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

From: Sven Schnelle <[email-masked]>
Date: Mon, 29 Nov 2021 15:21:23 +0100

Hi,

Yafang Shao <[email-masked]> writes:

> On Mon, Nov 29, 2021 at 6:13 PM Sven Schnelle <[email-masked]> 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.

Regards
Sven