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 11:13:31 +0100

Hi,

Yafang Shao <[email-masked]> writes:

> As the sched:sched_switch tracepoint args are derived from the kernel,
> we'd better make it same with the kernel. So the macro TASK_COMM_LEN is
> converted to type enum, then all the BPF programs can get it through BTF.
>
> The BPF program which wants to use TASK_COMM_LEN should include the header
> vmlinux.h. Regarding the test_stacktrace_map and test_tracepoint, as the
> type defined in linux/bpf.h are also defined in vmlinux.h, so we don't
> need to include linux/bpf.h again.
>
> Signed-off-by: Yafang Shao <[email-masked]>
> Acked-by: Andrii Nakryiko <[email-masked]>
> Acked-by: David Hildenbrand <[email-masked]>
> Cc: Mathieu Desnoyers <[email-masked]>
> Cc: Arnaldo Carvalho de Melo <[email-masked]>
> Cc: Andrii Nakryiko <[email-masked]>
> Cc: Michal Miroslaw <[email-masked]>
> Cc: Peter Zijlstra <[email-masked]>
> Cc: Steven Rostedt <[email-masked]>
> Cc: Matthew Wilcox <[email-masked]>
> Cc: David Hildenbrand <[email-masked]>
> Cc: Al Viro <[email-masked]>
> Cc: Kees Cook <[email-masked]>
> Cc: Petr Mladek <[email-masked]>
> ---
>  include/linux/sched.h                                   | 9 +++++++--
>  tools/testing/selftests/bpf/progs/test_stacktrace_map.c | 6 +++---
>  tools/testing/selftests/bpf/progs/test_tracepoint.c     | 6 +++---
>  3 files changed, 13 insertions(+), 8 deletions(-)
>
> 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.

Regards
Sven