linux-kernel - Re: [PATCH v2 7/7] tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM

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Date:   Wed, 8 Feb 2023 21:55:50 +0000
From:   John Stultz <jstultz@...gle.com>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     akpm@...ux-foundation.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, oliver.sang@...el.com, lkp@...el.com,
        Andrii Nakryiko <andrii@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Michal Miroslaw <mirq-linux@...e.qmqm.pl>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Petr Mladek <pmladek@...e.com>,
        Kajetan Puchalski <kajetan.puchalski@....com>,
        Lukasz Luba <lukasz.luba@....com>,
        Qais Yousef <qyousef@...gle.com>,
        Daniele Di Proietto <ddiproietto@...gle.com>
Subject: Re: [PATCH v2 7/7] tools/testing/selftests/bpf: replace open-coded
 16 with TASK_COMM_LEN

On Sat, Nov 20, 2021 at 11:27:38AM +0000, Yafang Shao wrote:
> As the sched:sched_switch tracepoint args are derived from the kernel,
> we'd better make it same with the kernel. So the macro TASK_COMM_LEN is
> converted to type enum, then all the BPF programs can get it through BTF.
> 
> The BPF program which wants to use TASK_COMM_LEN should include the header
> vmlinux.h. Regarding the test_stacktrace_map and test_tracepoint, as the
> type defined in linux/bpf.h are also defined in vmlinux.h, so we don't
> need to include linux/bpf.h again.
> 
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> Acked-by: Andrii Nakryiko <andrii@...nel.org>
> Acked-by: David Hildenbrand <david@...hat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
> Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
> Cc: Michal Miroslaw <mirq-linux@...e.qmqm.pl>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Petr Mladek <pmladek@...e.com>
> ---
>  include/linux/sched.h                                   | 9 +++++++--
>  tools/testing/selftests/bpf/progs/test_stacktrace_map.c | 6 +++---
>  tools/testing/selftests/bpf/progs/test_tracepoint.c     | 6 +++---
>  3 files changed, 13 insertions(+), 8 deletions(-)

Hey all,
  I know this is a little late, but I recently got a report that
this change was causiing older versions of perfetto to stop
working. 

Apparently newer versions of perfetto has worked around this
via the following changes:
  https://android.googlesource.com/platform/external/perfetto/+/c717c93131b1b6e3705a11092a70ac47c78b731d%5E%21/
  https://android.googlesource.com/platform/external/perfetto/+/160a504ad5c91a227e55f84d3e5d3fe22af7c2bb%5E%21/

But for older versions of perfetto, reverting upstream commit
3087c61ed2c4 ("tools/testing/selftests/bpf: replace open-coded 16
with TASK_COMM_LEN") is necessary to get it back to working.

I haven't dug very far into the details, and obviously this doesn't
break with the updated perfetto, but from a high level this does
seem to be a breaking-userland regression.

So I wanted to reach out to see if there was more context for this
breakage? I don't want to raise a unnecessary stink if this was
an unfortuante but forced situation.

thanks
-john