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Subject: Re: [PATCH bpf-next] selftests/bpf: Fix return value checks in
 perf_event_stackmap.c
To:     Yuntao Wang <ytcoode@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>
Cc:     Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
        Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Shuah Khan <shuah@kernel.org>,
        Hengqi Chen <hengqi.chen@gmail.com>, netdev@vger.kernel.org,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Shuah Khan <skhan@linuxfoundation.org>
References: <20220407153814.104914-1-ytcoode@gmail.com>
From:   Shuah Khan <skhan@linuxfoundation.org>
Message-ID: <7ac36fbe-aa44-9311-320b-1e953c29a3c4@linuxfoundation.org>
Date:   Thu, 7 Apr 2022 13:23:10 -0600
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On 4/7/22 9:38 AM, Yuntao Wang wrote:
> The bpf_get_stackid() function may also return 0 on success.

Can you add couple of sentences to describe what this patch
does? bpf_get_stackid() may also return doesn't really say
anything about why this patch is needed.

> 
> Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
> ---
>   tools/testing/selftests/bpf/progs/perf_event_stackmap.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/progs/perf_event_stackmap.c b/tools/testing/selftests/bpf/progs/perf_event_stackmap.c
> index b3fcb5274ee0..f793280a3238 100644
> --- a/tools/testing/selftests/bpf/progs/perf_event_stackmap.c
> +++ b/tools/testing/selftests/bpf/progs/perf_event_stackmap.c
> @@ -35,10 +35,10 @@ int oncpu(void *ctx)
>   	long val;
>   
>   	val = bpf_get_stackid(ctx, &stackmap, 0);
> -	if (val > 0)
> +	if (val >= 0)
>   		stackid_kernel = 2;
>   	val = bpf_get_stackid(ctx, &stackmap, BPF_F_USER_STACK);
> -	if (val > 0)
> +	if (val >= 0)
>   		stackid_user = 2;
>   
>   	trace = bpf_map_lookup_elem(&stackdata_map, &key);
> 
Linux 5.18-rc1 shows a couple of more bpf_get_stackid() in this function.
Removed in bpf-next - I assume.

The change is good. I would like to see it explained better in the
commit log.

With the commit log fixed to explain why this change is needed and
what happens if val equals to 0 condition isn't checked:

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>

thanks,
-- Shuah