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To: Benjamin Morel <benjamin.morel@gmail.com>, "bishop@php.net"
	<bishop@php.net>
CC: PHP Internals <internals@lists.php.net>
Thread-Topic: [PHP-DEV] How to debug a segmentation fault?
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Date: Thu, 30 Jan 2020 02:43:35 +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 =
it=0A=
>> hard for me to pinpoint the code that triggers the segfault.=0A=
>=0A=
> I'm not sure if I could gather helpful enough info about the issue, but=
=0A=
> anyway I filed a bug here:=0A=
> https://bugs.php.net/bug.php?id=3D79194=0A=
=0A=
>> gdb output for the segfault: (only showing the first 10 entries, it goes=
 on and on for more than 20000 entries=97there may be many more, I gave up =
after that)=0A=
=0A=
> #0  0x00007f7ea5fdc6d1 in bf_zend_execute () from /usr/lib64/php/modules/=
blackfire.so=0A=
> #1  0x0000560bec0b20c0 in ZEND_DO_FCALL_SPEC_RETVAL_USED_HANDLER () at /u=
sr/src/debug/php-7.4.2-1.fc31.remi.x86_64/Zend/zend_vm_execute.h:1714=0A=
> #2  execute_ex (ex=3D0x7f7e99d6b2c0) at /usr/src/debug/php-7.4.2-1.fc31.r=
emi.x86_64/Zend/zend_vm_execute.h:53821=0A=
> #3  0x00007f7ea5fdc6f8 in bf_zend_execute () from /usr/lib64/php/modules/=
blackfire.so=0A=
> #4  0x0000560bec0b20c0 in ZEND_DO_FCALL_SPEC_RETVAL_USED_HANDLER () at /u=
sr/src/debug/php-7.4.2-1.fc31.remi.x86_64/Zend/zend_vm_execute.h:1714=0A=
> #5  execute_ex (ex=3D0x7f7e99d6b2c0) at /usr/src/debug/php-7.4.2-1.fc31.r=
emi.x86_64/Zend/zend_vm_execute.h:53821=0A=
=0A=
The symptoms sounds like you have a global function (each call handled in Z=
END_DO_FCALL_SPEC_RETVAL_USED_HANDLER) that's recursively calling itself, i=
nfinitely. The stack trace doesn't say which.=0A=
When blackfire is used, it seems to use C's call stack, which leads to a st=
ack overflow.=0A=
When blackfire is used, the C PHP engine manages a different stack and does=
n't recurse, so it doesn't have a stack overflow, and the memory leak would=
 be slower.=0A=
=0A=
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.=0A=
=0A=
You may also want to try running the same code with xdebug - Xdebug will th=
row/log an error if you recurse more than max_nesting_level(defaults to 100=
).=0A=
https://xdebug.org/docs/basic=0A=
That will give you the php function that calls itself.=0A=
=0A=
If you can reproduce the issue consistently, you may also want to look into=
 https://github.com/adsr/phpspy (It has a top-like mode:)=