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

On Mon, Nov 29, 2021 at 6:13 PM Sven Schnelle [off-list ref] wrote:

Hi,

Yafang Shao [off-list ref] writes:

quoted

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 <redacted>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: David Hildenbrand <redacted>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Arnaldo Carvalho de Melo <redacted>
Cc: Andrii Nakryiko <redacted>
Cc: Michal Miroslaw <mirq-linux@rere.qmqm.pl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Hildenbrand <redacted>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <redacted>
Cc: Petr Mladek <pmladek@suse.com>
---
 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.

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,
 };

-- 
Thanks
Yafang