Backport TLS padding extension from master.

(cherry picked from commit 8c6d8c2a49)

Conflicts:

	CHANGES
	ssl/t1_lib.c

CHANGES

@@ -4,7 +4,24 @@
 
  Changes between 1.0.1f and 1.0.1g [xx XXX xxxx]
 
-  *)
+  *) TLS pad extension: draft-agl-tls-padding-02
+
+     Workaround for the "TLS hang bug" (see FAQ and PR#2771): if the
+     TLS client Hello record length value would otherwise be > 255 and
+     less that 512 pad with a dummy extension containing zeroes so it
+     is at least 512 bytes long.
+
+     To enable it use an unused extension number (for example chrome uses
+     35655) using:
+
+     e.g. -DTLSEXT_TYPE_padding=35655
+
+     Since the extension is ignored the actual number doesn't matter as long
+     as it doesn't clash with any existing extension.
+
+     This will be updated when the extension gets an official number.
+
+     [Adam Langley, Steve Henson]
 
  Changes between 1.0.1e and 1.0.1f [6 Jan 2014]
 

ssl/t1_lib.c

@@ -662,6 +662,36 @@ unsigned char *ssl_add_clienthello_tlsext(SSL *s, unsigned char *p, unsigned cha
                 }
 #endif
 
+#ifdef TLSEXT_TYPE_padding
+	/* Add padding to workaround bugs in F5 terminators.
+	 * See https://tools.ietf.org/html/draft-agl-tls-padding-02
+	 *
+	 * NB: because this code works out the length of all existing
+	 * extensions it MUST always appear last.
+	 */
+	{
+	int hlen = ret - (unsigned char *)s->init_buf->data;
+	/* The code in s23_clnt.c to build ClientHello messages includes the
+	 * 5-byte record header in the buffer, while the code in s3_clnt.c does
+	 * not. */
+	if (s->state == SSL23_ST_CW_CLNT_HELLO_A)
+		hlen -= 5;
+	if (hlen > 0xff && hlen < 0x200)
+		{
+		hlen = 0x200 - hlen;
+		if (hlen >= 4)
+			hlen -= 4;
+		else
+			hlen = 0;
+
+		s2n(TLSEXT_TYPE_padding, ret);
+		s2n(hlen, ret);
+		memset(ret, 0, hlen);
+		ret += hlen;
+		}
+	}
+#endif
+
 	if ((extdatalen = ret-p-2)== 0) 
 		return p;