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From: David Hildenbrand <david@redhat.com>
To: Sven Schnelle <svens@linux.ibm.com>, Yafang Shao <laoar.shao@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	"linux-perf-use." <linux-perf-users@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel test robot <oliver.sang@intel.com>,
	kbuild test robot <lkp@intel.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Michal Miroslaw <mirq-linux@rere.qmqm.pl>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Matthew Wilcox <willy@infradead.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Kees Cook <keescook@chromium.org>, Petr Mladek <pmladek@suse.com>
Subject: Re: [PATCH v2 7/7] tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN
Date: Mon, 29 Nov 2021 15:32:26 +0100	[thread overview]
Message-ID: <54e1b56c-e424-a4b3-4d61-3018aa095f36@redhat.com> (raw)
In-Reply-To: <yt9d35nfvy8s.fsf@linux.ibm.com>

On 29.11.21 15:21, Sven Schnelle wrote:
> Hi,
> 
> Yafang Shao <laoar.shao@gmail.com> writes:
> 
>> On Mon, Nov 29, 2021 at 6:13 PM Sven Schnelle <svens@linux.ibm.com> wrote:
>>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>>> index 78c351e35fec..cecd4806edc6 100644
>>>> --- a/include/linux/sched.h
>>>> +++ b/include/linux/sched.h
>>>> @@ -274,8 +274,13 @@ struct task_group;
>>>>
>>>>  #define get_current_state()  READ_ONCE(current->__state)
>>>>
>>>> -/* Task command name length: */
>>>> -#define TASK_COMM_LEN                        16
>>>> +/*
>>>> + * Define the task command name length as enum, then it can be visible to
>>>> + * BPF programs.
>>>> + */
>>>> +enum {
>>>> +     TASK_COMM_LEN = 16,
>>>> +};
>>>
>>> This breaks the trigger-field-variable-support.tc from the ftrace test
>>> suite at least on s390:
>>>
>>> echo
>>> 'hist:keys=next_comm:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,sched.sched_waking.prio,next_comm)
>>> if next_comm=="ping"'
>>> linux/tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-field-variable-support.tc: line 15: echo: write error: Invalid argument
>>>
>>> I added a debugging line into check_synth_field():
>>>
>>> [   44.091037] field->size 16, hist_field->size 16, field->is_signed 1, hist_field->is_signed 0
>>>
>>> Note the difference in the signed field.
>>>
>>
>> Hi Sven,
>>
>> 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.

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
... :)

-- 
Thanks,

David / dhildenb


  reply	other threads:[~2021-11-29 14:34 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-20 11:27 [PATCH v2 0/7] task comm cleanups Yafang Shao
2021-11-20 11:27 ` [PATCH v2 1/7] fs/exec: replace strlcpy with strscpy_pad in __set_task_comm Yafang Shao
2021-11-20 11:27 ` [PATCH v2 2/7] fs/exec: replace strncpy with strscpy_pad in __get_task_comm Yafang Shao
2021-11-20 11:27 ` [PATCH v2 3/7] drivers/infiniband: replace open-coded string copy with get_task_comm Yafang Shao
2021-11-20 11:27 ` [PATCH v2 4/7] fs/binfmt_elf: " Yafang Shao
2021-11-29 16:01   ` Steven Rostedt
2021-11-30  3:01     ` Yafang Shao
2021-11-30 14:22       ` Steven Rostedt
2021-11-30 15:53         ` Yafang Shao
2021-11-20 11:27 ` [PATCH v2 5/7] samples/bpf/test_overhead_kprobe_kern: replace bpf_probe_read_kernel with bpf_probe_read_kernel_str to get task comm Yafang Shao
2021-11-20 11:27 ` [PATCH v2 6/7] tools/bpf/bpftool/skeleton: " Yafang Shao
2021-11-20 11:27 ` [PATCH v2 7/7] tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN Yafang Shao
2021-11-29 10:13   ` Sven Schnelle
2021-11-29 13:41     ` Yafang Shao
2021-11-29 14:21       ` Sven Schnelle
2021-11-29 14:32         ` David Hildenbrand [this message]
2021-11-29 14:38           ` Sven Schnelle
2021-11-29 15:33             ` Yafang Shao
2021-11-29 16:07               ` Steven Rostedt
2021-11-29 16:08                 ` Steven Rostedt
2021-11-29 15:28         ` Yafang Shao
2021-11-29 17:30     ` Steven Rostedt
2021-11-29 17:56       ` Sven Schnelle
2021-11-30  3:03       ` Yafang Shao
2021-11-30 14:23         ` Steven Rostedt
2021-11-30 15:46           ` Yafang Shao
2023-02-08 21:55   ` John Stultz
2023-02-09  0:10     ` Alexei Starovoitov
2023-02-09  0:54       ` John Stultz
2023-02-09  2:06         ` Mathieu Desnoyers
2023-02-09  6:20           ` Yafang Shao
2023-02-09 14:27             ` Kajetan Puchalski
2023-02-09 15:37               ` Yafang Shao
2023-02-10 18:09                 ` Kajetan Puchalski
2023-02-11 16:51                 ` Qais Yousef
2023-02-12  3:19                   ` Yafang Shao
2023-02-09  2:28         ` Steven Rostedt
2023-02-09  2:33           ` Steven Rostedt
2023-02-11 19:00             ` Steven Rostedt
2023-02-12  3:38               ` Yafang Shao
2023-02-12  3:44                 ` Steven Rostedt
2023-02-13 17:43                   ` Namhyung Kim
2023-02-13 17:46                     ` Mathieu Desnoyers

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