The post Generate PHP core dumps on segfaults in PHP-FPM appeared first on ma.ttias.be.
The PHP documentation is pretty clear on how to get a backtrace in PHP, but some of the explanations are confusing and seem focused on mod_php, instead of PHP-FPM. So here's the steps you can take to enable core dumps in PHP-FPM pools.
Enable core dumps on Linux
Chances are, your current Linux config doesn't support core dumps yet. You can enable them and set the location where the kernel will dump the core files.
$ echo '/tmp/coredump-%e.%p' > /proc/sys/kernel/core_pattern
You can use many different kinds of core dump variables for the filename, such as;
%% a single % character
%c core file size soft resource limit of crashing process (since
Linux 2.6.24)
%d dump mode—same as value returned by prctl(2) PR_GET_DUMPABLE
(since Linux 3.7)
%e executable filename (without path prefix)
%E pathname of executable, with slashes ('/') replaced by
exclamation marks ('!') (since Linux 3.0).
%g (numeric) real GID of dumped process
%h hostname (same as nodename returned by uname(2))
%p PID of dumped process, as seen in the PID namespace in which
the process resides
%P PID of dumped process, as seen in the initial PID namespace
(since Linux 3.12)
%s number of signal causing dump
%t time of dump, expressed as seconds since the Epoch,
1970-01-01 00:00:00 +0000 (UTC)
%u (numeric) real UID of dumped process
The example above will use the executable name (%e) and the pidfile (%p) in the filename. It'll dump in /tmp, as that will be writable to any kind of user.
Now that your kernel knows where to save the core dumps, it's time to change PHP-FPM.
Enable PHP-FPM core dumps per pool
To enable a core dump on a SIGSEGV, you can enable the rlimit_core option per PHP-FPM pool. Open your pool configuration and add the following.
rlimit_core = unlimited
Restart your PHP-FPM daemon (service php-fpm restart) to activate the config. Next time a SIGSEGV happens, your PHP-FPM logs will show you some more information.
WARNING: [pool poolname] child 20076 exited on signal 11 (SIGSEGV - core dumped) after 8.775895 seconds from start
You can find the core-dump in /tmp/coredump*.
$ ls /tmp/coredump* -rw------- 1 user group 220M /tmp/coredump-php-fpm.2393
The filename shows the program (php-fpm) and the PID (2393).
Reading the core dumps
This is one part that the PHP docs are pretty clear about, so just a copy paste with modified/updated paths.
First, you need gdb installed (yum install gdb) to get the backtraces. You then start the gdb binary like gdb $program-path $coredump-path. Since our program is php-fpm, which resides in /usr/sbin/php-fpm, we call the gdb binary like this.
$ gdb /usr/sbin/php-fpm /tmp/coredump-php-fpm.2393 (gdb loading all symbols ... ) ... Reading symbols from /usr/lib64/php/modules/memcache.so...(no debugging symbols found)...done. Loaded symbols for /usr/lib64/php/modules/memcache.so ... (gdb) bt #0 0x00007f8a8b6d7c37 in mmc_value_handler_multi () from /usr/lib64/php/modules/memcache.so #1 0x00007f8a8b6db9ad in mmc_unpack_value () from /usr/lib64/php/modules/memcache.so #2 0x00007f8a8b6e0637 in ?? () from /usr/lib64/php/modules/memcache.so #3 0x00007f8a8b6dd55b in mmc_pool_select () from /usr/lib64/php/modules/memcache.so #4 0x00007f8a8b6ddcc8 in mmc_pool_run () from /usr/lib64/php/modules/memcache.so #5 0x00007f8a8b6d7e92 in ?? () from /usr/lib64/php/modules/memcache.so #6 0x00007f8a8ac335cf in nr_php_curl_setopt () at /home/hudson/slave-workspace/workspace/PHP_Release_Agent/label/centos5-64-nrcamp/agent/php_curl.c:202 #7 0x0000000002b14fe0 in ?? () #8 0x0000000000000000 in ?? ()
The bt command will show you the PHP backtrace on the moment of the core dump. To exit gdb, just type quit.
The post Generate PHP core dumps on segfaults in PHP-FPM appeared first on ma.ttias.be.