Re: [PATCH] io_uring/zcrx: return early from io_zcrx_recv_skb if readlen is 0

[Jens Axboe <axboe@kernel.dk>]

On 4/1/25 1:10 PM, David Wei wrote:
> On 2025-04-01 11:56, Jens Axboe wrote:
>> On 4/1/25 12:28 PM, David Wei wrote:
>>> When readlen is set for a recvzc request, tcp_read_sock() will call
>>> io_zcrx_recv_skb() one final time with len == desc->count == 0. This is
>>> caused by the !desc->count check happening too late. The offset + 1 !=
>>> skb->len happens earlier and causes the while loop to continue.
>>>
>>> Fix this in io_zcrx_recv_skb() instead of tcp_read_sock(). Return early
>>> if len is 0 i.e. the read is done.
>>
>> Needs a Fixes tag, which looks like it should be:
>>
>> Fixes: 6699ec9a23f8 ("io_uring/zcrx: add a read limit to recvzc requests")
>>
>> ?
> 
> Sorry I missed that, will add the tag.
> 
>>
>>> Signed-off-by: David Wei <dw@davidwei.uk>
>>> ---
>>>  io_uring/zcrx.c | 2 ++
>>>  1 file changed, 2 insertions(+)
>>>
>>> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
>>> index 9c95b5b6ec4e..d1dd25e7cf4a 100644
>>> --- a/io_uring/zcrx.c
>>> +++ b/io_uring/zcrx.c
>>> @@ -818,6 +818,8 @@ io_zcrx_recv_skb(read_descriptor_t *desc, struct sk_buff *skb,
>>>  	int ret = 0;
>>>  
>>>  	len = min_t(size_t, len, desc->count);
>>> +	if (!len)
>>> +		goto out;
>>
>> just return 0 here? Jumping to out would make more sense if there
>> are things to fixup/account at this point, but it's just going
>> to find offset == start_off and return 'ret', which is 0 anyway.
>>
> 
> Makes sense, yeah. I'll return 0 here early.

Probably augment that with an unlikely() as well for v2, it's definitely
an error/unexpected case.

-- 
Jens Axboe