linux-kernel - Re: [PATCH bpf-next] bpf: Fix a typo in comment for DFS algorithm

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Date:   Wed, 26 Oct 2022 20:04:59 +0800
From:   Xu Kuohai <xukuohai@...wei.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Xu Kuohai <xukuohai@...weicloud.com>
CC:     bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH bpf-next] bpf: Fix a typo in comment for DFS algorithm

On 10/26/2022 2:32 PM, Alexei Starovoitov wrote:
> On Tue, Oct 25, 2022 at 7:42 PM Xu Kuohai <xukuohai@...weicloud.com> wrote:
>>
>> From: Xu Kuohai <xukuohai@...wei.com>
>>
>> There is a typo in comment for DFS algorithm in bpf/verifier.c. The top
>> element should not be popped until all its neighbors have been checked.
>> Fix it.
>>
>> Fixes: 475fb78fbf48 ("bpf: verifier (add branch/goto checks)")
>> Signed-off-by: Xu Kuohai <xukuohai@...wei.com>
>> ---
>>   kernel/bpf/verifier.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index b83a8d420520..96ba5ea6d1a6 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -10662,7 +10662,7 @@ static int check_return_code(struct bpf_verifier_env *env)
>>    * 3      let S be a stack
>>    * 4      S.push(v)
>>    * 5      while S is not empty
>> - * 6            t <- S.pop()
>> + * 6            t <- S.top()
> 
> Even with this fix the comment is not quite accurate.
> I wonder whether we should keep it or delete it completely.

The comment describes the non-recursive DFS algorithm used by the C code.
Although it does not describe the full details, it helps us to understand
the code, so I think it should be kept.

> At least please use 'peek' instead of 'top'.

OK.