Re: [PATCH] sandbox: Support signal handling only when requested

6 Jun 2021


      Am 6. Juni 2021 20:07:31 MESZ schrieb Sean Anderson seanga2@gmail.com:
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On 6/6/21 1:57 PM, Heinrich Schuchardt wrote:
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On 6/6/21 7:52 PM, Sean Anderson wrote:
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On 6/6/21 1:28 PM, Heinrich Schuchardt wrote:
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On 6/6/21 6:44 PM, Simon Glass wrote:
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Hi Heinrich,
On Mon, 22 Mar 2021 at 18:56, Simon Glass sjg@chromium.org
wrote:
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Hi Heinrich,
On Mon, 22 Mar 2021 at 23:02, Heinrich Schuchardt
xypron.glpk@gmx.de wrote:
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> 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
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>> stuck
>> in a loop until the stack overflows:
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>> Segmentation violation
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>> Segmentation violation
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>> Segmentation violation
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>> Segmentation violation
>>
>> Segmentation violation
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>> 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
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when run directly.
BTW this sems to expose some rather nasty bug in dlmalloc or how
it is
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used. I notice that as soon as the first test is run, the 'top'
value
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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.
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> Corrupting gd could cause an endless recursive loop, as these
lines
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> follow printing the observed string:
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>          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,
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> recursion cannot occur anymore. If a segmentation violation
occurs
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> inside the handler it will be delegated to the default handler.
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> Furthermore we could consider removing the signal handler at the
start
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> of os_signal_action().
The issue is that if you get a segfault you really don't know if
you
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can continue and do anything else.
What is the goal with the signal handler? I don't think the user
can
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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.
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Can't you just rerun with gdb?
This would require that the problem is easily reproducible which may
not
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be the case.
Hm, perhaps you could keep track of how many times we've tried to catch
a signal, and bail if this is the second time around. E.g. something
like
Removing SA_NODEFER from the signal mask will let the OS kick in if an exception occurs in a signal handler.
No counter is needed.
Best regards
Heinrich
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static void os_signal_handler(int sig, siginfo_t *info, void *con)
{
   /* other variables */
   static int level = 0;
switch (level++) {
   case 0:
   	break;
   case 1:
   	sandbox_exit();
   default:
   	os_exit(0);
   }
/* rest of the handler */
}
--Sean
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Best regards
Heinrich
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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