[PATCH] sandbox: Support signal handling only when requested

[Heinrich Schuchardt]

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.

>
> 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