Re: [PATCH 02/16] x86/dumpstack: Add get_stack_info() support for the SYSENTER stack

From: Andy Lutomirski
Date: Mon Nov 20 2017 - 16:30:51 EST

On Mon, Nov 20, 2017 at 1:27 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Mon, Nov 20, 2017 at 01:07:16PM -0800, Andy Lutomirski wrote:
>> >> but, more importantly, the OOPS unwinder will just bail without this
>> >> patch. With the patch, we get a valid unwind, except that everything
>> >> has a ? in front.
>> >
>> > Hm. I can't even fathom how that's possible. Are you talking about the
>> > "unwind from NMI to SYSENTER stack" path? Or any unwind to a syscall?
>> > Either way I'm baffled... If the unwinder only encounters the SYSENTER
>> > stack at the end, how could that cause everything beforehand to have a
>> > question mark?
>> I mean that, if I put a ud2 or other bug in the code that runs on the
>> SYSENTER stack, without this patch, I get a totally blank call trace.
> I would expect a blank call trace either way...

Try making sync_regs use a few kB of stack space or, better yet, call
a non-inlined function that uses too much stack.

> In fact I just added a ud1 after your RDI save, and got a blank call
> trace.
> Also the RIP printout isn't very helpful, I guess kallsyms doesn't know
> about the trampoline?

Um, right. I should fix that.