Re: [PATCH 3/8] Deliver signal in hardware single step

[Pedro Alves <palves@redhat.com>]

On 03/11/2016 11:05 AM, Pedro Alves wrote:
> On 03/04/2016 10:44 AM, Yao Qi wrote:
>> GDBserver doesn't deliver signal when stepping over a breakpoint even
>> hardware single step is used.  When GDBserver started to step over
>> (thread creation) breakpoint for mutlit-threaded debugging in 2002 [1],
>> GDBserver behaves this way.
>>
>> This behaviour gets trouble on conditional breakpoints on branch to
>> self instruction like this,
>>
>>      0x00000000004005b6 <+29>:	jmp    0x4005b6 <main+29>
>>
>> and I set breakpoint
>>
>> $(gdb) break branch-to-self.c:43 if counter > 3
>>
>> and the variable counter will be set to 5 in SIGALRM signal handler.
>> Since GDBserver keeps stepping over breakpoint, the SIGALRM can never
>> be dequeued and delivered to the inferior, so the program can't stop.
>> The test can be found in gdb.base/branch-to-self.exp.
>>
>> I can understand why does GDBserver queue signal for software single
>> step, but I can't figure out a reason we should queue signal for
>> hardware single step.  With this patch applied, GDBserver forward the
>> signal to inferior and the program can stop correctly.
>>
>> [1] PATCH: Multithreaded debugging for gdbserver
>>       https://sourceware.org/ml/gdb-patches/2002-06/msg00157.html
>>
>
> Because the signal handler might recurse and call the same code
> that had the breakpoint (or some other removed breakpoint), and thus
> we'd miss a breakpoint hit in the signal handler.

Hmm, no, I got confused.  We'll stop in first instruction in the signal 
handler.  Let me go back and take a fresh look.

Thanks,
Pedro Alves