linux-kernel - Re: [RFC PATCH 06/10] x86/entry: add CFI hint undwarf annotations

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Message-ID: <CALCETrWtMy2y5kyZGVCBvSEYKPkrhwUTm-t6BqpefpVthOYttg@mail.gmail.com>
Date:   Thu, 1 Jun 2017 07:39:38 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.cz>, Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 06/10] x86/entry: add CFI hint undwarf annotations

On Thu, Jun 1, 2017 at 7:28 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Thu, Jun 01, 2017 at 09:23:58AM -0500, Josh Poimboeuf wrote:
>> On Thu, Jun 01, 2017 at 07:03:18AM -0700, Andy Lutomirski wrote:
>> > On Wed, May 31, 2017 at 10:44 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>> > Just to make sure I understand this, if we unwind from...
>> >
>> > > @@ -112,6 +114,7 @@ For 32-bit we have the following conventions - kernel is built with
>> > >         movq %rdx, 12*8+\offset(%rsp)
>> > >         movq %rsi, 13*8+\offset(%rsp)
>> >
>> > ...here..., will objtool think that rdx and rsi (etc) still live in
>> > their respective regs, or will it find them in the on-stack data given
>> > by CFI_REGS?  If the former, how does undwarf deal with the
>> > corresponding pops?
>>
>> It will find them in their respective registers, which is fine because
>> they haven't been clobbered yet.
>
> Sorry, I hit send too soon.  Which pops are you referring to?
>

If we do push, push, push, CFI_REGS, and then, later, we pop all those
saved regs, how does undwarf figure out that those pops are moving a
saved reg from the stack back to a register?  Is objtool just that
smart, or did I fail to notice an annotation somewhere, or does it not
matter?