Re: [PATCH 16/17] netfilter: nft_byteorder: provide 64bit le/be conversion

David Laight [off-list ref] wrote:

From: Florian Westphal

quoted

Sent: 08 January 2016 16:24
To: David Laight
David Laight [off-list ref] wrote:

quoted

From: Pablo Neira Ayuso

quoted

Sent: 08 January 2016 14:02
From: Florian Westphal <fw@strlen.de>

Needed to convert the (64bit) conntrack counters to BE ordering.

...

quoted

 	switch (priv->size) {
+	case 8: {
+		u64 src64;
+
+		switch (priv->op) {
+		case NFT_BYTEORDER_NTOH:
+			for (i = 0; i < priv->len / 8; i++) {
+				src64 = get_unaligned_be64(&src[i]);
+				src64 = be64_to_cpu((__force __be64)src64);
+				put_unaligned_be64(src64, &dst[i]);
+			}
+			break;
+		case NFT_BYTEORDER_HTON:
+			for (i = 0; i < priv->len / 8; i++) {
+				src64 = get_unaligned_be64(&src[i]);
+				src64 = (__force u64)cpu_to_be64(src64);
+				put_unaligned_be64(src64, &dst[i]);
+			}
+			break;
+		}
+		break;

That is horrid.

Yes, sorry for this, however ...

quoted

On a little-endian system you are byteswapping the data 3 times.
Image the code on a cpu that doesn't support misaligned transfers
and doesn't have a byteswap instruction.

diff --git a/net/netfilter/nft_byteorder.c b/net/netfilter/nft_byteorder.c
--- a/net/netfilter/nft_byteorder.c
+++ b/net/netfilter/nft_byteorder.c

@@ -46,16 +46,16 @@ static void nft_byteorder_eval(const struct nft_expr *expr,
 		switch (priv->op) {
 		case NFT_BYTEORDER_NTOH:
 			for (i = 0; i < priv->len / 8; i++) {
-				src64 = get_unaligned_be64(&src[i]);
+				src64 = get_unaligned((u64 *)&src[i]);
 				src64 = be64_to_cpu((__force __be64)src64);
-				put_unaligned_be64(src64, &dst[i]);
+				put_unaligned(src64, (u64 *)&dst[i]);

Why not just:
				src64 = get_unaligned_be64(&src[i]);
				put_unaligned(src64, (u64 *)&dst[i]);

Sure.