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Date:   Fri, 3 Nov 2023 20:37:04 +0100
From:   Florian Westphal <fw@strlen.de>
To:     Brian Davidson <davidson.brian@gmail.com>
Cc:     netfilter@vger.kernel.org, pablo@netfilter.org
Subject: Re: ip6 dscp fails map lookup
Message-ID: <20231103193704.GI8035@breakpoint.cc>
References: <CAOoaxdEpJDAQMYxRCC6rUVc=sMY=YXxF42LSeDvMQUPC5sahLA@mail.gmail.com>
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Brian Davidson <davidson.brian@gmail.com> wrote:
> Using 'ip6 dscp' in a map lookup does not give expected results. It
> seems to always match the zero value (cs0). It appears in the first
> rule that the byteorder is not being converted between the two bitwise
> operations, which is what happens when using ip6 dscp directly in the
> second rule.
> 
> # nft -f - <<EOF
> add table ip6 t
> add chain ip6 t c
> add map ip6 t mapv6 { typeof ip6 dscp : meta mark; }
> EOF
> 
> # nft -d netlink add rule ip6 t c meta mark set ip6 dscp map @mapv6
> ip6 t c
>   [ payload load 2b @ network header + 0 => reg 1 ]
>   [ bitwise reg 1 = ( reg 1 & 0x0000c00f ) ^ 0x00000000 ]
>   [ bitwise reg 1 = ( reg 1 >> 0x00000006 ) ]
>   [ lookup reg 1 set mapv6 dreg 1 ]
>   [ meta set mark with reg 1 ]
> 
> # nft -d netlink add rule ip6 t c meta mark set ip6 dscp
> ip6 t c
>   [ payload load 2b @ network header + 0 => reg 1 ]
>   [ bitwise reg 1 = ( reg 1 & 0x0000c00f ) ^ 0x00000000 ]
>   [ byteorder reg 1 = ntoh(reg 1, 2, 2) ]
>   [ bitwise reg 1 = ( reg 1 >> 0x00000006 ) ]
>   [ meta set mark with reg 1 ]

Uhm.  Pablo, any idea why the byte-swap-or-not logic depends
on something *other* than if the mask length is > 8 bit or not?

diff --git a/src/evaluate.c b/src/evaluate.c
--- a/src/evaluate.c
+++ b/src/evaluate.c
@@ -545,7 +545,7 @@ static void expr_evaluate_bits(struct eval_ctx *ctx, struct expr **exprp)
 	and->len	= masklen;
 
 	if (shift) {
-		if (ctx->stmt_len > 0 && div_round_up(masklen, BITS_PER_BYTE) > 1) {
+		if (masklen > BITS_PER_BYTE) {
 			int op = byteorder_conversion_op(expr, BYTEORDER_HOST_ENDIAN);
 			and = unary_expr_alloc(&expr->location, op, and);
 			and->len = masklen;