linux-kernel - Re: [PATCH V3 5/9] objtool: Add return address unwind hints

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Date:   Tue, 14 Apr 2020 21:48:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        jthierry@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH V3 5/9] objtool: Add return address unwind hints

On Tue, Apr 14, 2020 at 09:27:27PM +0200, Alexandre Chartre wrote:
> This provides a generic code to handle any intra-function call. Currently we have
> the RSB stuffing ones which are forgotten with the UNWIND_HINT_TYPE_RADDR_DELETE
> directive. And for retpoline, they will not return if we have an indirect jump
> (JMP_NOSPEC) but they will return if we have an indirect call (CALL_NOSPEC). The
> code can handle both cases. For example, if we were to have a CALL_NOSPEC invocation
> which is not in an alternative then objtool can now correctly handle it.

The specialness of CALL_NOSPEC goes away with my proposed retpoline
rework as well. I really don't think we need something as complicated as
this.

Fundamentally validate_branch() will continue after a CALL instruction;
so I'm thikning the worst that can happen from not following a
(theoretical direct return) is a false-positive unreachable code
warning, and we can trivially fix those with exisiting hints.