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

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Message-ID: <CALOAHbDkMhnO_OfQiV4gA8rGnLpyQ27nUcWSnN_-8TXkfQ1Eyw@mail.gmail.com>
Date:   Mon, 29 Nov 2021 23:33:33 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Sven Schnelle <svens@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        "linux-perf-use." <linux-perf-users@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel test robot <oliver.sang@...el.com>,
        kbuild test robot <lkp@...el.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        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>
Subject: Re: [PATCH v2 7/7] tools/testing/selftests/bpf: replace open-coded 16
 with TASK_COMM_LEN

On Mon, Nov 29, 2021 at 10:38 PM Sven Schnelle <svens@...ux.ibm.com> wrote:
>
> Hi,
>
> David Hildenbrand <david@...hat.com> writes:
> > On 29.11.21 15:21, Sven Schnelle wrote:
> >> Yafang Shao <laoar.shao@...il.com> writes:
> >>> Thanks for the report and debugging!
> >>> Seems we should explicitly define it as signed ?
> >>> Could you pls. help verify it?
> >>>
> >>> diff --git a/include/linux/sched.h b/include/linux/sched.h
> >>> index cecd4806edc6..44d36c6af3e1 100644
> >>> --- a/include/linux/sched.h
> >>> +++ b/include/linux/sched.h
> >>> @@ -278,7 +278,7 @@ struct task_group;
> >>>   * Define the task command name length as enum, then it can be visible to
> >>>   * BPF programs.
> >>>   */
> >>> -enum {
> >>> +enum SignedEnum {
> >>>         TASK_COMM_LEN = 16,
> >>>  };
> >>
> >> Umm no. What you're doing here is to define the name of the enum as
> >> 'SignedEnum'. This doesn't change the type. I think before C++0x you
> >> couldn't force an enum type.
> >
> > I think there are only some "hacks" to modify the type with GCC. For
> > example, with "__attribute__((packed))" we can instruct GCC to use the
> > smallest type possible for the defined enum values.
>
> Yes, i meant no way that the standard defines. You could force it to
> signed by having a negative member.
>
> > I think with some fake entries one can eventually instruct GCC to use an
> > unsigned type in some cases:
> >
> > https://stackoverflow.com/questions/14635833/is-there-a-way-to-make-an-enum-unsigned-in-the-c90-standard-misra-c-2004-compl
> >
> > enum {
> >       TASK_COMM_LEN = 16,
> >       TASK_FORCE_UNSIGNED = 0x80000000,
> > };
> >
> > Haven't tested it, though, and I'm not sure if we should really do that
> > ... :)
>
> TBH, i would vote for reverting the change. defining an array size as
> enum feels really odd.
>

We changed it to enum because the BTF can't parse macro while it can
parse the enum type.
Anyway I don't insist on keeping this change if you think reverting it
is better.

Andrew, would you pls. help drop this patch from the -mm tree (the
other 6 patches in this series can be kept) ?


--
Thanks
Yafang