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

[David Wei <dw@davidwei.uk>]

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.