Accessing Thread Information in kernel crash dumps with ddd+gdb

Vivek Goyal vgoyal at
Fri Apr 18 10:10:28 EDT 2008

On Thu, Apr 17, 2008 at 05:16:55PM -0700, Piet Delaney wrote:
> Hey Guys:
> I've been using kgdb for a while with our 2.6.12 and now 2.6.16 kernel
> as well as kdump/kexec with our 2.6.16 kernel. I'm a bit disappointed
> with the visibility of local variables on the threads/tasks not currently
> running on CPUs. Both crash, and the gdb macros that you guys wrote,
> show the most important stuff but I'd prefer to be able to see everything
> with gdb/ddd as I can with kgdb; including all local variables and formal
> parameters at each stack frame.
> A long time ago I used gdb on SunOS 4.1.4 and use to simply set $fp
> and $sp from the saved information in the U-block to view a process.
> I wish gdb would allow be to run your macros, btt for example, and extract
> the stackp from task.thread.esp assign it temporally to $sp for the 
> current task,
> do the backtrace command and see everything. Changing $sp and $fp for a 
> while
> like I use to do with gdb on SunOS 4.1.4 and then using ddd+gdb to 
> browse the
> stack formals and locals would be nice. Just doing a 'set write on' 
> isn't sufficient,
> gdb wants a process and I can't see to satisfy it with simply setting 
> the current
> thread.
> I was wondering if any of you guys have been thinking of anything like this
> and had and hacks or ideas on how to see the locals and formals for all 
> tasks.
> One thought I had was a minor hack of the kexec code to do something 
> like your gdb macros
> and walk thru the task list and then append a ELF Notes, like done by 
> crash_save_this_cpu(),
> for each task. I have no idea if gdb has a limit on the number of 
> elf_prstatus structures
> that can be provided. I suppose I'd leave it a KEXEC config variable to 
> enable this, as
> some would argue that it's not as save as simply saving the regs for the 
> active CPUs.
> This would leave 'info threads' with gdb similar to 'ps' with crash and 
> virtually identical
> to the experience with kgdb.

IIUC, you are suggesting that we create elf notes even for non-active
tasks in vmcore. We should not be doing that.

- It is not safe to traverse through task list after system has crashed.
- We reserve the memory for elf notes at system boot. At that time we
  have no idea how many task system will have at the time of crash.

I think following can be a way forward for your requirement.

- Either gdb should provide a SunOS kind of facility where one can 
  provide stack pointer and switch the task context. ( I don't know
  if there is already a way to do that).

- Or one can write a user space tool, which parses original vmcore,
  walks through task list, prepare elf notes for all the tasks and emit
  a new vmcore which is fetched to gdb.


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