Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220422150507.222488-1-namhyung@kernel.org> <20220422150507.222488-5-namhyung@kernel.org>
 <CAEf4Bzbdh-wbQQLzoXGGKkqqE=+qz19C4tCq4Ynb-_PXzRYM1w@mail.gmail.com>
 <CAM9d7chos3xgxPMOMwgSh6nCNfqk8k2tXO=0JsdL4KgN_yngCA@mail.gmail.com>
 <CAEf4BzZ-RwXV8NoWk4rLyLWyxJhQ6b96ieVCy0kkjLCq8cVxqw@mail.gmail.com> <CAM9d7ciZcsTD7oK5JQA5PJ3gDHcN+Fzon=gVoPvyRb4yLzVF7w@mail.gmail.com>
In-Reply-To: <CAM9d7ciZcsTD7oK5JQA5PJ3gDHcN+Fzon=gVoPvyRb4yLzVF7w@mail.gmail.com>
From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Fri, 6 May 2022 17:14:27 -0700
Message-ID: <CAEf4Bzb0SHV7O7iDkYg0ad6JQcOii7jdLLHj5NW9dB+H=vD_kw@mail.gmail.com>
Subject: Re: [PATCH 4/4] perf record: Handle argument change in sched_switch
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Ian Rogers <irogers@google.com>,
        Song Liu <songliubraving@fb.com>, Hao Luo <haoluo@google.com>,
        bpf <bpf@vger.kernel.org>,
        "linux-perf-use." <linux-perf-users@vger.kernel.org>,
        Blake Jones <blakejones@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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