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

[Jon Maloy <jmaloy@redhat.com>]

On 2024-01-21 17:16, Stefano Brivio wrote:
> On Thu, 18 Jan 2024 17:22:52 -0500
> Jon Maloy <jmaloy@redhat.com> wrote:
>
>> On 2024-01-16 05:49, Paolo Abeni wrote:
>>> On Thu, 2024-01-11 at 18:00 -0500, jmaloy@redhat.com wrote:
>>>> From: Jon Maloy <jmaloy@redhat.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.
> So, out of curiosity, I actually tried: the current behaviour is
> recvmsg() failing with EFAULT, only as data is received (!), for TCP
> and UDP with AF_INET, and for AF_UNIX (both datagram and stream).
>
> EFAULT, however, is not in the list of "shall fail", nor "may fail"
> conditions described by POSIX.1-2008, so there isn't really anything
> that mandates it API-wise.
>
> Likewise, POSIX doesn't require any signal to be delivered (and no
> signals are delivered on Linux in any case: note that iov_base is not
> dereferenced).
>
> For TCP sockets only, passing a NULL buffer is already supported by
> recv() with MSG_TRUNC (same here, Linux extension). This change would
> finally make recvmsg() consistent with that TCP-specific bit.
>
>> This is a change to TCP only, at least until somebody decides to
>> implement it elsewhere (why not?)
> Side note, I can't really think of a reasonable use case for UDP -- it
> doesn't quite fit with the notion of message boundaries.
>
> Even letting alone the fact that passt(1) and pasta(1) don't need this
> for UDP (no acknowledgement means no need to keep unacknowledged data
> anywhere), if another application wants to do something conceptually
> similar, we should probably target recvmmsg().
>
>>> What about using/implementing SO_PEEK_OFF support instead?
>> I looked at SO_PEEK_OFF, and it honestly looks both awkward and limited.
> I think it's rather intended to skip headers with fixed size or
> suchlike.
>
>> We would have to make frequent calls to setsockopt(), something that
>> would beat much of the purpose of this feature.
> ...right, we would need to reset the SO_PEEK_OFF value at every
> recvmsg(), which is probably even worse than the current overhead.
>
>> I stand by my opinion here.
>> This feature is simple, non-intrusive, totally backwards compatible and
>> implies no changes to the API or BPI.
> My thoughts as well, plus the advantage for our user-mode networking
> case is quite remarkable given how simple the change is.

After pondering more upon this, and also some team internal discussions, 
I have decided to give it a try with SO_PEEK_OFF, just to see to see the 
outcome, both at kernel level and in user space.
So please wait with any possible application of this , if that ever 
happens with RFCs.

///jon
>
>> I would love to hear other opinions on this, though.
>>
>> Regards
>> /jon
>>
>>> Cheers,
>>>
>>> Paolo