008-12-18 22:44:04 +0000 (Thu, 18 Dec 2008)
>>
>> Log Message:
>> -----------
>> User-mode GDB stub improvements - handle fork
>>
>> Close gdbserver in child processes, so that only one stub tries to talk
>> to GDB at a time.  Updated from an earlier patch by Paul Brook.
>>
>> Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
>> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
>>
>> Modified Paths:
>> --------------
>>     trunk/gdbstub.c
>>     trunk/gdbstub.h
>>     trunk/linux-user/main.c
>>     trunk/linux-user/syscall.c
>>
>> Modified: trunk/gdbstub.c
>> ===================================================================
>> --- trunk/gdbstub.c	2008-12-18 22:43:56 UTC (rev 6094)
>> +++ trunk/gdbstub.c	2008-12-18 22:44:04 UTC (rev 6095)
>> @@ -1996,6 +1996,18 @@
>>      gdb_accept();
>>      return 0;
>>  }
>> +
>> +/* Disable gdb stub for child processes.  */
>> +void gdbserver_fork(CPUState *env)
>> +{
>> +    GDBState *s = gdbserver_state;
>> +    if (s->fd < 0)
>> +      return;
>> +    close(s->fd);
>> +    s->fd = -1;
>> +    cpu_breakpoint_remove_all(env, BP_GDB);
>> +    cpu_watchpoint_remove_all(env, BP_GDB);
>> +}
>>   
>>     
> In case of USE_NPTL is defined, it seems that either s or env can be
> NULL, so I get a exception (yes, nptl works on arm in user mode).
> My understanding is that this might be a race condition of the first
> fork() call coming even before env or gdbserver_state is initialised
> towards !NULL.
>   
The reason why I wrote this and did not propose a fix like Laurent did
now was the fact that:

- gdbserver_fork() is called unconditionally before gdbserver_start()
has ever been called
- gdbserver_fork() as result accesses gdbserver data structures before
they are inited, so we have NULL pointer access

Another issue is that gdbserver_fork() is only defined when
CONFIG_USER_ONLY is not defined but gdbserver_fork() is called when
always when USE_NPTL is defined.
So switching on CONFIG_USER_ONLY and USE_NPTL will result in
gdbserver_fork() beeing undefined. That was why my impression was that
the code is somewhat incomplete.

Also, does debugging work in user mode at all if USE_NPTL is on? If not
I would remove the call to gdbserver_fork() in this case completely.

Martin