bpf: Replace bpf_lpm_trie_key 0-length array with flexible array

PATCH Superseded Kees Cook 2 participants 4 messages Feb 20, 2024 – Feb 21, 2024

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Networking Network drivers Documentation Kselftest +3 more subsystems

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Kees Cook

author

2024-02-20 18:54 UTC #0 lore raw

PATCH v4 1/1 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/

Acked-by Gustavo A. R. Silva <[email protected]>

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

---
 Documentation/bpf/map_lpm_trie.rst            |  2 +-
 include/uapi/linux/bpf.h                      | 15 +++++++++++++-
 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                | 15 +++++++++++++-
 .../selftests/bpf/progs/map_ptr_kern.c        |  2 +-
 tools/testing/selftests/bpf/test_lpm_map.c    | 18 ++++++++---------
 8 files changed, 51 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 +14 -1

index 754e68ca8744..31e9bdd4641e 100644
@@ -8,6 +8,7 @@
 #ifndef _UAPI__LINUX_BPF_H__
 #define _UAPI__LINUX_BPF_H__
 
+#include <linux/stddef.h>
 #include <linux/types.h>
 #include <linux/bpf_common.h>

@@ -77,12 +78,24 @@ 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 +14 -1

index 7f24d898efbb..76b1b41c0dce 100644
@@ -8,6 +8,7 @@
 #ifndef _UAPI__LINUX_BPF_H__
 #define _UAPI__LINUX_BPF_H__
 
+#include <linux/stddef.h>
 #include <linux/types.h>
 #include <linux/bpf_common.h>

@@ -77,12 +78,24 @@ 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 */

Signature · 1 line

2.34.1

Daniel Borkmann

maintainer

2024-02-21 16:40 UTC #1 lore raw

Quoted 58 lines · Kees Cook

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.

Quoted 5 lines

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/
Acked-by Gustavo A. R. Silva <[email protected]>
Signed-off-by Kees Cook <[email protected]>

[...]

The build in BPF CI is still broken, did you try to build selftests?

https://github.com/kernel-patches/bpf/actions/runs/7978647641

[...]
GEN-SKEL [test_progs] linked_funcs.skel.h
LINK-BPF [test_progs] test_usdt.bpf.o
GEN-SKEL [test_progs-no_alu32] profiler1.skel.h
GEN-SKEL [test_progs] test_usdt.skel.h
In file included from /tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:11,
from test_cpp.cpp:4:
/tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:92:17: error: ‘struct bpf_lpm_trie_key_u8::<unnamed union>::bpf_lpm_trie_key_hdr’ invalid; an anonymous union may only have public non-static data members [-fpermissive]

     92 |  __struct_group(bpf_lpm_trie_key_hdr, hdr, /* no attrs */,
        |                 ^~~~~~~~~~~~~~~~~~~~
  /tmp/work/bpf/bpf/tools/include/uapi/linux/stddef.h:29:10: note: in definition of macro ‘__struct_group’
     29 |   struct TAG { MEMBERS } ATTRS NAME; \
        |          ^~~
    BINARY   bench

make: *** [Makefile:703: /tmp/work/bpf/bpf/tools/testing/selftests/bpf/test_cpp] Error 1
make: *** Waiting for unfinished jobs....
make: Leaving directory '/tmp/work/bpf/bpf/tools/testing/selftests/bpf'
Error: Process completed with exit code 2.

Quoted 38 lines

include/uapi/linux/bpf.h +14 -1

index 754e68ca8744..31e9bdd4641e 100644
@@ -8,6 +8,7 @@
  #ifndef _UAPI__LINUX_BPF_H__
  #define _UAPI__LINUX_BPF_H__
 
+#include <linux/stddef.h>
  #include <linux/types.h>
  #include <linux/bpf_common.h>

@@ -77,12 +78,24 @@ 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) */

Kees Cook

author

2024-02-21 21:38 UTC #2 lore raw

Quoted 59 lines · Daniel Borkmann

Quoted 58 lines · Kees Cook
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.

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Quoted 5 lines
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/
Acked-by Gustavo A. R. Silva <[email protected]>
Signed-off-by Kees Cook <[email protected]>
[...]
The build in BPF CI is still broken, did you try to build selftests?

I did! I didn't have this error. :(

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https://github.com/kernel-patches/bpf/actions/runs/7978647641

Thanks for the pointer. It took a bit of digging, but I found this:
https://github.com/libbpf/ci/blob/main/build-selftests/build_selftests.sh
which is much more involved than just "make -C tools/testing/selftests/bpf"

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[...]
GEN-SKEL [test_progs] linked_funcs.skel.h
LINK-BPF [test_progs] test_usdt.bpf.o
GEN-SKEL [test_progs-no_alu32] profiler1.skel.h
GEN-SKEL [test_progs] test_usdt.skel.h
In file included from /tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:11,
from test_cpp.cpp:4:
/tmp/work/bpf/bpf/tools/include/uapi/linux/bpf.h:92:17: error: ‘struct bpf_lpm_trie_key_u8::<unnamed union>::bpf_lpm_trie_key_hdr’ invalid; an anonymous union may only have public non-static data members [-fpermissive]

Quoted 5 lines

     92 |  __struct_group(bpf_lpm_trie_key_hdr, hdr, /* no attrs */,
        |                 ^~~~~~~~~~~~~~~~~~~~
  /tmp/work/bpf/bpf/tools/include/uapi/linux/stddef.h:29:10: note: in definition of macro ‘__struct_group’
     29 |   struct TAG { MEMBERS } ATTRS NAME; \
        |          ^~~

I'll see if I can figure out where this is coming from. This sounds like something is being built with an unexpectedly strict option. (The above report seems weird -- this isn't coming from -fpermissive, and is actually an _error_ not a warning, which is the opposite of what -fpermissive is supposed to do.) Also the mention of "public" is scary here... that implies a C++ compiler is involved? Maybe that's why my builds didn't catch this?

Quoted 1 line

make: *** [Makefile:703: /tmp/work/bpf/bpf/tools/testing/selftests/bpf/test_cpp] Error 1

Ah yes, cpp. Fun. I will try to reproduce this failure.

Signature · 1 line

Kees Cook

Kees Cook

author

2024-02-21 22:01 UTC #3 lore raw

In reply to Daniel Borkmann

Quoted 1 line · Daniel Borkmann

The build in BPF CI is still broken, did you try to build selftests?

Okay, I give up. How is a mortal supposed to build these?

If I try to follow what I see in
https://github.com/libbpf/ci/blob/main/build-selftests/build_selftests.sh
I just get more and more kinds of errors:

In file included from progs/cb_refs.c:5:
progs/../bpf_testmod/bpf_testmod_kfunc.h:29:8: error: redefinition of 'prog_test_pass1'
29 | struct prog_test_pass1 {
| ^
/srv/code/tools/testing/selftests/bpf/tools/include/vmlinux.h:106850:8: note: previous definition is
here
106850 | struct prog_test_pass1 {
| ^

Messing around with deleting vmlinux.h seems to get me further, but later:

/srv/code/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c: In function 'bpf_testmod_ops_is_valid_access':
/srv/code/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c:535:16: error: implicit declaration of function 'bpf_tracing_btf_ctx_access' [-Werror=implicit-function-declaration]
535 | return bpf_tracing_btf_ctx_access(off, size, type, prog, info);

      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~

and then I'm stuck. It looks like the build isn't actually using KBUILD_OUTPUT for finding includes. If I try to add -I flags to the Makefile I just drown in new errors.

Signature · 1 line

Kees Cook