NFSD: Insulate nfsd4_encode_read_plus_data() from page boundaries in the encode buffer

Commit eeadcb7 ("NFSD: Simplify READ_PLUS") replaced the use of
write_bytes_to_xdr_buf(), copying what was in nfsd4_encode_read()
at the time.

However, the current code will corrupt the encoded data if the XDR
data items that are reserved early and then poked into the XDR
buffer later happen to fall on a page boundary in the XDR encoding
buffer.

__xdr_commit_encode can shift encoded data items in the encoding
buffer so that pointers returned from xdr_reserve_space() no longer
address the same part of the encoding stream.

Fixes: eeadcb7 ("NFSD: Simplify READ_PLUS")
Reviewed-by: NeilBrown <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>

Author: chucklever

fs/nfsd/nfs4xdr.c

@@ -5305,13 +5305,20 @@ nfsd4_encode_read_plus_data(struct nfsd4_compoundres *resp,
 	struct file *file = read->rd_nf->nf_file;
 	struct xdr_stream *xdr = resp->xdr;
 	bool splice_ok = argp->splice_ok;
+	unsigned int offset_offset;
+	__be32 nfserr, wire_count;
 	unsigned long maxcount;
-	__be32 nfserr, *p;
+	__be64 wire_offset;
 
-	/* Content type, offset, byte count */
-	p = xdr_reserve_space(xdr, 4 + 8 + 4);
-	if (!p)
+	if (xdr_stream_encode_u32(xdr, NFS4_CONTENT_DATA) != XDR_UNIT)
+		return nfserr_io;
+
+	offset_offset = xdr->buf->len;
+
+	/* Reserve space for the byte offset and count */
+	if (unlikely(!xdr_reserve_space(xdr, XDR_UNIT * 3)))
 		return nfserr_io;
+	xdr_commit_encode(xdr);
 
 	maxcount = min_t(unsigned long, read->rd_length,
 			 (xdr->buf->buflen - xdr->buf->len));
@@ -5323,10 +5330,12 @@ nfsd4_encode_read_plus_data(struct nfsd4_compoundres *resp,
 	if (nfserr)
 		return nfserr;
 
-	*p++ = cpu_to_be32(NFS4_CONTENT_DATA);
-	p = xdr_encode_hyper(p, read->rd_offset);
-	*p = cpu_to_be32(read->rd_length);
-
+	wire_offset = cpu_to_be64(read->rd_offset);
+	write_bytes_to_xdr_buf(xdr->buf, offset_offset, &wire_offset,
+			       XDR_UNIT * 2);
+	wire_count = cpu_to_be32(read->rd_length);
+	write_bytes_to_xdr_buf(xdr->buf, offset_offset + XDR_UNIT * 2,
+			       &wire_count, XDR_UNIT);
 	return nfs_ok;
 }