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

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Date:   Thu, 25 Nov 2021 09:47:56 +0800
From:   xufeng zhang <yunbo.xufeng@...ux.alibaba.com>
To:     Hou Tao <houtao1@...wei.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 Tao,

在 2021/11/24 下午4:49, Hou Tao 写道:
> 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.

This is a good point.

I think the reason why mainline has not such a helper yet is because a 
LLVM __builtin_memset() is

already available, but clearly this __builtin_memset() has too much 
limitation which can't meet all the needs,

there might be other concerns to implement such a memset helper which I 
don't know, but I think your suggestion

is a good idea.


Xufeng


>
> Regards,
> Tao
>
>>>        }
>>>          return len;
>> .