Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot

On Sep 2, 2021, at 1:56 PM, John Fastabend [off-list ref] wrote:

Song Liu wrote:

quoted

Introduce bpf_get_branch_snapshot(), which allows tracing pogram to get
branch trace from hardware (e.g. Intel LBR). To use the feature, the
user need to create perf_event with proper branch_record filtering
on each cpu, and then calls bpf_get_branch_snapshot in the bpf function.
On Intel CPUs, VLBR event (raw event 0x1b00) can be use for this.

Signed-off-by: Song Liu <redacted>
---

[...]

quoted

+BPF_CALL_3(bpf_get_branch_snapshot, void *, buf, u32, size, u64, flags)
+{
+#ifndef CONFIG_X86
+	return -ENOENT;
+#else
+	static const u32 br_entry_size = sizeof(struct perf_branch_entry);
+	u32 entry_cnt = size / br_entry_size;
+
+	if (unlikely(flags))
+		return -EINVAL;
+
+	if (!buf || (size % br_entry_size != 0))
+		return -EINVAL;

LGTM, but why fail if buffer is slightly larger than expected? I guess its a slightly
buggy program that would do this, but not actually harmful right?

This check was added because bpf_read_branch_records() has a similar check. 
I guess it is OK either way. 

Acked-by: John Fastabend <john.fastabend@gmail.com>

Thanks for the review!