split allocation out of sshbuf_reserve() into a separate

sshbuf_allocate() function; ok markus@

usr.bin/ssh/sshbuf.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: sshbuf.c,v 1.7 2016/09/12 01:22:38 deraadt Exp $	*/
+/*	$OpenBSD: sshbuf.c,v 1.8 2016/11/25 23:22:04 djm Exp $	*/
 /*
  * Copyright (c) 2011 Damien Miller
  *
@@ -314,53 +314,63 @@ sshbuf_check_reserve(const struct sshbuf *buf, size_t len)
 }
 
 int
-sshbuf_reserve(struct sshbuf *buf, size_t len, u_char **dpp)
+sshbuf_allocate(struct sshbuf *buf, size_t len)
 {
 	size_t rlen, need;
 	u_char *dp;
 	int r;
 
-	if (dpp != NULL)
-		*dpp = NULL;
-
-	SSHBUF_DBG(("reserve buf = %p len = %zu", buf, len));
+	SSHBUF_DBG(("allocate buf = %p len = %zu", buf, len));
 	if ((r = sshbuf_check_reserve(buf, len)) != 0)
 		return r;
 	/*
 	 * If the requested allocation appended would push us past max_size
 	 * then pack the buffer, zeroing buf->off.
 	 */
 	sshbuf_maybe_pack(buf, buf->size + len > buf->max_size);
-	SSHBUF_TELL("reserve");
-	if (len + buf->size > buf->alloc) {
-		/*
-		 * Prefer to alloc in SSHBUF_SIZE_INC units, but
-		 * allocate less if doing so would overflow max_size.
-		 */
-		need = len + buf->size - buf->alloc;
-		rlen = ROUNDUP(buf->alloc + need, SSHBUF_SIZE_INC);
-		SSHBUF_DBG(("need %zu initial rlen %zu", need, rlen));
-		if (rlen > buf->max_size)
-			rlen = buf->alloc + need;
-		SSHBUF_DBG(("adjusted rlen %zu", rlen));
-		if ((dp = realloc(buf->d, rlen)) == NULL) {
-			SSHBUF_DBG(("realloc fail"));
-			if (dpp != NULL)
-				*dpp = NULL;
-			return SSH_ERR_ALLOC_FAIL;
-		}
-		buf->alloc = rlen;
-		buf->cd = buf->d = dp;
-		if ((r = sshbuf_check_reserve(buf, len)) < 0) {
-			/* shouldn't fail */
-			if (dpp != NULL)
-				*dpp = NULL;
-			return r;
-		}
+	SSHBUF_TELL("allocate");
+	if (len + buf->size <= buf->alloc)
+		return 0; /* already have it. */
+
+	/*
+	 * Prefer to alloc in SSHBUF_SIZE_INC units, but
+	 * allocate less if doing so would overflow max_size.
+	 */
+	need = len + buf->size - buf->alloc;
+	rlen = ROUNDUP(buf->alloc + need, SSHBUF_SIZE_INC);
+	SSHBUF_DBG(("need %zu initial rlen %zu", need, rlen));
+	if (rlen > buf->max_size)
+		rlen = buf->alloc + need;
+	SSHBUF_DBG(("adjusted rlen %zu", rlen));
+	if ((dp = realloc(buf->d, rlen)) == NULL) {
+		SSHBUF_DBG(("realloc fail"));
+		return SSH_ERR_ALLOC_FAIL;
+	}
+	buf->alloc = rlen;
+	buf->cd = buf->d = dp;
+	if ((r = sshbuf_check_reserve(buf, len)) < 0) {
+		/* shouldn't fail */
+		return r;
 	}
+	SSHBUF_TELL("done");
+	return 0;
+}
+
+int
+sshbuf_reserve(struct sshbuf *buf, size_t len, u_char **dpp)
+{
+	u_char *dp;
+	int r;
+
+	if (dpp != NULL)
+		*dpp = NULL;
+
+	SSHBUF_DBG(("reserve buf = %p len = %zu", buf, len));
+	if ((r = sshbuf_allocate(buf, len)) != 0)
+		return r;
+
 	dp = buf->d + buf->size;
 	buf->size += len;
-	SSHBUF_TELL("done");
 	if (dpp != NULL)
 		*dpp = dp;
 	return 0;

usr.bin/ssh/sshbuf.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: sshbuf.h,v 1.7 2016/05/02 08:49:03 djm Exp $	*/
+/*	$OpenBSD: sshbuf.h,v 1.8 2016/11/25 23:22:04 djm Exp $	*/
 /*
  * Copyright (c) 2011 Damien Miller
  *
@@ -134,6 +134,14 @@ u_char *sshbuf_mutable_ptr(const struct sshbuf *buf);
  */
 int	sshbuf_check_reserve(const struct sshbuf *buf, size_t len);
 
+/*
+ * Preallocates len additional bytes in buf.
+ * Useful for cases where the caller knows how many bytes will ultimately be
+ * required to avoid realloc in the buffer code.
+ * Returns 0 on success, or a negative SSH_ERR_* error code on failure.
+ */
+int	sshbuf_allocate(struct sshbuf *buf, size_t len);
+
 /*
  * Reserve len bytes in buf.
  * Returns 0 on success and a pointer to the first reserved byte via the