How to debug a segmentation fault? View as Markdown

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?

I'm not sure if there are official recommendations, but what I've done before for httpd:

1. Do you have a line in your syslogs (e.g. /var/log/messages or elsewhere for ip (instruction pointer) segfaulted in libphp7.so or elsewhere, etc. If you run objdump --disassemble-all on the shared library where the crash occurs, you might know slightly more, but not very much, and would want to generate a core dump instead.

   For something like "segfault at ffffffff00000008 ip 00007f2ca2ef7121 sp 00007ffeef578850 error 5 in libphp7.so[7f2ca2a83000+b95000]", it'd have crashed at the instruction on 0x00007f2ca2ef7121 - 0x7f2ca2a83000 in libphp7.so

2. I haven't tried these instructions before, but you probably want to enable core dumps. https://ma.ttias.be/generate-php-core-dumps-segfaults-php-fpm/ (first google result) may help.
   Then, you can run 'gdb /usr/local/php/modules/libphp7.so /tmp/coredumpxxx' to see the stack trace where the dump occurred to include it, and include it in a bug report. (the path or shared library you want may differ)
   If you can install debugging symbols (I'm not sure what the package is called - something like php-debuginfo or running debuginfo-install ....?), or build php with --enable-debug and reproduce the crash, that would also help.
   Make sure to turn off core dumps and restart php-fpm when you're done.

Using a build of php configured with --enable-debug would also tell you if a ZEND_ASSERT failed before the segmentation fault (I think, I forget how it works with fpm)

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

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.).