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

[Chen Zhongjin]

Hi,

On 2022/9/21 0:49, Ard Biesheuvel wrote:
> 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.

Maybe not enough, I guess.

For switch jump tables we need to know its *own* jump targets so that we 
can go through all its branches. If there are more than one indirect 
jump inside one function, only marks targets with BTI J can't help 
matching the entry and its targets.


Anyway I think this job is more for compiler. Switch jump tables is 
different from other indirect jump/call. It have fixed control flow just 
as if/else flow and the indirect jump table is just a compiler 
optimization which hide this.


Best,

Chen