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

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Date:   Mon, 29 Nov 2021 15:38:43 +0100
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Yafang Shao <laoar.shao@...il.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

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.

Regards
Sven