Add support for initialising WPACKETs from a static buffer

Normally WPACKETs will use a BUF_MEM which can grow as required. Sometimes
though that may be overkill for what is needed - a static buffer may be
sufficient. This adds that capability.

Reviewed-by: Rich Salz <rsalz@openssl.org>

ssl/packet.c

diff --git a/ssl/packet.c b/ssl/packet.c
index 27462e9..5c55133 100644
--- a/ssl/packet.c
+++ b/ssl/packet.c
@@ -33,6 +33,9 @@
     return 1;
 }
 
+#define GETBUF(p)   (((p)->staticbuf != NULL) \
+                     ? (p)->staticbuf : (unsigned char *)(p)->buf->data)
+
 int WPACKET_reserve_bytes(WPACKET *pkt, size_t len, unsigned char **allocbytes)
 {
     /* Internal API, so should not fail */
@@ -43,7 +46,7 @@
     if (pkt->maxsize - pkt->written < len)
         return 0;
 
-    if (pkt->buf->length - pkt->written < len) {
+    if (pkt->staticbuf == NULL && (pkt->buf->length - pkt->written < len)) {
         size_t newlen;
         size_t reflen;
 
@@ -59,7 +62,7 @@
         if (BUF_MEM_grow(pkt->buf, newlen) == 0)
             return 0;
     }
-    *allocbytes = (unsigned char *)pkt->buf->data + pkt->curr;
+    *allocbytes = GETBUF(pkt) + pkt->curr;
 
     return 1;
 }
@@ -83,19 +86,12 @@
     return ((size_t)1 << (lenbytes * 8)) - 1 + lenbytes;
 }
 
-int WPACKET_init_len(WPACKET *pkt, BUF_MEM *buf, size_t lenbytes)
+static int wpacket_intern_init_len(WPACKET *pkt, size_t lenbytes)
 {
     unsigned char *lenchars;
 
-    /* Internal API, so should not fail */
-    assert(buf != NULL);
-    if (buf == NULL)
-        return 0;
-
-    pkt->buf = buf;
     pkt->curr = 0;
     pkt->written = 0;
-    pkt->maxsize = maxmaxsize(lenbytes);
 
     pkt->subs = OPENSSL_zalloc(sizeof(*pkt->subs));
     if (pkt->subs == NULL)
@@ -112,11 +108,42 @@
         pkt->subs = NULL;
         return 0;
     }
-    pkt->subs->packet_len = lenchars - (unsigned char *)pkt->buf->data;
+    pkt->subs->packet_len = lenchars - GETBUF(pkt);
 
     return 1;
 }
 
+int WPACKET_init_static_len(WPACKET *pkt, unsigned char *buf, size_t len,
+                            size_t lenbytes)
+{
+    size_t max = maxmaxsize(lenbytes);
+
+    /* Internal API, so should not fail */
+    assert(buf != NULL && len > 0);
+    if (buf == NULL || len == 0)
+        return 0;
+
+    pkt->staticbuf = buf;
+    pkt->buf = NULL;
+    pkt->maxsize = (max < len) ? max : len;
+
+    return wpacket_intern_init_len(pkt, lenbytes);
+}
+
+int WPACKET_init_len(WPACKET *pkt, BUF_MEM *buf, size_t lenbytes)
+{
+    /* Internal API, so should not fail */
+    assert(buf != NULL);
+    if (buf == NULL)
+        return 0;
+
+    pkt->staticbuf = NULL;
+    pkt->buf = buf;
+    pkt->maxsize = maxmaxsize(lenbytes);
+
+    return wpacket_intern_init_len(pkt, lenbytes);
+}
+
 int WPACKET_init(WPACKET *pkt, BUF_MEM *buf)
 {
     return WPACKET_init_len(pkt, buf, 0);
@@ -179,8 +206,8 @@
 
     /* Write out the WPACKET length if needed */
     if (sub->lenbytes > 0
-                && !put_value((unsigned char *)&pkt->buf->data[sub->packet_len],
-                              packlen, sub->lenbytes))
+                && !put_value(&GETBUF(pkt)[sub->packet_len], packlen,
+                              sub->lenbytes))
             return 0;
 
     pkt->subs = sub->parent;
@@ -248,7 +275,7 @@
     if (!WPACKET_allocate_bytes(pkt, lenbytes, &lenchars))
         return 0;
     /* Convert to an offset in case the underlying BUF_MEM gets realloc'd */
-    sub->packet_len = lenchars - (unsigned char *)pkt->buf->data;
+    sub->packet_len = lenchars - GETBUF(pkt);
 
     return 1;
 }

ssl/packet_locl.h

diff --git a/ssl/packet_locl.h b/ssl/packet_locl.h
index 94933c1..4658734 100644
--- a/ssl/packet_locl.h
+++ b/ssl/packet_locl.h
@@ -625,6 +625,9 @@
     /* The buffer where we store the output data */
     BUF_MEM *buf;
 
+    /* Fixed sized buffer which can be used as an alternative to buf */
+    unsigned char *staticbuf;
+
     /*
      * Offset into the buffer where we are currently writing. We use an offset
      * in case the buffer grows and gets reallocated.
@@ -671,6 +674,13 @@
 int WPACKET_init(WPACKET *pkt, BUF_MEM *buf);
 
 /*
+ * Same as WPACKET_init_len except we do not use a growable BUF_MEM structure.
+ * A fixed buffer of memory |buf| of size |len| is used instead. A failure will
+ * occur if you attempt to write beyond the end of the buffer
+ */
+int WPACKET_init_static_len(WPACKET *pkt, unsigned char *buf, size_t len,
+                            size_t lenbytes);
+/*
  * Set the flags to be applied to the current sub-packet
  */
 int WPACKET_set_flags(WPACKET *pkt, unsigned int flags);

test/wpackettest.c

diff --git a/test/wpackettest.c b/test/wpackettest.c
index aabf781..e082b18 100644
--- a/test/wpackettest.c
+++ b/test/wpackettest.c
@@ -20,6 +20,7 @@
 const static unsigned char empty = 0x00;
 const static unsigned char alloc[] = { 0x02, 0xfe, 0xff };
 const static unsigned char submem[] = { 0x03, 0x02, 0xfe, 0xff };
+const static unsigned char fixed[] = { 0xff, 0xff, 0xff };
 
 static BUF_MEM *buf;
 
@@ -34,6 +35,7 @@
     WPACKET pkt;
     int i;
     size_t written;
+    unsigned char sbuf[3];
 
     if (!WPACKET_init(&pkt, buf)
             || !WPACKET_put_bytes_u8(&pkt, 0xff)
@@ -95,6 +97,31 @@
         return 0;
     }
 
+    /* Test initialising from a fixed size buffer */
+    if (!WPACKET_init_static_len(&pkt, sbuf, sizeof(sbuf), 0)
+                /* Adding 3 bytes should succeed */
+            || !WPACKET_put_bytes_u24(&pkt, 0xffffff)
+                /* Adding 1 more byte should fail */
+            ||  WPACKET_put_bytes_u8(&pkt, 0xff)
+                /* Finishing the top level WPACKET should succeed */
+            || !WPACKET_finish(&pkt)
+            || !WPACKET_get_total_written(&pkt, &written)
+            ||  written != sizeof(fixed)
+            || memcmp(sbuf, fixed, sizeof(sbuf)) != 0
+                /* Initialise with 1 len byte */
+            || !WPACKET_init_static_len(&pkt, sbuf, sizeof(sbuf), 1)
+                /* Adding 2 bytes should succeed */
+            || !WPACKET_put_bytes_u16(&pkt, 0xfeff)
+                /* Adding 1 more byte should fail */
+            ||  WPACKET_put_bytes_u8(&pkt, 0xff)
+            || !WPACKET_finish(&pkt)
+            || !WPACKET_get_total_written(&pkt, &written)
+            ||  written != sizeof(alloc)
+            ||  memcmp(sbuf, alloc, written) != 0) {
+        testfail("test_WPACKET_init():5 failed\n", &pkt);
+        return 0;
+    }
+
     return 1;
 }