Re: [PATCH v2] bpf: Remove config check to enable bpf support for branch records

[kajoljain <kjain@linux.ibm.com>]

On 11/19/21 4:18 AM, Andrii Nakryiko wrote:
> On Thu, Nov 18, 2021 at 5:10 AM Kajol Jain <kjain@linux.ibm.com> wrote:
>>
>> Branch data available to bpf programs can be very useful to get
>> stack traces out of userspace application.
>>
>> Commit fff7b64355ea ("bpf: Add bpf_read_branch_records() helper")
>> added bpf support to capture branch records in x86. Enable this feature
>> for other architectures as well by removing check specific to x86.
>> Incase any platform didn't support branch stack, it will return with
>> -EINVAL.
>>
>> Selftest 'perf_branches' result on power9 machine with branch stacks
>> support.
>>
>> Before this patch changes:
>> [command]# ./test_progs -t perf_branches
>>  #88/1 perf_branches/perf_branches_hw:FAIL
>>  #88/2 perf_branches/perf_branches_no_hw:OK
>>  #88 perf_branches:FAIL
>> Summary: 0/1 PASSED, 0 SKIPPED, 1 FAILED
>>
>> After this patch changes:
>> [command]# ./test_progs -t perf_branches
>>  #88/1 perf_branches/perf_branches_hw:OK
>>  #88/2 perf_branches/perf_branches_no_hw:OK
>>  #88 perf_branches:OK
>> Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
>>
>> Selftest 'perf_branches' result on power9 machine which doesn't
>> support branch stack
>>
>> After this patch changes:
>> [command]# ./test_progs -t perf_branches
>>  #88/1 perf_branches/perf_branches_hw:SKIP
>>  #88/2 perf_branches/perf_branches_no_hw:OK
>>  #88 perf_branches:OK
>> Summary: 1/1 PASSED, 1 SKIPPED, 0 FAILED
>>
>> Fixes: fff7b64355eac ("bpf: Add bpf_read_branch_records() helper")
>> Suggested-by: Peter Zijlstra <peterz@infradead.org>
>> Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
>> ---
>>
>> Tested this patch changes on power9 machine using selftest
>> 'perf branches' which is added in commit 67306f84ca78 ("selftests/bpf:
>> Add bpf_read_branch_records()")
>>
>> Changelog:
>> v1 -> v2
>> - Inorder to add bpf support to capture branch record in
>>   powerpc, rather then adding config for powerpc, entirely
>>   remove config check from bpf_read_branch_records function
>>   as suggested by Peter Zijlstra
> 
> what will be returned for architectures that don't support branch
> records? Will it be zero instead of -ENOENT?
> 

Hi Andrii,
     Incase any architecture doesn't support branch records and if it
tries to do branch sampling with sample type as
PERF_SAMPLE_BRANCH_STACK, perf_event_open itself will fail.

And even if, perf_event_open succeeds  we have appropriate checks in
bpf_read_branch_records function, which will return -EINVAL for those
architectures.

Reference from linux/kernel/trace/bpf_trace.c

Here, br_stack will be empty, for unsupported architectures.

BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
	   void *, buf, u32, size, u64, flags)
{
.....
	if (unlikely(flags & ~BPF_F_GET_BRANCH_RECORDS_SIZE))
		return -EINVAL;

	if (unlikely(!br_stack))
		return -EINVAL;
....
}

Thanks,
Kajol Jain

>>
>> - Link to the v1 patch: https://lkml.org/lkml/2021/11/14/434
>>
>>  kernel/trace/bpf_trace.c | 4 ----
>>  1 file changed, 4 deletions(-)
>>
>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>> index 7396488793ff..5e445985c6b4 100644
>> --- a/kernel/trace/bpf_trace.c
>> +++ b/kernel/trace/bpf_trace.c
>> @@ -1402,9 +1402,6 @@ static const struct bpf_func_proto bpf_perf_prog_read_value_proto = {
>>  BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
>>            void *, buf, u32, size, u64, flags)
>>  {
>> -#ifndef CONFIG_X86
>> -       return -ENOENT;
>> -#else
>>         static const u32 br_entry_size = sizeof(struct perf_branch_entry);
>>         struct perf_branch_stack *br_stack = ctx->data->br_stack;
>>         u32 to_copy;
>> @@ -1425,7 +1422,6 @@ BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
>>         memcpy(buf, br_stack->entries, to_copy);
>>
>>         return to_copy;
>> -#endif
>>  }
>>
>>  static const struct bpf_func_proto bpf_read_branch_records_proto = {
>> --
>> 2.27.0
>>