Core files generated by linux kernel modules

Question

I am trying to load a kernel module (out-of-tree) and dmesg shows a panic. The kernel is still up though. I guess the module panic'd.

Where to find the core file? I want to use gdb and see whats the problem.

Answer 1

Where to find the core file?

Core files are strictly a user-space concept.

I want to use gdb and see whats the problem.

You may be looking for KGDB and/or Kdump/Kexec .

Answer 2

there won't be a core file. You should follow the stack trace in kernel messages. type dmesg to see it.

Answer 3

Normally, whenever the coredump was generated, it will state "core dumped". This could be one high level easy way to confirm whether coredump got generated however, this statement alone cannot guarantee on coredump file availability. The location where coredump is generated is specified through core_pattern to kernel via sysctl. You need to check the information present in core_pattern of your system. Also, note that in case of Ubuntu, it appears that the coredump file size is kept as zero by default which will avoid generation of coredump. So, you might need to check the corefile size ulimit and change it to 'ulimit -c unlimited', if it is zero. The manpage http://man7.org/linux/man-pages/man5/core.5.html explains about various reasons due to which coredump shall not get generated.

However, from your explanation, it appears that you are facing 'kernel oops' as the kernel is still up(unstable state) even though a particular module got panic'd/killed. In such cases, kernel shall print an oops message. Refer to link https://www.kernel.org/doc/Documentation/oops-tracing.txt that has information regarding the kernel oops messages.

Abstract from the link: Normally the Oops text is read from the kernel buffers by klogd and handed to syslogd which writes it to a syslog file, typically /var/log/messages (depends on /etc/syslog.conf). Sometimes klogd dies, in which case you can run dmesg > file to read the data from the kernel buffers and save it. Or you can cat /proc/kmsg > file, however you have to break in to stop the transfer, kmsg is a "never ending file".

printk is used for generating the oops messages. printk does tagging of severity by means of different loglevels /priorities and allows the classification of messages according to their severity. (Different priorities are defined in file linux/kernel.h or linux/kern_levels.h, in form of macros like KERN_EMERG, KERN_ALERT, KERN_CRIT etc..)So, you may need to check the default logging levels in system by using cat /proc/sys/kernel/printk and change it as per your requirement. Also, check whether the logging daemons are up and incase you want to debug kernel, ensure that the kernel is compiled with CONFIG_DEBUG_INFO.

The method to use GDB to find the location where the kernel panicked or oopsed in ubuntu is in the link https://wiki.ubuntu.com/Kernel/KernelDebuggingTricks which can be one of the method that can be used by you for debugging kernel oops.