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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 20 Sep 2022 18:49:33 +0200
Message-ID: <CAMj1kXGa7D6TLOQruYF+0czWwxcRxN7k1rWTrhB2xnjTQ32c9Q@mail.gmail.com>
Subject: Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Chen Zhongjin <chenzhongjin@huawei.com>,
        Michael Matz <matz@suse.de>, Borislav Petkov <bp@alien8.de>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        linux-toolchains@vger.kernel.org,
        Indu Bhagat <indu.bhagat@oracle.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-kernel@vger.kernel.org, "Jose E. Marchesi" <jemarch@gnu.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Mark Rutland <mark.rutland@arm.com>,
        Will Deacon <will@kernel.org>, x86@kernel.org,
        linux-arm-kernel@lists.infradead.org,
        live-patching@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Sathvika Vasireddy <sv@linux.ibm.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Mark Brown <broonie@kernel.org>
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On Thu, 15 Sept 2022 at 10:47, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Sep 15, 2022 at 10:56:58AM +0800, Chen Zhongjin wrote:
>
> > We have found some anonymous information on x86 in .rodata.
>
> Well yes, but that's still a bunch of heuristics on our side.
>
> > I'm not sure if those are *all* of Josh wanted on x86, however for arm64 we
> > did not found that in the same section so it is a problem on arm64 now.
>
> Nick found Bolt managed the ARM64 jumptables:
>
>   https://github.com/llvm/llvm-project/blob/main/bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp#L484
>
> But that does look like a less than ideal solution too.
>
> > Does the compiler will emit these for all arches? At lease I tried and
> > didn't find anything meaningful (maybe I omitted it).
>
> That's the question; can we get the compiler to help us here in a well
> defined manner.

Do BTI landing pads help at all here? I.e., I assume that objtool just
treats any indirect call as a dangling edge in the control flow graph,
and the problem is identifying the valid targets. In the BTI case,
those will all start with a 'BTI J' instruction.