[PATCH nft 05/12] evaluate: handle payload matching split in two bytes

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

When the bits are split between two bytes and the payload field is
smaller than one byte, we need to extend the expression length on both
sides (payload and constant) of the relational expression.

The existing trimming from the delinerization step handles the listing
for us, so no changes on that front.

This patch allows us to match the IPv6 DSCP field which falls into the
case that is described above.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 src/evaluate.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/src/evaluate.c b/src/evaluate.c
index ab73261..fcd4ecd 100644
--- a/src/evaluate.c
+++ b/src/evaluate.c
@@ -362,7 +362,8 @@ conflict_resolution_gen_dependency(struct eval_ctx *ctx, int protocol,
 	return 0;
 }
 
-static uint8_t expr_offset_shift(const struct expr *expr, unsigned int offset)
+static uint8_t expr_offset_shift(const struct expr *expr, unsigned int offset,
+				 unsigned int *extra_len)
 {
 	unsigned int new_offset, len;
 	int shift;
@@ -370,34 +371,38 @@ static uint8_t expr_offset_shift(const struct expr *expr, unsigned int offset)
 	new_offset = offset % BITS_PER_BYTE;
 	len = round_up(expr->len, BITS_PER_BYTE);
 	shift = len - (new_offset + expr->len);
-	assert(shift >= 0);
-
+	while (shift < 0) {
+		shift += BITS_PER_BYTE;
+		*extra_len += BITS_PER_BYTE;
+	}
 	return shift;
 }
 
 static void expr_evaluate_bits(struct eval_ctx *ctx, struct expr **exprp)
 {
 	struct expr *expr = *exprp, *and, *mask, *lshift, *off;
-	unsigned masklen;
+	unsigned masklen, len = expr->len, extra_len = 0;
 	uint8_t shift;
 	mpz_t bitmask;
 
 	switch (expr->ops->type) {
 	case EXPR_PAYLOAD:
-		shift = expr_offset_shift(expr, expr->payload.offset);
+		shift = expr_offset_shift(expr, expr->payload.offset,
+					  &extra_len);
 		break;
 	case EXPR_EXTHDR:
-		shift = expr_offset_shift(expr, expr->exthdr.tmpl->offset);
+		shift = expr_offset_shift(expr, expr->exthdr.tmpl->offset,
+					  &extra_len);
 		break;
 	default:
 		BUG("Unknown expression %s\n", expr->ops->name);
 	}
 
-	masklen = expr->len + shift;
+	masklen = len + shift;
 	assert(masklen <= NFT_REG_SIZE * BITS_PER_BYTE);
 
 	mpz_init2(bitmask, masklen);
-	mpz_bitmask(bitmask, expr->len);
+	mpz_bitmask(bitmask, len);
 	mpz_lshift_ui(bitmask, shift);
 
 	mask = constant_expr_alloc(&expr->location, expr_basetype(expr),
@@ -423,6 +428,9 @@ static void expr_evaluate_bits(struct eval_ctx *ctx, struct expr **exprp)
 		*exprp = lshift;
 	} else
 		*exprp = and;
+
+	if (extra_len)
+		expr->len += extra_len;
 }
 
 static int __expr_evaluate_exthdr(struct eval_ctx *ctx, struct expr **exprp)
-- 
2.1.4