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>

1 files changed, 5 insertions, 0 deletions

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;
 	}