[PATCH 08/12] netlink: cmp: shift rhs constant if lhs offset doesn't start on byte boundary

[Florian Westphal <fw@strlen.de>]

if we have payload(someoffset) == 42, then shift 42 in case someoffset
doesn't start on a byte boundary.

We already insert a mask instruction to only load those bits into
the register that we were interested in, but the cmp will fail without
also adjusting rhs accordingly.

Needs additional patch in reverse direction to undo the shift again
when dumping ruleset.

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 src/netlink_linearize.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/src/netlink_linearize.c b/src/netlink_linearize.c
index 0467872..044bac8 100644
--- a/src/netlink_linearize.c
+++ b/src/netlink_linearize.c
@@ -278,6 +278,15 @@ static void netlink_gen_range(struct netlink_linearize_ctx *ctx,
 			      const struct expr *expr,
 			      enum nft_registers dreg);
 
+static void payload_shift_value(const struct expr *left, struct expr *right)
+{
+	if (right->ops->type != EXPR_VALUE ||
+	    left->ops->type != EXPR_PAYLOAD)
+		return;
+
+	mpz_lshift_ui(right->value, left->payload.offset % BITS_PER_BYTE);
+}
+
 static void netlink_gen_cmp(struct netlink_linearize_ctx *ctx,
 			    const struct expr *expr,
 			    enum nft_registers dreg)
@@ -326,6 +335,7 @@ static void netlink_gen_cmp(struct netlink_linearize_ctx *ctx,
 	netlink_put_register(nle, NFT_EXPR_CMP_SREG, sreg);
 	nft_rule_expr_set_u32(nle, NFT_EXPR_CMP_OP,
 			      netlink_gen_cmp_op(expr->op));
+	payload_shift_value(expr->left, right);
 	netlink_gen_data(right, &nld);
 	nft_rule_expr_set(nle, NFT_EXPR_CMP_DATA, nld.value, nld.len);
 	release_register(ctx, expr->left);
-- 
2.0.5