Re: [PATCH v3] bpf: Replace bpf_lpm_trie_key 0-length array with flexible array

["Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx>]

On 2/19/24 17:41, Kees Cook wrote:
> 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.

Yep, this `_hdr` tagged struct adds flexibility (no pun intended :p)
to flexible structures currently nested in the middle of other
structs. We can just use the header members grouped together in the
`_hdr` struct, and avoid embedding the flexible-array member. :)

> 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]
> Reported-by: Mark Rutland <mark.rutland@xxxxxxx>
> Closes: https://paste.debian.net/hidden/ca500597/
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Acked-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>

Thanks!
--
Gustavo

> ---
> v3- create a new pair of structs -- leave old struct alone
> v2- https://lore.kernel.org/lkml/20240216235536.it.234-kees@xxxxxxxxxx/
> v1- https://lore.kernel.org/lkml/20230204183241.never.481-kees@xxxxxxxxxx/
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Cc: Martin KaFai Lau <martin.lau@xxxxxxxxx>
> Cc: Song Liu <song@xxxxxxxxxx>
> Cc: Yonghong Song <yhs@xxxxxx>
> Cc: John Fastabend <john.fastabend@xxxxxxxxx>
> Cc: KP Singh <kpsingh@xxxxxxxxxx>
> Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
> Cc: Hao Luo <haoluo@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Mykola Lysenko <mykolal@xxxxxx>
> Cc: Shuah Khan <shuah@xxxxxxxxxx>
> Cc: Haowen Bai <baihaowen@xxxxxxxxx>
> Cc: bpf@xxxxxxxxxxxxxxx
> Cc: linux-kselftest@xxxxxxxxxxxxxxx
> ---
>   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(-)
>
> diff --git a/Documentation/bpf/map_lpm_trie.rst b/Documentation/bpf/map_lpm_trie.rst
> index 74d64a30f500..f9cd579496c9 100644
> --- a/Documentation/bpf/map_lpm_trie.rst
> +++ b/Documentation/bpf/map_lpm_trie.rst
> @@ -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
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 754e68ca8744..c5a46d12076e 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -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) */
> diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
> index b32be680da6c..050fe1ebf0f7 100644
> --- a/kernel/bpf/lpm_trie.c
> +++ b/kernel/bpf/lpm_trie.c
> @@ -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;
>   }
> diff --git a/samples/bpf/map_perf_test_user.c b/samples/bpf/map_perf_test_user.c
> index d2fbcf963cdf..07ff471ed6ae 100644
> --- a/samples/bpf/map_perf_test_user.c
> +++ b/samples/bpf/map_perf_test_user.c
> @@ -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;
> diff --git a/samples/bpf/xdp_router_ipv4_user.c b/samples/bpf/xdp_router_ipv4_user.c
> index 9d41db09c480..266fdd0b025d 100644
> --- a/samples/bpf/xdp_router_ipv4_user.c
> +++ b/samples/bpf/xdp_router_ipv4_user.c
> @@ -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;
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 7f24d898efbb..c55244bf1a20 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -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) */
> diff --git a/tools/testing/selftests/bpf/progs/map_ptr_kern.c b/tools/testing/selftests/bpf/progs/map_ptr_kern.c
> index 3325da17ec81..efaf622c28dd 100644
> --- a/tools/testing/selftests/bpf/progs/map_ptr_kern.c
> +++ b/tools/testing/selftests/bpf/progs/map_ptr_kern.c
> @@ -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;
>   };
>   
> diff --git a/tools/testing/selftests/bpf/test_lpm_map.c b/tools/testing/selftests/bpf/test_lpm_map.c
> index c028d621c744..d98c72dc563e 100644
> --- a/tools/testing/selftests/bpf/test_lpm_map.c
> +++ b/tools/testing/selftests/bpf/test_lpm_map.c
> @@ -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 */