nfsd: Incoming xdr_bufs may have content in tail buffer

Since the beginning, svcsock has built a received RPC Call message
by populating the xdr_buf's head, then placing the remaining
message bytes in the xdr_buf's page list. The xdr_buf's tail is
never populated.

This means that an NFSv4 COMPOUND containing an NFS WRITE operation
plus trailing operations has a page list that contains the WRITE
data payload followed by the trailing operations. NFSv4 XDR decoders
will not look in the xdr_buf's tail, ever, because svcsock never put
anything there.

To support transports that can pass the write payload in the
xdr_buf's pagelist and trailing content in the xdr_buf's tail,
introduce logic in READ_BUF that switches to the xdr_buf's tail vec
when the decoder runs out of content in rq_arg.pages.

Signed-off-by: Chuck Lever <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs4xdr.c

···
 	 */
 	unsigned int avail = (char *)argp->end - (char *)argp->p;
 	__be32 *p;
+
+	if (argp->pagelen == 0) {
+		struct kvec *vec = &argp->rqstp->rq_arg.tail[0];
+
+		if (!argp->tail) {
+			argp->tail = true;
+			avail = vec->iov_len;
+			argp->p = vec->iov_base;
+			argp->end = vec->iov_base + avail;
+		}
+
+		if (avail < nbytes)
+			return NULL;
+
+		p = argp->p;
+		argp->p += XDR_QUADLEN(nbytes);
+		return p;
+	}
+
 	if (avail + argp->pagelen < nbytes)
 		return NULL;
 	if (avail + PAGE_SIZE < nbytes) /* need more than a page !! */
···
 	args->end = rqstp->rq_arg.head[0].iov_base + rqstp->rq_arg.head[0].iov_len;
 	args->pagelist = rqstp->rq_arg.pages;
 	args->pagelen = rqstp->rq_arg.page_len;
+	args->tail = false;
 	args->tmpp = NULL;
 	args->to_free = NULL;
 	args->ops = args->iops;

fs/nfsd/xdr4.h

···
 	__be32 *			end;
 	struct page **			pagelist;
 	int				pagelen;
+	bool				tail;
 	__be32				tmp[8];
 	__be32 *			tmpp;
 	struct svcxdr_tmpbuf		*to_free;