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How to debug a segmentation fault?

php.internals

Benjamin Morel

6 years ago

Hi internals,

I'm encountering a SIGSEGV in PHP-FPM on PHP 7.4.2 in a Symfony app. The
bug seems to happen during the rendering of a Twig template; this makes it
hard for me to pinpoint the code that triggers the segfault.

Could you please tell me what the procedure to collect information is, so
that I can file a useful bug report?

Thank you,
— Benjamin

tyson andre

6 years ago

Nikita Popov

6 years ago

On Tue, Jan 28, 2020 at 5:29 PM Benjamin Morel <benjamin.morel@gmail.com>
wrote:

> Hi internals,
>
> I'm encountering a SIGSEGV in PHP-FPM on PHP 7.4.2 in a Symfony app. The
> bug seems to happen during the rendering of a Twig template; this makes it
> hard for me to pinpoint the code that triggers the segfault.
>
> Could you please tell me what the procedure to collect information is, so
> that I can file a useful bug report?
>

As the first step, I would always suggest to reproduce this issue with the
CLI binary (this includes the builtin server under -S localhost:8000 -t
public). That way you don't have to deal with multi-process FPM.

Once that is done, the easiest way to get some useful debugging information
is to run PHP under valgrind, this is done using "USE_ZEND_ALLOC=0 valgrind
php ...".

Nikita

Bishop Bettini

6 years ago

On Tue, Jan 28, 2020 at 11:28 AM Benjamin Morel <benjamin.morel@gmail.com>
wrote:

> I'm encountering a SIGSEGV in PHP-FPM on PHP 7.4.2 in a Symfony app. The
> bug seems to happen during the rendering of a Twig template; this makes it
> hard for me to pinpoint the code that triggers the segfault.
>
> Could you please tell me what the procedure to collect information is, so
> that I can file a useful bug report?
>

Great advice so far. I would add, since you're using PHP-FPM, that the pool
configuration needs to have:

process.dumpable = 1

to get core dumps out of workers. Without this you might not get core
dumps, even when you have other core dump settings configured (ulimit -c
unlimited, chmod +r phpfpm, etc.).

Benjamin Morel

6 years ago

>
> I would add, since you're using PHP-FPM, that the pool configuration needs
> to have:
> process.dumpable = 1

Thank you! I could not generate a core dump before adding this
configuration option!
The generating a gdb backtrace
<https://bugs.php.net/bugs-generating-backtrace.php> page looks a big
outdated, it would be nice if someone could update it will all the helpful
info received in this thread.

I'm not sure if I could gather helpful enough info about the issue, but
anyway I filed a bug here:
https://bugs.php.net/bug.php?id=79194

Thank you,
Benjamin

On Wed, 29 Jan 2020 at 18:19, Bishop Bettini <bishop@php.net> wrote:

tyson andre

6 years ago

tyson andre

6 years ago

Sorry, I didn't notice that https://bugs.php.net/bug.php?id=79194 was already closed with a similar answer.

tyson andre

6 years ago

Benjamin Morel

6 years ago

>
> Infinite recursion leading to a segfault is **a known bug with no plans to
> be fixed that I know of,** which I've encountered many times.

That's unfortunate; as I said in the bug report, it's so easy to trigger
and so easy to miss when reviewing the code! And the process crashing
instead of reporting an error is really a pity.
An explicit error message would be nice, but as I understand this would
require setting a hard limit on the call stack depth, which may bring other
issues.

Thank you,
— Benjamin

On Thu, 30 Jan 2020 at 03:59, tyson andre <tysonandre775@hotmail.com> wrote: