[PATCH] sandbox: Support signal handling only when requested

[Sean Anderson]

On 6/6/21 1:28 PM, Heinrich Schuchardt wrote:
> On 6/6/21 6:44 PM, Simon Glass wrote:
>> Hi Heinrich,
>>
>> On Mon, 22 Mar 2021 at 18:56, Simon Glass <sjg at chromium.org> wrote:
>>>
>>> Hi Heinrich,
>>>
>>> On Mon, 22 Mar 2021 at 23:02, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>>>>
>>>> On 22.03.21 06:21, Simon Glass wrote:
>>>>> At present if sandbox crashes it prints a message and tries to exit. But
>>>>> with the recently introduced signal handler, it often seems to get stuck
>>>>> in a loop until the stack overflows:
>>>>>
>>>>> Segmentation violation
>>>>>
>>>>> Segmentation violation
>>>>>
>>>>> Segmentation violation
>>>>>
>>>>> Segmentation violation
>>>>>
>>>>> Segmentation violation
>>>>>
>>>>> Segmentation violation
>>>>>
>>>>> Segmentation violation
>>>>> ...
>>>>
>>>> Hello Simon,
>>>>
>>>> do you have a reproducible example? I never have seen this.
>>>
>>> https://source.denx.de/u-boot/custodians/u-boot-dm/-/jobs/242433
>>>
>>> You need to run that commit with pytest though...it does not happen
>>> when run directly.
>>>
>>> BTW this sems to expose some rather nasty bug in dlmalloc or how it is
>>> used. I notice that as soon as the first test is run, the 'top' value
>>> in dlmalloc is outside the range of the malloc pool, which seems
>>> wrong. I wonder if there is something broken with how
>>> dm_test_pre_run() and dm_test_post_run() work.
>>>
>>>>
>>>> Corrupting gd could cause an endless recursive loop, as these lines
>>>> follow printing the observed string:
>>>>
>>>>          printf("pc = 0x%lx, ", pc);
>>>>          printf("pc_reloc = 0x%lx\n\n", pc - gd->reloc_off);
>>>
>>> Yes I suspect printf() is dead.
>>>
>>>>
>>>> If we remove SA_NODEFER from the signal mask in arch/sandbox/cpu/os.c,
>>>> recursion cannot occur anymore. If a segmentation violation occurs
>>>> inside the handler it will be delegated to the default handler.
>>>>
>>>> Furthermore we could consider removing the signal handler at the start
>>>> of os_signal_action().
>>>
>>> The issue is that if you get a segfault you really don't know if you
>>> can continue and do anything else.
>>>
>>> What is the goal with the signal handler? I don't think the user can
>>> do anything about it.
> 
> Hello Simon,
> 
> the signal handler prints out the crash location and this makes
> analyzing problems much easier. It proved valuable to me several times.

Can't you just rerun with gdb?

--Sean

> 
>>
>> I keep hitting this problem during development with sandbox, so I
>> think I need to apply this patch.
>>
>> Does anything need to be updated in the tests?
>>
>> Regards,
>> Simon
>>
> 
> Did you try removing SA_NODEFER as proposed?
> 
> Best regards
> 
> Heinrich