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

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Date:   Mon, 20 Nov 2017 12:46:13 -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 12:42 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Mon, Nov 20, 2017 at 09:07:33AM -0800, Andy Lutomirski wrote:
>> +bool in_SYSENTER_stack(unsigned long *stack, struct stack_info *info)
>
> Can you make it lowercase for consistency with the other in_*_stack()
> functions?  For example, in_irq_stack() is all lowercase even though
> "IRQ" is normally written in uppercase.
>
> But also, I'm wondering whether this get_stack_info() support is even
> really needed.
>
> As currently written, the trampoline code doesn't have any ORC data
> associated with it.  So the unwinder would never have the need to
> actually read the SYSENTER stack.
>
> You _could_ add an UNWIND_HINT_IRET_REGS annotation after the simulated
> iret frame is written, which would allow the unwinder to dump those regs
> when unwinding from an NMI.

There's some ORC data in the non-trampoline  SYSENTER path 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.

>
> But there's only a tiny window where that would be possible: only a few
> instructions.  I'm not sure that would be worth the effort, unless we
> got to the point where we expect to have 100% unwinder coverage.  But
> that's currently unrealistic anyway because of generated code and
> runtime patching.

I tripped it myself several times when debugging this code.

>
> --
> Josh