[U-Boot] [PATCH] net/dns.c: Fix broken endian handling in dns command

[Bernhard Kaindl]

From: Bernhard Kaindl <bernhard.kaindl at thalesgroup.com>

The U-Boot dns command only worked in little-endian CPUs so far because it
was based on an antique version of the TADNS source which was using a broken
macro to read the shorts found in DNS reply messages by shifting the LSB from
the message into the CPU's MSB of a short int and the MSB from the stream into
the LSB part of the CPU's short int. So far, so twisted.

To correct the twisted bytes, the code used ntohs() as a byte-swapping function
to swap the MSB back where it belongs and vice versa.

This works fine, except that ntohs() naturally does nothing on big-endian CPUs.

So on big-endian CPUs, the MSB from the message stayed in the LSB of the CPU
and vice versa.

Ditch this brain-deadness by just shifting the MSB from the network byte
stream of the reply message into the right (MSB) location of a short int
and putting the LSB from the network byte stream as the lower byte of it,
and we are done with reading the short from the network stream for both
endianesses, no ntohs() or such!

The current TADNS source uses the same macro meanwhile.

Signed-off-by: Bernhard Kaindl <bernhard.kaindl at thalesgroup.com>
Cc: Pieter Voorthuijsen <pieter.voorthuijsen at prodrive.nl>
Cc: Robin Getz <rgetz at blackfin.uclinux.org>
---
 net/dns.c |   22 ++++++++++------------
 1 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/net/dns.c b/net/dns.c
index b51d1bd..7034183 100644
--- a/net/dns.c
+++ b/net/dns.c
@@ -28,6 +28,9 @@
 
 #include "dns.h"
 
+/* p is a char *, we shift the MSB up in our CPU endianness -> no ntohs used! */
+#define netbytes2hs(p) ((p)[0] << 8 | (p)[1])
+
 char *NetDNSResolve;	/* The host to resolve  */
 char *NetDNSenvvar;	/* The envvar to store the answer in */
 
@@ -109,7 +112,6 @@ DnsHandler(uchar *pkt, unsigned dest, IPaddr_t sip, unsigned src, unsigned len)
 	int found, stop, dlen;
 	char IPStr[22];
 	IPaddr_t IPAddress;
-	short tmp;
 
 
 	debug("%s\n", __func__);
@@ -120,14 +122,14 @@ DnsHandler(uchar *pkt, unsigned dest, IPaddr_t sip, unsigned src, unsigned len)
 		debug("0x%p - 0x%.2x  0x%.2x  0x%.2x  0x%.2x\n",
 			pkt+i, pkt[i], pkt[i+1], pkt[i+2], pkt[i+3]);
 
-	/* We sent 1 query. We want to see more that 1 answer. */
+	/* We sent one query. We want to have a single answer: */
 	header = (struct header *) pkt;
 	if (ntohs(header->nqueries) != 1)
 		return;
 
 	/* Received 0 answers */
 	if (header->nanswers == 0) {
-		puts("DNS server returned no answers\n");
+		puts("DNS: host not found\n");
 		NetState = NETLOOP_SUCCESS;
 		return;
 	}
@@ -139,9 +141,8 @@ DnsHandler(uchar *pkt, unsigned dest, IPaddr_t sip, unsigned src, unsigned len)
 		continue;
 
 	/* We sent query class 1, query type 1 */
-	tmp = p[1] | (p[2] << 8);
-	if (&p[5] > e || ntohs(tmp) != DNS_A_RECORD) {
-		puts("DNS response was not A record\n");
+	if (&p[5] > e || netbytes2hs(p+1) != DNS_A_RECORD) {
+		puts("DNS: response not an A record\n");
 		NetState = NETLOOP_SUCCESS;
 		return;
 	}
@@ -160,14 +161,12 @@ DnsHandler(uchar *pkt, unsigned dest, IPaddr_t sip, unsigned src, unsigned len)
 		}
 		debug("Name (Offset in header): %d\n", p[1]);
 
-		tmp = p[2] | (p[3] << 8);
-		type = ntohs(tmp);
+		type = netbytes2hs(p+2);
 		debug("type = %d\n", type);
 		if (type == DNS_CNAME_RECORD) {
 			/* CNAME answer. shift to the next section */
 			debug("Found canonical name\n");
-			tmp = p[10] | (p[11] << 8);
-			dlen = ntohs(tmp);
+			dlen = netbytes2hs(p+10);
 			debug("dlen = %d\n", dlen);
 			p += 12 + dlen;
 		} else if (type == DNS_A_RECORD) {
@@ -181,8 +180,7 @@ DnsHandler(uchar *pkt, unsigned dest, IPaddr_t sip, unsigned src, unsigned len)
 
 	if (found && &p[12] < e) {
 
-		tmp = p[10] | (p[11] << 8);
-		dlen = ntohs(tmp);
+		dlen = netbytes2hs(p+10);
 		p += 12;
 		memcpy(&IPAddress, p, 4);
 
-- 
1.7.3.4