netdev - Re: [PATCH net-next v5 6/7] tap: Keep hdr_len in tap_get

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Message-ID: <8b389981-c04a-4d4f-8a5a-043b4cd6e8db@daynix.com>
Date: Thu, 6 Feb 2025 16:04:11 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Jonathan Corbet <corbet@....net>, Jason Wang <jasowang@...hat.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 "Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo
 <xuanzhuo@...ux.alibaba.com>, Shuah Khan <shuah@...nel.org>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, kvm@...r.kernel.org,
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org,
 Yuri Benditovich <yuri.benditovich@...nix.com>,
 Andrew Melnychenko <andrew@...nix.com>,
 Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com,
 devel@...nix.com
Subject: Re: [PATCH net-next v5 6/7] tap: Keep hdr_len in tap_get_user()

On 2025/02/06 6:21, Willem de Bruijn wrote:
> Akihiko Odaki wrote:
>> hdr_len is repeatedly used so keep it in a local variable.
>>
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
> 
>> @@ -682,11 +683,8 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>>   	if (msg_control && sock_flag(&q->sk, SOCK_ZEROCOPY)) {
>>   		struct iov_iter i;
>>   
>> -		copylen = vnet_hdr.hdr_len ?
>> -			tap16_to_cpu(q, vnet_hdr.hdr_len) : GOODCOPY_LEN;
>> -		if (copylen > good_linear)
>> -			copylen = good_linear;
>> -		else if (copylen < ETH_HLEN)
>> +		copylen = min(hdr_len ? hdr_len : GOODCOPY_LEN, good_linear);
>> +		if (copylen < ETH_HLEN)
>>   			copylen = ETH_HLEN;
> 
> I forgot earlier: this can also use single line statement
> 
>      copylen = max(copylen, ETH_HLEN);
> 
> And perhaps easiest to follow is
> 
>      copylen = hdr_len ?: GOODCOPY_LEN;
>      copylen = min(copylen, good_linear);
>      copylen = max(copylen, ETH_HLEN);

I introduced the min() usage as it now neatly fits in a line, but I 
found even clamp() fits so I'll use it in the next version:
copylen = clamp(hdr_len ?: GOODCOPY_LEN, ETH_HLEN, good_linear);

Please tell me if you prefer hdr_len ?: GOODCOPY_LEN in a separate line:
copylen = hdr_len ?: GOODCOPY_LEN;
copylen = clamp(copylen, ETH_HLEN, good_linear);

> 
>>   		linear = copylen;
>>   		i = *from;
>> @@ -697,11 +695,9 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>>   
>>   	if (!zerocopy) {
>>   		copylen = len;
>> -		linear = tap16_to_cpu(q, vnet_hdr.hdr_len);
>> -		if (linear > good_linear)
>> -			linear = good_linear;
>> -		else if (linear < ETH_HLEN)
>> -			linear = ETH_HLEN;
>> +		linear = min(hdr_len, good_linear);
>> +		if (copylen < ETH_HLEN)
>> +			copylen = ETH_HLEN;> > Same

I realized I mistakenly replaced linear with copylen here. Using clamp() 
will remove redundant variable references and fix the bug.