Re: [PATCH nft] proto: fix VLAN header definition

[Pablo Neira Ayuso <pablo@netfilter.org>]

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On Sun, Nov 29, 2015 at 11:37:43PM +0100, Florian Westphal wrote:
> Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > On Sun, Nov 29, 2015 at 01:09:29AM +0100, Florian Westphal wrote:
> > > Thanks for looking at this.  I'll take a closer look tomorrow,
> > > your patch works fine for ip version/hdrlength but seems it messes
> > > with endianess somewhere.
> > 
> > I forgot to update payload_shift_value() too, to skip the shift when
> > not needed, sorry, new patch attached.
> 
> Almost there. Again, with Patricks patch to fix VLAN header:
> 
> # src/nft --debug=netlink add rule bridge raw prerouting ether type vlan vlan type ip vlan id 4094 ip version 4 counter 
> bridge raw prerouting

OK, new try, the idea behind is to calculate this shift through:

        x = offset % BITS_PER_BYTE
        y = len % BITS_PER_BYTE

to get both offset and length at byte level.

Then calculate the shift based on this:

        shift = BITS_PER_BYTE - (x + y)

Does this look good to you?

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diff --git a/src/netlink_linearize.c b/src/netlink_linearize.c
index 9840cd4..96f4dda 100644
--- a/src/netlink_linearize.c
+++ b/src/netlink_linearize.c
@@ -113,7 +113,8 @@ static void netlink_gen_payload_mask(struct netlink_linearize_ctx *ctx,
 	mpz_t mask;
 
 	offset = expr->payload.offset % BITS_PER_BYTE;
-	shift = (expr->len % BITS_PER_BYTE) - offset;
+	len = expr->len % BITS_PER_BYTE;
+	shift = BITS_PER_BYTE - (offset + len);
 	masklen = expr->len + shift;
 
 	if (masklen > 128)
@@ -291,7 +292,7 @@ static void netlink_gen_range(struct netlink_linearize_ctx *ctx,
 
 static void payload_shift_value(const struct expr *left, struct expr *right)
 {
-	unsigned int shift, offset;
+	unsigned int shift, offset, len;
 
 	if (right->ops->type != EXPR_VALUE ||
 	    left->ops->type != EXPR_PAYLOAD)
@@ -300,7 +301,8 @@ static void payload_shift_value(const struct expr *left, struct expr *right)
 	if (left->payload.offset % BITS_PER_BYTE ||
 	    (left->payload.offset + left->len) % BITS_PER_BYTE) {
 		offset = left->payload.offset % BITS_PER_BYTE;
-		shift = (left->len % BITS_PER_BYTE) - offset;
+		len = left->len % BITS_PER_BYTE;
+		shift = BITS_PER_BYTE - (offset + len);
 		mpz_lshift_ui(right->value, shift);
 	}
 }