On 29.11.21 15:21, Sven Schnelle wrote:
> Hi,
>
> Yafang Shao <laoar.shao@xxxxxxxxx> writes:
>
>> On Mon, Nov 29, 2021 at 6:13 PM Sven Schnelle <svens@xxxxxxxxxxxxx> 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
