How is Linux kernel live debugging done and what tools are used?

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Another option is to use an ICE or JTAG controller, and GDB. This ‘hardware’ solution is especially used with embedded systems.

But for instance QEMU offers similar features:

  • start QEMU with a GDB ‘remote’ stub which listens on ‘localhost:1234’ : qemu -s ...,

  • then with GDB, you open the kernel file vmlinux compiled with debug information (you can take a look a this mailing list thread where they discuss the unoptimization of the kernel).

  • connect GDB and QEMU: target remote localhost:1234

  • see your live kernel:

      (gdb) where
  #0  cpu_v7_do_idle () at arch/arm/mm/proc-v7.S:77
  #1  0xc0029728 in arch_idle () atarm/mach-realview/include/mach/system.h:36
  #2  default_idle () at arm/kernel/process.c:166
  #3  0xc00298a8 in cpu_idle () at arch/arm/kernel/process.c:199
  #4  0xc00089c0 in start_kernel () at init/main.c:713

Unfortunately, user-space debugging is not possible so far with GDB (no task list information, no memory management unit reprogramming to see different process contexts, …), but if you stay in kernel-space, that’s quite convenient.

  • info threads will give you the list and states of the different CPUs

You can get more details about the procedure in this PDF:

Debugging Linux systems using GDB and QEMU.

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