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

From: Kajol Jain
Date: Tue Nov 23 2021 - 04:51:51 EST

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

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()")

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;
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;

if (unlikely(!br_stack))
- return -EINVAL;
+ return -ENOENT;

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;

static const struct bpf_func_proto bpf_read_branch_records_proto = {