nameserver/compression: avoid pointers on pointers

src/libknot/packet/rrset-wire.c

@@ -238,7 +238,7 @@ static int compr_put_dname(const knot_dname_t *dname, uint8_t *dst, uint16_t max
 		if (written + sizeof(uint16_t) > max) {
 			return KNOT_ESPACE;
 		}
-		knot_wire_put_pointer(dst + written, compr_ptr - compr->wire);
+		knot_wire_put_pointer_check(dst + written, compr_ptr - compr->wire, compr->wire);
 		written += sizeof(uint16_t);
 	}
 
@@ -280,14 +280,14 @@ static int write_owner(const knot_rrset_t *rrset, uint8_t **dst, size_t *dst_ava
 	// Write result.
 	if (owner_pointer > 0) {
 		WRITE_OWNER_CHECK(sizeof(uint16_t), dst_avail);
-		knot_wire_put_pointer(*dst, owner_pointer);
+		knot_wire_put_pointer_check(*dst, owner_pointer, compr->wire);
 		WRITE_OWNER_INCR(dst, dst_avail, sizeof(uint16_t));
 	// Check for coincidence with previous RR set.
 	} else if (compr != NULL && compr->suffix.pos != 0 && *rrset->owner != '\0' &&
 	           dname_equal_wire(rrset->owner, compr->wire + compr->suffix.pos,
 	                            compr->wire)) {
 		WRITE_OWNER_CHECK(sizeof(uint16_t), dst_avail);
-		knot_wire_put_pointer(*dst, compr->suffix.pos);
+		knot_wire_put_pointer_check(*dst, compr->suffix.pos, compr->wire);
 		compr_set_ptr(compr, KNOT_COMPR_HINT_OWNER,
 		              compr->wire + compr->suffix.pos,
 		              knot_dname_size(rrset->owner));

src/libknot/packet/wire.h

@@ -1017,6 +1017,17 @@ static inline void knot_wire_put_pointer(uint8_t *pos, uint16_t ptr)
 	pos[0] |= KNOT_WIRE_PTR;		// Add pointer mark.
 }
 
+/*! \brief Put pointer, but avoid pointing at other pointer. */
+static inline void knot_wire_put_pointer_check(uint8_t *pos, uint16_t ptr, const uint8_t *wire)
+{
+	if (knot_wire_is_pointer(wire + ptr)) {
+		// copy the referenced pointer instead
+		memcpy(pos, wire + ptr, sizeof(ptr));
+	} else {
+		knot_wire_put_pointer(pos, ptr);
+	}
+}
+
 static inline uint16_t knot_wire_get_pointer(const uint8_t *pos)
 {
 	assert(knot_wire_is_pointer(pos));			// Check pointer.