使用gdb调试PHP段错误 - ≈正念≈

最近在灰度测试PHP7的过程中，php-fpm出现间歇性的段错误。系统的错误信息如下：php-fpm[7664]: segfault at 7f6ff4600000 ip 00007f6ff782176f sp 00007fff2e9c2fe8 error 4 in libc-2.12.so[7f6ff7798000+18a000]为了排查出错的原因，我们接下来需要进行调试。由于错误间歇性出现在php-fpm处理请求的过程中，因此，我们需要获取获取Linux的core dumps文件。打开Linux的core dumps一般情况下，Linux默认core dumps是关闭状态。我们可以将其打开并且重定向到我们指定的文件。$ echo '/tmp/coredump-%e.%p' > /proc/sys/kernel/core_patterncore dumps文件支持变量：%% 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这个例子中，我们把错误文件重定向到/tmp目录下。配置php-fpm支持core dumps为了让php-fpm支持core dumps，我们需要打开php-fpm连接池的rlimit_core配置，在配置文件中设置。rlimit_core = unlimited重启php-fpm进程，当SIGSEGV信号量产生时，将会在你指定的core dumps目录产生指定的文件：$ ls /tmp/coredump*-rw------- 1 user group 220M /tmp/coredump-php-fpm.2393使用gdb读取core dumps文件首先，确认你的机器中正确安装了gdb调试工具（yum install gdb）。然后，你将使用gdb $program-path $coredump-path这样的命令格式调试。由于我们的程序运行在php-fpm，我们将使用以下的命令调试：$ gdb /usr/local/services/php7/sbin/php-fpm core.6054GNU gdb (GDB) Red Hat Enterprise Linux (7.2-60.el6_4.1)Copyright (C) 2010 Free Software Foundation, Inc.License GPLv3+: GNU GPL version 3 or later <http://g...Core was generated by `php-fpm: pool www '.Program terminated with signal 11, Segmentation fault.#0 0x00007f54017dc76f in memcpy () from /lib64/libc.so.6...(gdb) bt#0 0x00007f54017dc76f in memcpy () from /lib64/libc.so.6#1 0x00007f53fdf96443 in zend_string_init (execute_data=0x7f53fe416fc0) at /usr/local/services/php7/include/php/Zend/zend_string.h:159#2 hp_execute_ex (execute_data=0x7f53fe416fc0) at /usr/local/src/xhprof-php7/extension/xhprof.c:1476#3 0x00000000008c28b0 in ZEND_DO_FCALL_SPEC_HANDLER () at /data/software/php-7.0.6/Zend/zend_vm_execute.h:800#4 0x00000000008851cb in execute_ex (ex=Unhandled dwarf expression opcode 0xf3) at /data/software/php-7.0.6/Zend/zend_vm_execute.h:414bt命令将会显示core dumps文件的调用栈。到此为止，我们定位到问题出现在/usr/local/src/xhprof-php7/extension/xhprof.c文件在调用memcpy()方法时，出现内存段错误。结语目前PHP7官方并未支持xhprof扩展，为了分析性能，我们安装了第三方编译过的版本。将该扩展从灰度环境中下线，段错误问题就不再出现了。