Re: [RFC] situation with csum_and_copy_... API

From: Eric Dumazet
Date: Thu Nov 20 2014 - 17:54:09 EST

On Thu, 2014-11-20 at 22:25 +0000, Al Viro wrote:

> Yes, it is. You are breaking several _other_ kernel_sendmsg() users.
> They are already slightly broken, but that'll make breakage much more
> common.
> Please, don't - the right thing to do is to have iov_iter in msghdr
> (we already have the kernel and userland ones with different types and
> we do not assume their layouts to be identical - currently they are,
> but it's easy to change), keep iovec constant in all cases and advance
> ->msg_iter. Also in all cases.
> Note that direct manipulations of what's currently in ->msg_iov are
> wrong - all those loops over vector elements, etc., belong in low-level
> primitives. The main missing ones right now are csum_and_copy_{from,to}_iter()
> - I have those in local queue, but I'm still trying to get a reasonably
> clean mm/iov_iter.c without ridiculous amounts of boilerplating. A bit more
> massage is needed there...
> Seriously, take a look at vfs.git#iov_iter-net; it's preparations for the
> one that'll introduce ->msg_iter. Right now that branch has local iov_iter
> declared and initialized in several ->sendmsg() and ->recvmsg() instances and
> fed to primitives that work with it; after the conversion it'll be in
> msg->msg_iter and it will be initialized by sock_sendmsg()/sock_recvmsg().
> The tricky part is how to get through that without temporary breaking the
> existing sendmsg/recvmsg users in the kernel *and* without a patch size from
> hell. I more or less see how to carve the remaining steps into
> reasonably-sized chunks; iscsi is one of the tricky ones and it, AFAICS,
> is genuinely broken in mainline and will need fixes that can go into -stable.
> And no, your solution doesn't work. Sorry. You'll break e.g. smb_send_kvec()
> that way. ceph_tcp_sendmsg() as well, IIRC.

Nowhere in tcp_sendmsg() the iov had const qualifier.

If it was declared as const, this discussion would not happen,
we would know we are not allowed to modify it.

iov_iter is nice, but not a single time it is used in net/

Please make sure to add const where appropriate.


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