Core dumps

I was working on qmail (patching it heavily ) and at one point I was getting a segmentation fault. Trying to trace it in a busy environment is hard, especially in qmail that is very modular and processes come and go away all the time.

So I thought a core dump could be the answer.The only problem was my system wouldn't dump the core of the processes where the 'segmentation fault' occurred. I have set the core size file high enough ( ulimit -c 10000000 ) in the qmail start script but still not core dump so I decided to do some research.

From core man page it seems like a core could not be dumped for a number of reasons:

  1. the process owner doesn't have write permission to the directory where the core file should be written.
  2. the core file size limits ( RLIMIT_CORE , RLIMIT_FSIZE ) are less then the size of the core that would be dumped
  3. the process was setuid and in this case dumping the core would depend on the setting in /proc/sys/fs/suid_dumpable ( proc(5) )
  4. the file being executed doesn't have read permission

In my case #2 and #4 were not an issue so it must have been #1 and/or #3 .

To make sure #1 was not an issue I created a directory /tmp/cores, made it 777 to make sure any process can write to it ( not safe in a multiuser environment but works ) and then set /proc/sys/kernel/core_pattern to /tmp/cores/core so now all core files would be in /tmp/cores instead of the process working directory as it is by default. It's also a good idea to make sure /proc/sys/kernel/core_uses_pid is set to 1 so the pid of the process will be appended to the core file name.

To fix #3 I set /proc/sys/fs/suid_dumpable to 2 . I restarted qmail and there I had the core in /tmp/cores .

2 thoughts on “Core dumps

  1. Hi,

    I am using 2.6 kernel and facing same problem. For #3 I don’t have the suid_dumpable file present in my /proc/sys/fs dir. And my qmail-queue is having setuid bit set.

    How to generate the core file for such system

    uname -a
    Linux hosting_pro_stage.rediff.com 2.6.9-55.0.12.ELsmp #1 SMP Fri Nov 2 11:19:08 EDT 2007 i686 i686 i386 GNU/Linux

    1. The suid_dumpable feature was introduced in kernel 2.6.13 . Before this you could not dump the core if your process changes credentials by calling setuid or setgid .
      Seems like your only option is to do your debugging on a system with a more recent kernel.

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