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To: Benjamin Morel <benjamin.morel@gmail.com>, PHP Internals
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Thread-Topic: [PHP-DEV] How to debug a segmentation fault?
Thread-Index: AQHV1fgMRGyLBah1gUGPr6kq4bV9NagATidQ
Date: Tue, 28 Jan 2020 17:14:57 +0000
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Subject: Re: [PHP-DEV] How to debug a segmentation fault?
From: tysonandre775@hotmail.com (tyson andre)

> I'm encountering a SIGSEGV in PHP-FPM on PHP 7.4.2 in a Symfony app. The=
=0A=
> bug seems to happen during the rendering of a Twig template; this makes i=
t=0A=
> hard for me to pinpoint the code that triggers the segfault.=0A=
> =0A=
> Could you please tell me what the procedure to collect information is, so=
=0A=
> that I can file a useful bug report?=0A=
=0A=
I'm not sure if there are official recommendations, but what I've done befo=
re for httpd:=0A=
=0A=
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 oc=
curs, you might know slightly more, but not very much, and would want to ge=
nerate a core dump instead.=0A=
=0A=
   For something like "segfault at ffffffff00000008 ip 00007f2ca2ef7121 sp =
00007ffeef578850 error 5 in libphp7.so[7f2ca2a83000+b95000]", it'd have cra=
shed at the instruction on 0x00007f2ca2ef7121 - 0x7f2ca2a83000 in libphp7.s=
o=0A=
=0A=
2. I haven't tried these instructions before, but you probably want to enab=
le core dumps. https://ma.ttias.be/generate-php-core-dumps-segfaults-php-fp=
m/ (first google result) may help.=0A=
   Then, you can run 'gdb /usr/local/php/modules/libphp7.so /tmp/coredumpxx=
x' to see the stack trace where the dump occurred to include it, and includ=
e it in a bug report. (the path or shared library you want may differ)=0A=
   If you can install debugging symbols (I'm not sure what the package is c=
alled - something like php-debuginfo or running debuginfo-install ....?), o=
r build php with --enable-debug and reproduce the crash, that would also he=
lp.=0A=
   Make sure to turn off core dumps and restart php-fpm when you're done.=
=0A=
=0A=
  Using a build of php configured with --enable-debug would also tell you i=
f a ZEND_ASSERT failed before the segmentation fault (I think, I forget how=
 it works with fpm)=