gss_krb5: Added and improved code comments

Signed-off-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth_gss/auth_gss.c

@@ -1316,15 +1316,21 @@ gss_wrap_req_priv(struct rpc_cred *cred, struct gss_cl_ctx *ctx,
 	inpages = snd_buf->pages + first;
 	snd_buf->pages = rqstp->rq_enc_pages;
 	snd_buf->page_base -= first << PAGE_CACHE_SHIFT;
-	/* Give the tail its own page, in case we need extra space in the
-	 * head when wrapping: */
+	/*
+	 * Give the tail its own page, in case we need extra space in the
+	 * head when wrapping:
+	 *
+	 * call_allocate() allocates twice the slack space required
+	 * by the authentication flavor to rq_callsize.
+	 * For GSS, slack is GSS_CRED_SLACK.
+	 */
 	if (snd_buf->page_len || snd_buf->tail[0].iov_len) {
 		tmp = page_address(rqstp->rq_enc_pages[rqstp->rq_enc_pages_num - 1]);
 		memcpy(tmp, snd_buf->tail[0].iov_base, snd_buf->tail[0].iov_len);
 		snd_buf->tail[0].iov_base = tmp;
 	}
 	maj_stat = gss_wrap(ctx->gc_gss_ctx, offset, snd_buf, inpages);
-	/* RPC_SLACK_SPACE should prevent this ever happening: */
+	/* slack space should prevent this ever happening: */
 	BUG_ON(snd_buf->len > snd_buf->buflen);
 	status = -EIO;
 	/* We're assuming that when GSS_S_CONTEXT_EXPIRED, the encryption was

net/sunrpc/auth_gss/gss_mech_switch.c

@@ -285,6 +285,20 @@ gss_verify_mic(struct gss_ctx		*context_handle,
 				 mic_token);
 }
 
+/*
+ * This function is called from both the client and server code.
+ * Each makes guarantees about how much "slack" space is available
+ * for the underlying function in "buf"'s head and tail while
+ * performing the wrap.
+ *
+ * The client and server code allocate RPC_MAX_AUTH_SIZE extra
+ * space in both the head and tail which is available for use by
+ * the wrap function.
+ *
+ * Underlying functions should verify they do not use more than
+ * RPC_MAX_AUTH_SIZE of extra space in either the head or tail
+ * when performing the wrap.
+ */
 u32
 gss_wrap(struct gss_ctx	*ctx_id,
 	 int		offset,

net/sunrpc/auth_gss/svcauth_gss.c

@@ -1315,6 +1315,14 @@ svcauth_gss_wrap_resp_priv(struct svc_rqst *rqstp)
 	inpages = resbuf->pages;
 	/* XXX: Would be better to write some xdr helper functions for
 	 * nfs{2,3,4}xdr.c that place the data right, instead of copying: */
+
+	/*
+	 * If there is currently tail data, make sure there is
+	 * room for the head, tail, and 2 * RPC_MAX_AUTH_SIZE in
+	 * the page, and move the current tail data such that
+	 * there is RPC_MAX_AUTH_SIZE slack space available in
+	 * both the head and tail.
+	 */
 	if (resbuf->tail[0].iov_base) {
 		BUG_ON(resbuf->tail[0].iov_base >= resbuf->head[0].iov_base
 							+ PAGE_SIZE);
@@ -1327,6 +1335,13 @@ svcauth_gss_wrap_resp_priv(struct svc_rqst *rqstp)
 			resbuf->tail[0].iov_len);
 		resbuf->tail[0].iov_base += RPC_MAX_AUTH_SIZE;
 	}
+	/*
+	 * If there is no current tail data, make sure there is
+	 * room for the head data, and 2 * RPC_MAX_AUTH_SIZE in the
+	 * allotted page, and set up tail information such that there
+	 * is RPC_MAX_AUTH_SIZE slack space available in both the
+	 * head and tail.
+	 */
 	if (resbuf->tail[0].iov_base == NULL) {
 		if (resbuf->head[0].iov_len + 2*RPC_MAX_AUTH_SIZE > PAGE_SIZE)
 			return -ENOMEM;