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From:   Marek Majkowski <marek@cloudflare.com>
Date:   Tue, 11 Feb 2020 10:06:35 +0000
Message-ID: <CAJPywTKuu+RPsspAT4Z_243KvtchTe7p7c4DpvG07Nv5A67fnw@mail.gmail.com>
Subject: Re: perf not picking up symbols for namespaced processes
To:     Jiri Olsa <jolsa@redhat.com>
Cc:     Ivan Babrou <ivan@cloudflare.com>,
        kernel-team <kernel-team@cloudflare.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>, sashal@kernel.org,
        Kenton Varda <kenton@cloudflare.com>
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Jirka,

On Tue, Feb 4, 2020 at 7:27 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > 11913 openat(AT_FDCWD, "/proc/9512/ns/mnt", O_RDONLY) = 197
> > 11913 setns(197, CLONE_NEWNS) = 0
> > 11913 stat("/home/marek/bin/runsc-debug", 0x7fffffff8480) = -1 ENOENT
> > (No such file or directory)
> > 11913 setns(196, CLONE_NEWNS) = 0
>
> hi,
> could you guys please share more details on what you run exactly,
> and perhaps that change you mentioned?

I was debugging gvisor (runsc), which does execve(/proc/self/exe), and
then messes up with its mount namespace. The effect is that the binary
running doesn't exist in the mount namespace. This confuses perf,
which fails to load symbols for that process.

To my understanding, by default, perf looks for the binary in the
process mount namespace. In this case clearly the binary wasn't there.
Ivan wrote a rough patch [1], which I just confirmed works. The patch
attempts, after a failure to load binary from pids mount namespace, to
load binary from the default mount namespace (the one running perf).

[1] https://lkml.org/lkml/2019/12/5/878

Marek