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

[Ard Biesheuvel]

On Thu, 15 Sept 2022 at 10:47, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.