src: Support odd-sized payload matches

When expanding a payload match, don't disregard oversized templates at
the right offset. A more flexible user may extract less bytes from the
packet if only parts of a field are interesting, e.g. only the prefix of
source/destination address. Support that by using the template, but fix
the length. Later when creating a relational expression for it, detect
the unusually small payload expression length and turn the RHS value
into a prefix expression.

Signed-off-by: Phil Sutter <phil@nwl.cc>

2 files changed, 11 insertions, 0 deletions

diff --git a/src/netlink_delinearize.c b/src/netlink_delinearize.c
index 43d7ff82..b7876a8d 100644
--- a/src/netlink_delinearize.c
+++ b/src/netlink_delinearize.c
@@ -1776,6 +1776,12 @@ static void payload_match_expand(struct rule_pp_ctx *ctx,
 		tmp = constant_expr_splice(right, left->len);
 		expr_set_type(tmp, left->dtype, left->byteorder);
 
+		if (left->payload.tmpl && (left->len < left->payload.tmpl->len)) {
+			mpz_lshift_ui(tmp->value, left->payload.tmpl->len - left->len);
+			tmp->len = left->payload.tmpl->len;
+			tmp = prefix_expr_alloc(&tmp->location, tmp, left->len);
+		}
+
 		nexpr = relational_expr_alloc(&expr->location, expr->op,
 					      left, tmp);
 		if (expr->op == OP_EQ)
diff --git a/src/payload.c b/src/payload.c
index ca422d5b..e51c5797 100644
--- a/src/payload.c
+++ b/src/payload.c
@@ -822,6 +822,11 @@ void payload_expr_expand(struct list_head *list, struct expr *expr,
 			expr->payload.offset += tmpl->len;
 			if (expr->len == 0)
 				return;
+		} else if (expr->len > 0) {
+			new = payload_expr_alloc(&expr->location, desc, i);
+			new->len = expr->len;
+			list_add_tail(&new->list, list);
+			return;
 		} else
 			break;
 	}