Re: [PATCH v3] bpf: Remove config check to enable bpf support for branch records

[kajoljain]

On 11/23/21 5:33 PM, Daniel Borkmann wrote:
> On 11/23/21 10:51 AM, Kajol Jain 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 architecture doesn't support branch records,
>> bpf_read_branch_records still have appropriate checks and it
>> will return error number -EINVAL in that scenario. But based on
>> documentation there in include/uapi/linux/bpf.h file, incase of
>> unsupported archs, this function should return -ENOENT. Hence update
>> the appropriate checks to return -ENOENT instead.
>>
>> Selftest 'perf_branches' result on power9 machine which has 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
>> have branch stack report.
>>
>> 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@xxxxxxxxxxxxx>
>> Signed-off-by: Kajol Jain <kjain@xxxxxxxxxxxxx>
>> ---
>>
>> 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:
>> v2 -> v3
>> - Change the return error number for bpf_read_branch_records
>>    function from -EINVAL to -ENOENT for appropriate checks
>>    as suggested by Daniel Borkmann.
>>
>> - Link to the v2 patch: https://lkml.org/lkml/2021/11/18/510
>>
>> 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
>>
>> - Link to the v1 patch: https://lkml.org/lkml/2021/11/14/434
>>
>>   kernel/trace/bpf_trace.c | 8 ++------
>>   1 file changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>> index 7396488793ff..b94a00f92759 100644
>> --- a/kernel/trace/bpf_trace.c
>> +++ b/kernel/trace/bpf_trace.c
>> @@ -1402,18 +1402,15 @@ 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;
>>         if (unlikely(flags & ~BPF_F_GET_BRANCH_RECORDS_SIZE))
>> -        return -EINVAL;
>> +        return -ENOENT;
> 
> What's the rationale for also changing the above? Invalid/unsupported
> flags should
> still return -EINVAL as they did before ...

Thanks for pointing it, I will make this change in the next version.

Thanks,
Kajol Jain

> 
>>       if (unlikely(!br_stack))
>> -        return -EINVAL;
>> +        return -ENOENT;
> 
> ... meaning only this one here was necessary.
> 
>>       if (flags & BPF_F_GET_BRANCH_RECORDS_SIZE)
>>           return br_stack->nr * br_entry_size;
>> @@ -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 = {
>>
>