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

From: Florian Westphal <[email-masked]>
Date: Fri, 8 Jan 2016 17:23:51 +0100

David Laight <[email-masked]> wrote:
> From: Pablo Neira Ayuso
> > Sent: 08 January 2016 14:02
> > From: Florian Westphal <[email-masked]>
> > 
> > Needed to convert the (64bit) conntrack counters to BE ordering.
> > 
> ...
> >  	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 ...

> 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]);
 			}
 			break;
 		case NFT_BYTEORDER_HTON:
 			for (i = 0; i < priv->len / 8; i++) {
-				src64 = get_unaligned_be64(&src[i]);
+				src64 = get_unaligned((u64 *)&src[i]);
 				src64 = (__force u64)cpu_to_be64(src64);
-				put_unaligned_be64(src64, &dst[i]);
+				put_unaligned(src64, (u64 *)&dst[i]);
 			}
 			break;
 		}

Results in identical object code.