netdev - Re: [RFC] [PATCH bpf-next 1/1] bpf: Clear the noisy tail buffer for bpf_d

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Date:   Wed, 24 Nov 2021 16:49:45 +0800
From:   Hou Tao <houtao1@...wei.com>
To:     xufeng zhang <yunbo.xufeng@...ux.alibaba.com>, <jolsa@...nel.org>,
        <kpsingh@...gle.com>
CC:     <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <bpf@...r.kernel.org>, <daniel@...earbox.net>, <ast@...nel.org>,
        <andriin@...com>
Subject: Re: [RFC] [PATCH bpf-next 1/1] bpf: Clear the noisy tail buffer for
 bpf_d_path() helper

Hi,

On 11/24/2021 12:15 PM, xufeng zhang wrote:
> Jiri and KP,
>
> Any suggestion?
>
>
> Thanks in advance!
>
> Xufeng
>
> 在 2021/11/20 下午1:18, Xufeng Zhang 写道:
>> From: "Xufeng Zhang" <yunbo.xufeng@...ux.alibaba.com>
>>
>> The motivation behind this change is to use the returned full path
>> for lookup keys in BPF_MAP_TYPE_HASH map.
>> bpf_d_path() prepend the path string from the end of the input
>> buffer, and call memmove() to copy the full path from the tail
>> buffer to the head of buffer before return. So although the
>> returned buffer string is NULL terminated, there is still
>> noise data at the tail of buffer.
>> If using the returned full path buffer as the key of hash map,
>> the noise data is also calculated and makes map lookup failed.
>> To resolve this problem, we could memset the noisy tail buffer
>> before return.
>>
>> Signed-off-by: Xufeng Zhang <yunbo.xufeng@...ux.alibaba.com>
>> ---
>>   kernel/trace/bpf_trace.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>> index 25ea521fb8f1..ec4a6823c024 100644
>> --- a/kernel/trace/bpf_trace.c
>> +++ b/kernel/trace/bpf_trace.c
>> @@ -903,6 +903,8 @@ BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf,
>> u32, sz)
>>       } else {
>>           len = buf + sz - p;
>>           memmove(buf, p, len);
>> +        /* Clear the noisy tail buffer before return */
>> +        memset(buf + len, 0, sz - len);
Is implementing bpf_memset() helper a better idea ? So those who need to
clear the buffer after the terminated null character can use the helper to
do that.

Regards,
Tao

>>       }
>>         return len;
> .