linux-kernel - Re: [PATCH 3/3] objtool: Support stack layout changes in alternatives

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Date:   Mon, 4 Jan 2021 15:09:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, Miroslav Benes <mbenes@...e.cz>,
        Shinichiro Kawasaki <shinichiro.kawasaki@....com>
Subject: Re: [PATCH 3/3] objtool: Support stack layout changes in alternatives

On Tue, Dec 22, 2020 at 11:18:10PM -0600, Josh Poimboeuf wrote:

> For example, this scenario is allowed:
> 
>           Alt1                    Alt2                    Alt3
> 
>    0x00   CALL *pv_ops.save_fl    CALL xen_save_fl        PUSHF
>    0x01                                                   POP %RAX
>    0x02                                                   NOP
>    ...
>    0x05                           NOP
>    ...
>    0x07   <insn>
> 

> This scenario is NOT allowed:
> 
>           Alt1                    Alt2
> 
>    0x00   CALL *pv_ops.save_fl    PUSHF
>    0x01                           NOP6
>    ...
>    0x07   NOP                     POP %RAX
> 

> The problem here is that offset-0x7, which is an instruction boundary in
> both possible instruction patch streams, has two conflicting stack
> layouts.

There's another fun scenario:

  0x00	CALL *pv_ops.save_fl		PUSHF
  0x01					NOP2
  ..
  0x03					NOP5
  ..
  0x07	NOP2
  0x08					POP %RAX
  0x09	<insn>

No conflicting boundary at 0x07, but still buggered.

Let me go read the actual patch to see if this is handled.