Re: [PATCH net-next v6 01/18] net: Declare MSG_SPLICE_PAGES internal sendmsg() flag

[Al Viro <viro@zeniv.linux.org.uk>]

On Thu, Apr 13, 2023 at 01:51:29AM +0100, Al Viro wrote:
> On Tue, Apr 11, 2023 at 05:08:45PM +0100, David Howells wrote:
> 
> > @@ -2483,6 +2484,7 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
> >  	}
> >  	msg_sys->msg_flags = flags;
> >  
> > +	flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
> >  	if (sock->file->f_flags & O_NONBLOCK)
> >  		msg_sys->msg_flags |= MSG_DONTWAIT;
> 
> A bit too late, innit?  There's no users of 'flags' downstream of that
> assignment to ->msg_flags, so your &= is a no-op; it should be done
> *before* that assignment...

While we are at it, io-uring has this:
int io_sendmsg_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
{
        struct io_sr_msg *sr = io_kiocb_to_cmd(req, struct io_sr_msg);
...
        sr->msg_flags = READ_ONCE(sqe->msg_flags) | MSG_NOSIGNAL;

and

int io_send(struct io_kiocb *req, unsigned int issue_flags)
{
        struct sockaddr_storage __address;
        struct io_sr_msg *sr = io_kiocb_to_cmd(req, struct io_sr_msg);

...
        flags = sr->msg_flags;
        if (issue_flags & IO_URING_F_NONBLOCK)
                flags |= MSG_DONTWAIT;
        if (flags & MSG_WAITALL)
                min_ret = iov_iter_count(&msg.msg_iter);

        msg.msg_flags = flags;
        ret = sock_sendmsg(sock, &msg);

Note that io_sendmsg_prep() handles both IORING_OP_SENDMSG and IORING_OP_SEND,
so this pair of functions can hit the same request.  And sqe->msg_flags is
not sanitized at all - it comes straight from user buffer.