August 31, 2015

Linux Kernel Coredump Analysis with Crash Utility

There are excellent tutorials around using crash utility to analyze Linux kernel coredump. And this is yet another one …

Before we start …

  1. Capture the memory core dump file (in VMware environment).
    • Suspending a virtual machine on ESX/ESXi to collect diagnostic information (2005831),

    This basically means it is not necessary to enable kexec/kdump in a virtualization environment.

  2. Converting a snapshot file to memory dump using the vmss2core tool (2003941),

    Actually, crash utility can understand VMware vmss file from version 7.10

  3. Find the linux kernel symbols/source code which I have written in another post.

here is a typical command looks like:



The “How to” Guide

  • How to figure out the kernel version?
strings vmss.core | grep "Linux version [0-9]\.[0-9]" -m 1
  • How to solve the vmlinux and vmss.core do not match problem?

If you are sure that vmlinux contains the match debug information, then most likely you need to specify the phys_base machine dependent parameter. and experience shows that RHEL 5 kernels 2.6.18-* need –machdep phys_base=2M
Dave, the crash utility author, explained the problem in a Redhat Bug, crash fails to read RHEL-5 FV core dump files collected from xm dump-core.

  • How to list source code?
crash> gdb list schedule
3515    kernel/sched.c: No such file or directory.
crash> directory /mysrc/kernel-2.6.18/linux-2.6.18-371.1.2.el5.x86_64/
crash> gdb list schedule
warning: Source file is more recent than executable.
3516    /*
3517     * schedule() is the main scheduler function.
3518     */
3519    asmlinkage void __sched schedule(void)
3520    {
3521            struct task_struct *prev, *next;
3522            struct prio_array *array;
3523            struct list_head *queue;
3524            unsigned long long now;
crash> dis -l schedule
crash> gdb disas /m schedule
  • How to load module symbol?
crash> mod -s ext4 usr/lib/debug/lib/modules/2.6.32-220.el6.x86_64/kernel/fs/ext4/ext4.ko.debug
     MODULE       NAME              SIZE  OBJECT FILE
ffffffffa01181e0  ext4            364410  usr/lib/debug/lib/modules/2.6.32-220.el6.x86_64/kernel/fs/ext4/ext4.ko.debug
crash> whatis ext4_file_write
ssize_t ext4_file_write(struct kiocb *, const struct iovec *, unsigned long, loff_t);
crash> mod -d ext4
crash> whatis ext4_file_write
whatis: gdb request failed: whatis ext4_file_write
  • How to extract parameter information from backtrace?

    • For AMD64, RDI/RSI/RDX/RCX are the first four parameters according to X86_calling_conventions
    • Sometimes you need to search the value from the stack. bt -f will be more helpful in that case.
    • gdb disas /m <function> the disassembled code will give more information how the parameter are passed around.
    • crash command help page bt has an example to extract 32bit process parameters.
    • crash extension fp - Obtaining functions’ parameters from stack frames - is designed to simplify the process but does not work reliably.
  • Howto get user process backtrace? or extract a userland process core dump?

Extended Reading

  • volatility,
  • crash extension modules,
  • A Clarification on Linux Addressing,