linux-kernel - Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}

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Message-ID: <CAMj1kXGKHJHHvKRvZgq6Dy5BaR4vMUx8QXaw4XAqOhHxE3sb3Q@mail.gmail.com>
Date:   Sun, 11 Sep 2022 16:31:27 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>,
        linux-toolchains@...r.kernel.org,
        Indu Bhagat <indu.bhagat@...cle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, "Jose E. Marchesi" <jemarch@....org>,
        Miroslav Benes <mbenes@...e.cz>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Chen Zhongjin <chenzhongjin@...wei.com>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}

On Sun, 11 Sept 2022 at 16:26, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Sep 09, 2022 at 11:07:04AM -0700, Josh Poimboeuf wrote:
> > Alternatives
> > ------------
> >
> > Another idea which has been floated in the past is for objtool to read
> > DWARF (or .eh_frame) to help it figure out the control flow.  That
> > hasn't been tried yet, but would be considerably more difficult and
> > fragile IMO.
>
> I though Ard played around with that a bit on ARM64. And yes, given that
> most toolchains consider DWARF itself best-effort, I'm not holding my
> breath there.
>

I have patches out that use unwind data to locate pointer auth
sign/authenticate instructions in the code, in order to patch them to
shadow call stack pushes and pops at runtime if pointer authentication
is not supported by the hardware. This has little to do with objtool
or reliable stack traces.

I still think DWARF could help to make objtool's job a bit easier, but
I don't think it will be of any use with jump tables or noreturn
functions in particular.