String matching: Fix long postfix string match

Postfix matching is done by reversing all the postfixes and storing them
in a LPM TRIE map, and then reversing the string to match on and looking
it up in the map. If the string to match is longer than the postfix max
length then the string would not be matched, leading to long strings not
being subject to the match logic.

This commit modifies the look up to handle strings longer than the
postfix max length. It fixes the length to copy to the postfix max
length and supplies an offset from where to start copying. The offset is
calculated as the difference between the original length and the capped
length.

Signed-off-by: Kevin Sheldrake <[email protected]>

bpf/process/types/basic.h

@@ -795,8 +795,11 @@ filter_char_buf_prefix(struct selector_arg_filter *filter, char *arg_str, uint a
 	return !!pass;
 }
 
+// Define a mask for the maximum path length on Linux.
+#define PATH_MASK (4096 - 1)
+
 static inline __attribute__((always_inline)) void
-copy_reverse(__u8 *dest, uint len, __u8 *src)
+copy_reverse(__u8 *dest, uint len, __u8 *src, uint offset)
 {
 	uint i;
 
@@ -815,8 +818,8 @@ copy_reverse(__u8 *dest, uint len, __u8 *src)
 	// Alternative (prettier) fixes resulted in a confused verifier
 	// unfortunately.
 	for (i = 0; i < (STRING_POSTFIX_MAX_MATCH_LENGTH - 1); i++) {
-		dest[i & STRING_POSTFIX_MAX_MASK] = src[(len - 1 - i) & STRING_POSTFIX_MAX_MASK];
-		if (len == (i + 1))
+		dest[i & STRING_POSTFIX_MAX_MASK] = src[(len + offset - 1 - i) & PATH_MASK];
+		if (len + offset == (i + 1))
 			return;
 	}
 }
@@ -827,21 +830,22 @@ filter_char_buf_postfix(struct selector_arg_filter *filter, char *arg_str, uint
 	void *addrmap;
 	__u32 map_idx = *(__u32 *)&filter->value;
 	struct string_postfix_lpm_trie *arg;
+	uint orig_len = arg_len;
 	int zero = 0;
 
 	addrmap = map_lookup_elem(&string_postfix_maps, &map_idx);
-	if (!addrmap)
+	if (!addrmap || !arg_len)
 		return 0;
 
-	if (arg_len > STRING_POSTFIX_MAX_LENGTH || !arg_len)
-		return 0;
+	if (arg_len >= STRING_POSTFIX_MAX_MATCH_LENGTH)
+		arg_len = STRING_POSTFIX_MAX_MATCH_LENGTH - 1;
 
 	arg = (struct string_postfix_lpm_trie *)map_lookup_elem(&string_postfix_maps_heap, &zero);
 	if (!arg)
 		return 0;
 
 	arg->prefixlen = arg_len * 8; // prefix is in bits
-	copy_reverse(arg->data, arg_len, (__u8 *)arg_str);
+	copy_reverse(arg->data, arg_len, (__u8 *)arg_str, orig_len - arg_len);
 
 	__u8 *pass = map_lookup_elem(addrmap, arg);