Re: [PATCH 4/4] perf record: Handle argument change in sched_switch

Andrii Nakryiko
2022-05-07T00:14:47Z

On Thu, Apr 28, 2022 at 4:58 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> On Wed, Apr 27, 2022 at 12:26 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Wed, Apr 27, 2022 at 11:15 AM Namhyung Kim <namhyung@kernel.org> wrote:
> > > Actually I tried something similar but it was with a variable (in bss)
> > > so the verifier in an old kernel rejected it due to invalid arg access.
> > >
> > > I guess now the const makes the verifier ignore the branch as if
> > > it's dead but the compiler still generates the code, right?
> >
> >
> > yes, exactly
>
> Then I'm curious how it'd work on newer kernels.
> The verifier sees the false branch and detects type mismatch
> for the second argument then it'd reject the program?
>

Verifier will know which branch is never taken, and will just ignore
and remove any code in it as dead code.

> Thanks,
> Namhyung