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

[Daniel Borkmann]

On 11/19/21 10:35 AM, kajoljain wrote:
> On 11/19/21 4:18 AM, Andrii Nakryiko wrote:
> > On Thu, Nov 18, 2021 at 5:10 AM Kajol Jain <kjain@xxxxxxxxxxxxx> 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@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:
> > > 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;

In that case for unsupported archs we should probably bail out with -ENOENT here
as helper doc says '**-ENOENT** if architecture does not support branch records'
(see bpf_read_branch_records() doc in include/uapi/linux/bpf.h).

> ....
> }
>
> Thanks,
> Kajol Jain