linux-kernel - Re: [PATCH 4/4] perf record: Handle argument change in sched

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Date:   Fri, 6 May 2022 17:14:27 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Song Liu <songliubraving@...com>, Hao Luo <haoluo@...gle.com>,
        bpf <bpf@...r.kernel.org>,
        "linux-perf-use." <linux-perf-users@...r.kernel.org>,
        Blake Jones <blakejones@...gle.com>
Subject: Re: [PATCH 4/4] perf record: Handle argument change in sched_switch

On Thu, Apr 28, 2022 at 4:58 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Wed, Apr 27, 2022 at 12:26 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Wed, Apr 27, 2022 at 11:15 AM Namhyung Kim <namhyung@...nel.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