bpf: Replace bpf_lpm_trie_key 0-length array with flexible array

Replace deprecated 0-length array in struct bpf_lpm_trie_key with flexible array. Found with GCC 13:

../kernel/bpf/lpm_trie.c:207:51: warning: array subscript i is outside array bounds of 'const __u8[0]' {aka 'const unsigned char[]'} [-Warray-bounds=]
207 | *(__be16 *)&key->data[i]);
| ^~~~~~~~~~~~~
../include/uapi/linux/swab.h:102:54: note: in definition of macro '__swab16'
102 | #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x))
| ^
../include/linux/byteorder/generic.h:97:21: note: in expansion of macro '__be16_to_cpu'

    97 | #define be16_to_cpu __be16_to_cpu
       |                     ^~~~~~~~~~~~~
../kernel/bpf/lpm_trie.c:206:28: note: in expansion of macro 'be16_to_cpu'
   206 |                 u16 diff = be16_to_cpu(*(__be16 *)&node->data[i]
^
       |                            ^~~~~~~~~~~
In file included from ../include/linux/bpf.h:7:
../include/uapi/linux/bpf.h:82:17: note: while referencing 'data'
    82 |         __u8    data[0];        /* Arbitrary size */
       |                 ^~~~

And found at run-time under CONFIG_FORTIFY_SOURCE:

   UBSAN: array-index-out-of-bounds in kernel/bpf/lpm_trie.c:218:49
   index 0 is out of range for type '__u8 [*]'

Changing struct bpf_lpm_trie_key is difficult since has been used by userspace. For example, in Cilium:

	struct egress_gw_policy_key {
	        struct bpf_lpm_trie_key lpm_key;
	        __u32 saddr;
	        __u32 daddr;
	};

While direct references to the "data" member haven't been found, there are static initializers what include the final member. For example, the "{}" here:

         struct egress_gw_policy_key in_key = {
                 .lpm_key = { 32 + 24, {} },
                 .saddr   = CLIENT_IP,
                 .daddr   = EXTERNAL_SVC_IP & 0Xffffff,
         };

To avoid the build time and run time warnings seen with a 0-sized trailing array for struct bpf_lpm_trie_key, introduce a new struct that correctly uses a flexible array for the trailing bytes, struct bpf_lpm_trie_key_u8. As part of this, include the "header" portion (which is just the "prefixlen" member), so it can be used by anything building a bpf_lpr_trie_key that has trailing members that aren't a u8 flexible array (like the self-test[1]), which is named struct bpf_lpm_trie_key_hdr.

Adjust the kernel code to use struct bpf_lpm_trie_key_u8 through-out, and for the selftest to use struct bpf_lpm_trie_key_hdr. Add a comment to the UAPI header directing folks to the two new options.

Link https://lore.kernel.org/all/202206281009.4332AA33@keescook/ [1] open here

Reported-by Mark Rutland <[email protected]>

Closes https://paste.debian.net/hidden/ca500597/

Signed-off-by Kees Cook <[email protected]>

---
v3- create a new pair of structs -- leave old struct alone
v2- https://lore.kernel.org/lkml/[email protected]/
v1- https://lore.kernel.org/lkml/[email protected]/
Cc: Daniel Borkmann <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Martin KaFai Lau <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Yonghong Song <[email protected]>
Cc: John Fastabend <[email protected]>
Cc: KP Singh <[email protected]>
Cc: Stanislav Fomichev <[email protected]>
Cc: Hao Luo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mykola Lysenko <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Haowen Bai <[email protected]>
Cc: [email protected]
Cc: [email protected]

---
  Documentation/bpf/map_lpm_trie.rst            |  2 +-
  include/uapi/linux/bpf.h                      | 14 ++++++++++++-
  kernel/bpf/lpm_trie.c                         | 20 +++++++++----------
  samples/bpf/map_perf_test_user.c              |  2 +-
  samples/bpf/xdp_router_ipv4_user.c            |  2 +-
  tools/include/uapi/linux/bpf.h                | 14 ++++++++++++-
  .../selftests/bpf/progs/map_ptr_kern.c        |  2 +-
  tools/testing/selftests/bpf/test_lpm_map.c    | 18 ++++++++---------
  8 files changed, 49 insertions(+), 25 deletions(-)

Documentation/bpf/map_lpm_trie.rst +1 -1

index 74d64a30f500..f9cd579496c9 100644
@@ -17,7 +17,7 @@ significant byte.
 
  LPM tries may be created with a maximum prefix length that is a multiple
  of 8, in the range from 8 to 2048. The key used for lookup and update
-operations is a ``struct bpf_lpm_trie_key``, extended by
+operations is a ``struct bpf_lpm_trie_key_u8``, extended by
  ``max_prefixlen/8`` bytes.
 
  - For IPv4 addresses the data length is 4 bytes

include/uapi/linux/bpf.h +13 -1

index 754e68ca8744..c5a46d12076e 100644
@@ -77,12 +77,24 @@ struct bpf_insn {
  	__s32	imm;		/* signed immediate constant */
  };
 
-/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
+/* Deprecated: use struct bpf_lpm_trie_key_u8 (when the "data" member is needed for
+ * byte access) or struct bpf_lpm_trie_key_hdr (when using an alternative type for
+ * the trailing flexible array member) instead.
+ */
  struct bpf_lpm_trie_key {
  	__u32	prefixlen;	/* up to 32 for AF_INET, 128 for AF_INET6 */
  	__u8	data[0];	/* Arbitrary size */
  };
 
+/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry, with trailing byte array. */
+struct bpf_lpm_trie_key_u8 {
+	__struct_group(bpf_lpm_trie_key_hdr, hdr, /* no attrs */,
+		/* up to 32 for AF_INET, 128 for AF_INET6 */
+		__u32	prefixlen;
+	);
+	__u8	data[];		/* Arbitrary size */
+};
+
  struct bpf_cgroup_storage_key {
  	__u64	cgroup_inode_id;	/* cgroup inode id */
  	__u32	attach_type;		/* program attach type (enum bpf_attach_type) */

kernel/bpf/lpm_trie.c +10 -10

index b32be680da6c..050fe1ebf0f7 100644
@@ -164,13 +164,13 @@ static inline int extract_bit(const u8 *data, size_t index)
   */
  static size_t longest_prefix_match(const struct lpm_trie *trie,
  				   const struct lpm_trie_node *node,
-				   const struct bpf_lpm_trie_key *key)
+				   const struct bpf_lpm_trie_key_u8 *key)
  {
  	u32 limit = min(node->prefixlen, key->prefixlen);
  	u32 prefixlen = 0, i = 0;
 
  	BUILD_BUG_ON(offsetof(struct lpm_trie_node, data) % sizeof(u32));
-	BUILD_BUG_ON(offsetof(struct bpf_lpm_trie_key, data) % sizeof(u32));
+	BUILD_BUG_ON(offsetof(struct bpf_lpm_trie_key_u8, data) % sizeof(u32));
 
  #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && defined(CONFIG_64BIT)
 

@@ -229,7 +229,7 @@ static void *trie_lookup_elem(struct bpf_map *map, void *_key)
  {
  	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
  	struct lpm_trie_node *node, *found = NULL;
-	struct bpf_lpm_trie_key *key = _key;
+	struct bpf_lpm_trie_key_u8 *key = _key;
 
  	if (key->prefixlen > trie->max_prefixlen)
  		return NULL;

@@ -309,7 +309,7 @@ static long trie_update_elem(struct bpf_map *map,
  	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
  	struct lpm_trie_node *node, *im_node = NULL, *new_node = NULL;
  	struct lpm_trie_node __rcu **slot;
-	struct bpf_lpm_trie_key *key = _key;
+	struct bpf_lpm_trie_key_u8 *key = _key;
  	unsigned long irq_flags;
  	unsigned int next_bit;
  	size_t matchlen = 0;

@@ -437,7 +437,7 @@ static long trie_update_elem(struct bpf_map *map,
  static long trie_delete_elem(struct bpf_map *map, void *_key)
  {
  	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
-	struct bpf_lpm_trie_key *key = _key;
+	struct bpf_lpm_trie_key_u8 *key = _key;
  	struct lpm_trie_node __rcu **trim, **trim2;
  	struct lpm_trie_node *node, *parent;
  	unsigned long irq_flags;

@@ -536,7 +536,7 @@ static long trie_delete_elem(struct bpf_map *map, void *_key)
  				 sizeof(struct lpm_trie_node))
  #define LPM_VAL_SIZE_MIN	1
 
-#define LPM_KEY_SIZE(X)		(sizeof(struct bpf_lpm_trie_key) + (X))
+#define LPM_KEY_SIZE(X)		(sizeof(struct bpf_lpm_trie_key_u8) + (X))
  #define LPM_KEY_SIZE_MAX	LPM_KEY_SIZE(LPM_DATA_SIZE_MAX)
  #define LPM_KEY_SIZE_MIN	LPM_KEY_SIZE(LPM_DATA_SIZE_MIN)
 

@@ -565,7 +565,7 @@ static struct bpf_map *trie_alloc(union bpf_attr *attr)
  	/* copy mandatory map attributes */
  	bpf_map_init_from_attr(&trie->map, attr);
  	trie->data_size = attr->key_size -
-			  offsetof(struct bpf_lpm_trie_key, data);
+			  offsetof(struct bpf_lpm_trie_key_u8, data);
  	trie->max_prefixlen = trie->data_size * 8;
 
  	spin_lock_init(&trie->lock);

@@ -616,7 +616,7 @@ static int trie_get_next_key(struct bpf_map *map, void *_key, void *_next_key)
  {
  	struct lpm_trie_node *node, *next_node = NULL, *parent, *search_root;
  	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
-	struct bpf_lpm_trie_key *key = _key, *next_key = _next_key;
+	struct bpf_lpm_trie_key_u8 *key = _key, *next_key = _next_key;
  	struct lpm_trie_node **node_stack = NULL;
  	int err = 0, stack_ptr = -1;
  	unsigned int next_bit;

@@ -703,7 +703,7 @@ static int trie_get_next_key(struct bpf_map *map, void *_key, void *_next_key)
  	}
  do_copy:
  	next_key->prefixlen = next_node->prefixlen;
-	memcpy((void *)next_key + offsetof(struct bpf_lpm_trie_key, data),
+	memcpy((void *)next_key + offsetof(struct bpf_lpm_trie_key_u8, data),
  	       next_node->data, trie->data_size);
  free_stack:
  	kfree(node_stack);

@@ -715,7 +715,7 @@ static int trie_check_btf(const struct bpf_map *map,
  			  const struct btf_type *key_type,
  			  const struct btf_type *value_type)
  {
-	/* Keys must have struct bpf_lpm_trie_key embedded. */
+	/* Keys must have struct bpf_lpm_trie_key_u8 embedded. */
  	return BTF_INFO_KIND(key_type->info) != BTF_KIND_STRUCT ?
  	       -EINVAL : 0;
  }

samples/bpf/map_perf_test_user.c +1 -1

index d2fbcf963cdf..07ff471ed6ae 100644
@@ -370,7 +370,7 @@ static void run_perf_test(int tasks)
 
  static void fill_lpm_trie(void)
  {
-	struct bpf_lpm_trie_key *key;
+	struct bpf_lpm_trie_key_u8 *key;
  	unsigned long value = 0;
  	unsigned int i;
  	int r;

samples/bpf/xdp_router_ipv4_user.c +1 -1

index 9d41db09c480..266fdd0b025d 100644
@@ -91,7 +91,7 @@ static int recv_msg(struct sockaddr_nl sock_addr, int sock)
  static void read_route(struct nlmsghdr *nh, int nll)
  {
  	char dsts[24], gws[24], ifs[16], dsts_len[24], metrics[24];
-	struct bpf_lpm_trie_key *prefix_key;
+	struct bpf_lpm_trie_key_u8 *prefix_key;
  	struct rtattr *rt_attr;
  	struct rtmsg *rt_msg;
  	int rtm_family;

tools/include/uapi/linux/bpf.h +13 -1

index 7f24d898efbb..c55244bf1a20 100644
@@ -77,12 +77,24 @@ struct bpf_insn {
  	__s32	imm;		/* signed immediate constant */
  };
 
-/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
+/* Deprecated: use struct bpf_lpm_trie_key_u8 (when the "data" member is needed for
+ * byte access) or struct bpf_lpm_trie_key_hdr (when using an alternative type for
+ * the trailing flexible array member) instead.
+ */
  struct bpf_lpm_trie_key {
  	__u32	prefixlen;	/* up to 32 for AF_INET, 128 for AF_INET6 */
  	__u8	data[0];	/* Arbitrary size */
  };
 
+/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry, with trailing byte array. */
+struct bpf_lpm_trie_key_u8 {
+	__struct_group(bpf_lpm_trie_key_hdr, hdr, /* no attrs */,
+		/* up to 32 for AF_INET, 128 for AF_INET6 */
+		__u32	prefixlen;
+	);
+	__u8	data[];		/* Arbitrary size */
+};
+
  struct bpf_cgroup_storage_key {
  	__u64	cgroup_inode_id;	/* cgroup inode id */
  	__u32	attach_type;		/* program attach type (enum bpf_attach_type) */

tools/testing/selftests/bpf/progs/map_ptr_kern.c +1 -1

index 3325da17ec81..efaf622c28dd 100644
@@ -316,7 +316,7 @@ struct lpm_trie {
  } __attribute__((preserve_access_index));
 
  struct lpm_key {
-	struct bpf_lpm_trie_key trie_key;
+	struct bpf_lpm_trie_key_hdr trie_key;
  	__u32 data;
  };
 

tools/testing/selftests/bpf/test_lpm_map.c +9 -9

index c028d621c744..d98c72dc563e 100644
@@ -211,7 +211,7 @@ static void test_lpm_map(int keysize)
  	volatile size_t n_matches, n_matches_after_delete;
  	size_t i, j, n_nodes, n_lookups;
  	struct tlpm_node *t, *list = NULL;
-	struct bpf_lpm_trie_key *key;
+	struct bpf_lpm_trie_key_u8 *key;
  	uint8_t *data, *value;
  	int r, map;
 

@@ -331,8 +331,8 @@ static void test_lpm_map(int keysize)
  static void test_lpm_ipaddr(void)
  {
  	LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_NO_PREALLOC);
-	struct bpf_lpm_trie_key *key_ipv4;
-	struct bpf_lpm_trie_key *key_ipv6;
+	struct bpf_lpm_trie_key_u8 *key_ipv4;
+	struct bpf_lpm_trie_key_u8 *key_ipv6;
  	size_t key_size_ipv4;
  	size_t key_size_ipv6;
  	int map_fd_ipv4;

@@ -423,7 +423,7 @@ static void test_lpm_ipaddr(void)
  static void test_lpm_delete(void)
  {
  	LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_NO_PREALLOC);
-	struct bpf_lpm_trie_key *key;
+	struct bpf_lpm_trie_key_u8 *key;
  	size_t key_size;
  	int map_fd;
  	__u64 value;

@@ -532,7 +532,7 @@ static void test_lpm_delete(void)
  static void test_lpm_get_next_key(void)
  {
  	LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_NO_PREALLOC);
-	struct bpf_lpm_trie_key *key_p, *next_key_p;
+	struct bpf_lpm_trie_key_u8 *key_p, *next_key_p;
  	size_t key_size;
  	__u32 value = 0;
  	int map_fd;

@@ -693,9 +693,9 @@ static void *lpm_test_command(void *arg)
  {
  	int i, j, ret, iter, key_size;
  	struct lpm_mt_test_info *info = arg;
-	struct bpf_lpm_trie_key *key_p;
+	struct bpf_lpm_trie_key_u8 *key_p;
 
-	key_size = sizeof(struct bpf_lpm_trie_key) + sizeof(__u32);
+	key_size = sizeof(*key_p) + sizeof(__u32);
  	key_p = alloca(key_size);
  	for (iter = 0; iter < info->iter; iter++)
  		for (i = 0; i < MAX_TEST_KEYS; i++) {

@@ -717,7 +717,7 @@ static void *lpm_test_command(void *arg)
  				ret = bpf_map_lookup_elem(info->map_fd, key_p, &value);
  				assert(ret == 0 || errno == ENOENT);
  			} else {
-				struct bpf_lpm_trie_key *next_key_p = alloca(key_size);
+				struct bpf_lpm_trie_key_u8 *next_key_p = alloca(key_size);
  				ret = bpf_map_get_next_key(info->map_fd, key_p, next_key_p);
  				assert(ret == 0 || errno == ENOENT || errno == ENOMEM);
  			}

@@ -752,7 +752,7 @@ static void test_lpm_multi_thread(void)
 
  	/* create a trie */
  	value_size = sizeof(__u32);
-	key_size = sizeof(struct bpf_lpm_trie_key) + value_size;
+	key_size = sizeof(struct bpf_lpm_trie_key_hdr) + value_size;
  	map_fd = bpf_map_create(BPF_MAP_TYPE_LPM_TRIE, NULL, key_size, value_size, 100, &opts);
 
  	/* create 4 threads to test update, delete, lookup and get_next_key */

Replace deprecated 0-length array in struct bpf_lpm_trie_key with flexible array. Found with GCC 13:

../kernel/bpf/lpm_trie.c:207:51: warning: array subscript i is outside array bounds of 'const __u8[0]' {aka 'const unsigned char[]'} [-Warray-bounds=]
207 | *(__be16 *)&key->data[i]);
| ^~~~~~~~~~~~~
../include/uapi/linux/swab.h:102:54: note: in definition of macro '__swab16'
102 | #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x))
| ^
../include/linux/byteorder/generic.h:97:21: note: in expansion of macro '__be16_to_cpu'

    97 | #define be16_to_cpu __be16_to_cpu
       |                     ^~~~~~~~~~~~~
../kernel/bpf/lpm_trie.c:206:28: note: in expansion of macro 'be16_to_cpu'
   206 |                 u16 diff = be16_to_cpu(*(__be16 *)&node->data[i]
^
       |                            ^~~~~~~~~~~
In file included from ../include/linux/bpf.h:7:
../include/uapi/linux/bpf.h:82:17: note: while referencing 'data'
    82 |         __u8    data[0];        /* Arbitrary size */
       |                 ^~~~

And found at run-time under CONFIG_FORTIFY_SOURCE:

   UBSAN: array-index-out-of-bounds in kernel/bpf/lpm_trie.c:218:49
   index 0 is out of range for type '__u8 [*]'

Changing struct bpf_lpm_trie_key is difficult since has been used by userspace. For example, in Cilium:

	struct egress_gw_policy_key {
	        struct bpf_lpm_trie_key lpm_key;
	        __u32 saddr;
	        __u32 daddr;
	};

While direct references to the "data" member haven't been found, there are static initializers what include the final member. For example, the "{}" here:

         struct egress_gw_policy_key in_key = {
                 .lpm_key = { 32 + 24, {} },
                 .saddr   = CLIENT_IP,
                 .daddr   = EXTERNAL_SVC_IP & 0Xffffff,
         };

To avoid the build time and run time warnings seen with a 0-sized trailing array for struct bpf_lpm_trie_key, introduce a new struct that correctly uses a flexible array for the trailing bytes, struct bpf_lpm_trie_key_u8. As part of this, include the "header" portion (which is just the "prefixlen" member), so it can be used by anything building a bpf_lpr_trie_key that has trailing members that aren't a u8 flexible array (like the self-test[1]), which is named struct bpf_lpm_trie_key_hdr.

Adjust the kernel code to use struct bpf_lpm_trie_key_u8 through-out, and for the selftest to use struct bpf_lpm_trie_key_hdr. Add a comment to the UAPI header directing folks to the two new options.

Link https://lore.kernel.org/all/202206281009.4332AA33@keescook/ [1] open here

Reported-by Mark Rutland <[email protected]>

Closes https://paste.debian.net/hidden/ca500597/

Signed-off-by Kees Cook <[email protected]>

---
v3- create a new pair of structs -- leave old struct alone
v2- https://lore.kernel.org/lkml/[email protected]/
v1- https://lore.kernel.org/lkml/[email protected]/
Cc: Daniel Borkmann <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Martin KaFai Lau <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Yonghong Song <[email protected]>
Cc: John Fastabend <[email protected]>
Cc: KP Singh <[email protected]>
Cc: Stanislav Fomichev <[email protected]>
Cc: Hao Luo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mykola Lysenko <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Haowen Bai <[email protected]>
Cc: [email protected]
Cc: [email protected]

---
  Documentation/bpf/map_lpm_trie.rst            |  2 +-
  include/uapi/linux/bpf.h                      | 14 ++++++++++++-
  kernel/bpf/lpm_trie.c                         | 20 +++++++++----------
  samples/bpf/map_perf_test_user.c              |  2 +-
  samples/bpf/xdp_router_ipv4_user.c            |  2 +-
  tools/include/uapi/linux/bpf.h                | 14 ++++++++++++-
  .../selftests/bpf/progs/map_ptr_kern.c        |  2 +-
  tools/testing/selftests/bpf/test_lpm_map.c    | 18 ++++++++---------
  8 files changed, 49 insertions(+), 25 deletions(-)

Documentation/bpf/map_lpm_trie.rst +1 -1

index 74d64a30f500..f9cd579496c9 100644
@@ -17,7 +17,7 @@ significant byte.
 
  LPM tries may be created with a maximum prefix length that is a multiple
  of 8, in the range from 8 to 2048. The key used for lookup and update
-operations is a ``struct bpf_lpm_trie_key``, extended by
+operations is a ``struct bpf_lpm_trie_key_u8``, extended by
  ``max_prefixlen/8`` bytes.
 
  - For IPv4 addresses the data length is 4 bytes

include/uapi/linux/bpf.h +13 -1

index 754e68ca8744..c5a46d12076e 100644
@@ -77,12 +77,24 @@ struct bpf_insn {
  	__s32	imm;		/* signed immediate constant */
  };
 
-/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
+/* Deprecated: use struct bpf_lpm_trie_key_u8 (when the "data" member is needed for
+ * byte access) or struct bpf_lpm_trie_key_hdr (when using an alternative type for
+ * the trailing flexible array member) instead.
+ */
  struct bpf_lpm_trie_key {
  	__u32	prefixlen;	/* up to 32 for AF_INET, 128 for AF_INET6 */
  	__u8	data[0];	/* Arbitrary size */
  };
 
+/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry, with trailing byte array. */
+struct bpf_lpm_trie_key_u8 {
+	__struct_group(bpf_lpm_trie_key_hdr, hdr, /* no attrs */,
+		/* up to 32 for AF_INET, 128 for AF_INET6 */
+		__u32	prefixlen;
+	);
+	__u8	data[];		/* Arbitrary size */
+};
+
  struct bpf_cgroup_storage_key {
  	__u64	cgroup_inode_id;	/* cgroup inode id */
  	__u32	attach_type;		/* program attach type (enum bpf_attach_type) */

kernel/bpf/lpm_trie.c +10 -10

index b32be680da6c..050fe1ebf0f7 100644
@@ -164,13 +164,13 @@ static inline int extract_bit(const u8 *data, size_t index)
   */
  static size_t longest_prefix_match(const struct lpm_trie *trie,
  				   const struct lpm_trie_node *node,
-				   const struct bpf_lpm_trie_key *key)
+				   const struct bpf_lpm_trie_key_u8 *key)
  {
  	u32 limit = min(node->prefixlen, key->prefixlen);
  	u32 prefixlen = 0, i = 0;
 
  	BUILD_BUG_ON(offsetof(struct lpm_trie_node, data) % sizeof(u32));
-	BUILD_BUG_ON(offsetof(struct bpf_lpm_trie_key, data) % sizeof(u32));
+	BUILD_BUG_ON(offsetof(struct bpf_lpm_trie_key_u8, data) % sizeof(u32));
 
  #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && defined(CONFIG_64BIT)
 

@@ -229,7 +229,7 @@ static void *trie_lookup_elem(struct bpf_map *map, void *_key)
  {
  	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
  	struct lpm_trie_node *node, *found = NULL;
-	struct bpf_lpm_trie_key *key = _key;
+	struct bpf_lpm_trie_key_u8 *key = _key;
 
  	if (key->prefixlen > trie->max_prefixlen)
  		return NULL;

@@ -309,7 +309,7 @@ static long trie_update_elem(struct bpf_map *map,
  	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
  	struct lpm_trie_node *node, *im_node = NULL, *new_node = NULL;
  	struct lpm_trie_node __rcu **slot;
-	struct bpf_lpm_trie_key *key = _key;
+	struct bpf_lpm_trie_key_u8 *key = _key;
  	unsigned long irq_flags;
  	unsigned int next_bit;
  	size_t matchlen = 0;

@@ -437,7 +437,7 @@ static long trie_update_elem(struct bpf_map *map,
  static long trie_delete_elem(struct bpf_map *map, void *_key)
  {
  	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
-	struct bpf_lpm_trie_key *key = _key;
+	struct bpf_lpm_trie_key_u8 *key = _key;
  	struct lpm_trie_node __rcu **trim, **trim2;
  	struct lpm_trie_node *node, *parent;
  	unsigned long irq_flags;

@@ -536,7 +536,7 @@ static long trie_delete_elem(struct bpf_map *map, void *_key)
  				 sizeof(struct lpm_trie_node))
  #define LPM_VAL_SIZE_MIN	1
 
-#define LPM_KEY_SIZE(X)		(sizeof(struct bpf_lpm_trie_key) + (X))
+#define LPM_KEY_SIZE(X)		(sizeof(struct bpf_lpm_trie_key_u8) + (X))
  #define LPM_KEY_SIZE_MAX	LPM_KEY_SIZE(LPM_DATA_SIZE_MAX)
  #define LPM_KEY_SIZE_MIN	LPM_KEY_SIZE(LPM_DATA_SIZE_MIN)
 

@@ -565,7 +565,7 @@ static struct bpf_map *trie_alloc(union bpf_attr *attr)
  	/* copy mandatory map attributes */
  	bpf_map_init_from_attr(&trie->map, attr);
  	trie->data_size = attr->key_size -
-			  offsetof(struct bpf_lpm_trie_key, data);
+			  offsetof(struct bpf_lpm_trie_key_u8, data);
  	trie->max_prefixlen = trie->data_size * 8;
 
  	spin_lock_init(&trie->lock);

@@ -616,7 +616,7 @@ static int trie_get_next_key(struct bpf_map *map, void *_key, void *_next_key)
  {
  	struct lpm_trie_node *node, *next_node = NULL, *parent, *search_root;
  	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
-	struct bpf_lpm_trie_key *key = _key, *next_key = _next_key;
+	struct bpf_lpm_trie_key_u8 *key = _key, *next_key = _next_key;
  	struct lpm_trie_node **node_stack = NULL;
  	int err = 0, stack_ptr = -1;
  	unsigned int next_bit;

@@ -703,7 +703,7 @@ static int trie_get_next_key(struct bpf_map *map, void *_key, void *_next_key)
  	}
  do_copy:
  	next_key->prefixlen = next_node->prefixlen;
-	memcpy((void *)next_key + offsetof(struct bpf_lpm_trie_key, data),
+	memcpy((void *)next_key + offsetof(struct bpf_lpm_trie_key_u8, data),
  	       next_node->data, trie->data_size);
  free_stack:
  	kfree(node_stack);

@@ -715,7 +715,7 @@ static int trie_check_btf(const struct bpf_map *map,
  			  const struct btf_type *key_type,
  			  const struct btf_type *value_type)
  {
-	/* Keys must have struct bpf_lpm_trie_key embedded. */
+	/* Keys must have struct bpf_lpm_trie_key_u8 embedded. */
  	return BTF_INFO_KIND(key_type->info) != BTF_KIND_STRUCT ?
  	       -EINVAL : 0;
  }

samples/bpf/map_perf_test_user.c +1 -1

index d2fbcf963cdf..07ff471ed6ae 100644
@@ -370,7 +370,7 @@ static void run_perf_test(int tasks)
 
  static void fill_lpm_trie(void)
  {
-	struct bpf_lpm_trie_key *key;
+	struct bpf_lpm_trie_key_u8 *key;
  	unsigned long value = 0;
  	unsigned int i;
  	int r;

samples/bpf/xdp_router_ipv4_user.c +1 -1

index 9d41db09c480..266fdd0b025d 100644
@@ -91,7 +91,7 @@ static int recv_msg(struct sockaddr_nl sock_addr, int sock)
  static void read_route(struct nlmsghdr *nh, int nll)
  {
  	char dsts[24], gws[24], ifs[16], dsts_len[24], metrics[24];
-	struct bpf_lpm_trie_key *prefix_key;
+	struct bpf_lpm_trie_key_u8 *prefix_key;
  	struct rtattr *rt_attr;
  	struct rtmsg *rt_msg;
  	int rtm_family;

tools/include/uapi/linux/bpf.h +6 -1

index 7f24d898efbb..c55244bf1a20 100644
@@ -77,12 +77,24 @@ struct bpf_insn {
  	__s32	imm;		/* signed immediate constant */
  };
 
-/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
+/* Deprecated: use struct bpf_lpm_trie_key_u8 (when the "data" member is needed for
+ * byte access) or struct bpf_lpm_trie_key_hdr (when using an alternative type for
+ * the trailing flexible array member) instead.
+ */
  struct bpf_lpm_trie_key {
  	__u32	prefixlen;	/* up to 32 for AF_INET, 128 for AF_INET6 */
  	__u8	data[0];	/* Arbitrary size */
  };
 
+/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry, with trailing byte array. */
+struct bpf_lpm_trie_key_u8 {

Quoted 2 lines · Kees Cook

Replace deprecated 0-length array in struct bpf_lpm_trie_key with flexible array. Found with GCC 13:

[...]
This fails the BPF CI :

[...]
INSTALL /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/include/bpf/skel_internal.h INSTALL /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/include/bpf/libbpf_version.h INSTALL /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/include/bpf/usdt.bpf.h
In file included from urandom_read_lib1.c:7:
In file included from /tmp/work/bpf/bpf/tools/lib/bpf/libbpf_internal.h:20:
In file included from /tmp/work/bpf/bpf/tools/lib/bpf/relo_core.h:7:
/tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:91:2: error: type name requires a specifier or qualifier

     91 |         __struct_group(bpf_lpm_trie_key_hdr, hdr, /* no attrs */,
        |         ^
  /tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:91:58: error: expected identifier
     91 |         __struct_group(bpf_lpm_trie_key_hdr, hdr, /* no attrs */,
        |                                                                 ^
  /tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:93:18: error: unexpected ';' before ')'
     93 |                 __u32   prefixlen;
        |                                  ^
  /tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:95:7: error: flexible array member 'data' not allowed in otherwise empty struct
     95 |         __u8    data[];         /* Arbitrary size */
        |                 ^