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

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Date:   Thu, 2 Sep 2021 22:04:05 +0000
From:   Song Liu <songliubraving@...com>
To:     John Fastabend <john.fastabend@...il.com>
CC:     bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Ziljstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Kajol Jain <kjain@...ux.ibm.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper
 bpf_get_branch_snapshot



> On Sep 2, 2021, at 1:56 PM, John Fastabend <john.fastabend@...il.com> wrote:
> 
> 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?

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@...il.com>

Thanks for the review!