linux-kernel - RE: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch

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Date:   Thu, 02 Sep 2021 13:56:47 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Song Liu <songliubraving@...com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     acme@...nel.org, peterz@...radead.org, mingo@...hat.com,
        kjain@...ux.ibm.com, kernel-team@...com,
        Song Liu <songliubraving@...com>
Subject: RE: [PATCH v5 bpf-next 2/3] bpf: introduce helper
 bpf_get_branch_snapshot

Song Liu wrote:
> 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 <songliubraving@...com>
> ---

[...]

>  
> +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?

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