CIFS: Calculate the correct request length based on page offset and tail size

It's possible that the page offset is non-zero in the pages in a request,
change the function to calculate the correct data buffer length.
Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/transport.c

@@ -212,10 +212,24 @@ rqst_len(struct smb_rqst *rqst)
 	for (i = 0; i < rqst->rq_nvec; i++)
 		buflen += iov[i].iov_len;
 
-	/* add in the page array if there is one */
+	/*
+	 * Add in the page array if there is one. The caller needs to make
+	 * sure rq_offset and rq_tailsz are set correctly. If a buffer of
+	 * multiple pages ends at page boundary, rq_tailsz needs to be set to
+	 * PAGE_SIZE.
+	 */
 	if (rqst->rq_npages) {
-		buflen += rqst->rq_pagesz * (rqst->rq_npages - 1);
-		buflen += rqst->rq_tailsz;
+		if (rqst->rq_npages == 1)
+			buflen += rqst->rq_tailsz;
+		else {
+			/*
+			 * If there is more than one page, calculate the
+			 * buffer length based on rq_offset and rq_tailsz
+			 */
+			buflen += rqst->rq_pagesz * (rqst->rq_npages - 1) -
+					rqst->rq_offset;
+			buflen += rqst->rq_tailsz;
+		}
 	}
 
 	return buflen;