linux-kernel - Re: [PATCH 02/16] x86/dumpstack: Add get_stack

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Date:   Mon, 20 Nov 2017 13:30:12 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 02/16] x86/dumpstack: Add get_stack_info() support for the
 SYSENTER stack

On Mon, Nov 20, 2017 at 1:27 PM, Josh Poimboeuf <jpoimboe@...hat.com> 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.