netdev - Re: [RFC net-next] tcp: add support for read with offset when using MSG

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Message-ID: <595d89f1-15b1-537d-f876-0ac4627db535@redhat.com>
Date: Thu, 18 Jan 2024 17:22:52 -0500
From: Jon Maloy <jmaloy@...hat.com>
To: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 davem@...emloft.net
Cc: kuba@...nel.org, passt-dev@...st.top, sbrivio@...hat.com,
 lvivier@...hat.com, dgibson@...hat.com
Subject: Re: [RFC net-next] tcp: add support for read with offset when using
 MSG_PEEK



On 2024-01-16 05:49, Paolo Abeni wrote:
> On Thu, 2024-01-11 at 18:00 -0500, jmaloy@...hat.com wrote:
>> From: Jon Maloy <jmaloy@...hat.com>
>>
>> When reading received messages from a socket with MSG_PEEK, we may want
>> to read the contents with an offset, like we can do with pread/preadv()
>> when reading files. Currently, it is not possible to do that.
[...]
>> +				err = -EINVAL;
>> +				goto out;
>> +			}
>> +			peek_offset = msg->msg_iter.__iov[0].iov_len;
>> +			msg->msg_iter.__iov = &msg->msg_iter.__iov[1];
>> +			msg->msg_iter.nr_segs -= 1;
>> +			msg->msg_iter.count -= peek_offset;
>> +			len -= peek_offset;
>> +			*seq += peek_offset;
>> +		}
> IMHO this does not look like the correct interface to expose such
> functionality. Doing the same with a different protocol should cause a
> SIGSEG or the like, right?
I would expect doing the same thing with a different protocol to cause 
an EFAULT, as it should. But I haven't tried it.
This is a change to TCP only, at least until somebody decides to 
implement it elsewhere (why not?)
>
> What about using/implementing SO_PEEK_OFF support instead?
I looked at SO_PEEK_OFF, and it honestly looks both awkward and limited.
We would have to make frequent calls to setsockopt(), something that 
would beat much of the purpose of this feature.
I stand by my opinion here.
This feature is simple, non-intrusive, totally backwards compatible and 
implies no changes to the API or BPI.

I would love to hear other opinions on this, though.

Regards
/jon


>
> Cheers,
>
> Paolo
>