On Thu, Feb 9, 2023 at 11:23 AM Kees Cook keescook@chromium.org wrote:
The kernel is globally removing the ambiguous 0-length and 1-element arrays in favor of flexible arrays, so that we can gain both compile-time and run-time array bounds checking[1]. Most cases of these changes are trivial, but this case in BPF is not. It faces some difficulties:
struct bpf_lpm_trie_key is part of UAPI so changes can be fragile in the sense that projects external to Linux may be impacted.
The struct is intended to be used as a header, which means it may be within another structure, resulting in the "data" array member overlapping with the surrounding structure's following members. When converting from [0]-style to []-style, this overlap elicits warnings under Clang, and GCC considers it a deprecated extension (and similarly warns under -pedantic): https://godbolt.org/z/vWzqs41h6
Both the kernel and userspace access the existing "data" member for more than just static initializers and offsetof() calculations. For example:
cilium: struct egress_gw_policy_key in_key = { .lpm_key = { 32 + 24, {} }, .saddr = CLIENT_IP, .daddr = EXTERNAL_SVC_IP & 0Xffffff, };
systemd: ipv6_map_fd = bpf_map_new( BPF_MAP_TYPE_LPM_TRIE, offsetof(struct bpf_lpm_trie_key, data) + sizeof(uint32_t)*4, sizeof(uint64_t), ...); ... struct bpf_lpm_trie_key *key_ipv4, *key_ipv6; ... memcpy(key_ipv4->data, &a->address, sizeof(uint32_t));
Searching for other uses in Debian Code Search seem to be just copies of UAPI headers: https://codesearch.debian.net/search?q=struct+bpf_lpm_trie_key&literal=1...
Introduce struct bpf_lpm_trie_key_u8 for the kernel (and future userspace) to use for walking the individual bytes following the header, and leave the "data" member of struct bpf_lpm_trie_key as-is (i.e. a [0]-style array). This will allow existing userspace code to continue to use "data" as a fake flexible array. The kernel (and future userspace code) building with -fstrict-flex-arrays=3 will see struct bpf_lpm_trie_key::data has having 0 bytes so there will be no overlap warnings, and can instead use struct bpf_lpm_trie_key_u8::data for accessing the actual byte array contents. The definition of struct bpf_lpm_trie_key_u8 uses a union with struct bpf_lpm_trie_key so that things like container_of() can be used instead of doing explicit casting, all while leaving the member names un-namespaced (i.e. key->prefixlen == key_u8->prefixlen, key->data == key_u8->data), allowing for trivial drop-in replacement without collateral member renaming.
This will avoid structure overlap warnings and array bounds warnings while enabling run-time array bounds checking under CONFIG_UBSAN_BOUNDS=y and -fstrict-flex-arrays=3.
For reference, the current warning under GCC 13 with -fstrict-flex-arrays=3 and -Warray-bounds is:
../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 */ | ^~~~
Additionally update the samples and selftests to use the new structure, for demonstrating best practices.
[1] For lots of details, see both: https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-... https://people.kernel.org/kees/bounded-flexible-arrays-in-c
Cc: Alexei Starovoitov ast@kernel.org Cc: Stanislav Fomichev sdf@google.com Cc: Daniel Borkmann daniel@iogearbox.net Cc: Andrii Nakryiko andrii@kernel.org Cc: Martin KaFai Lau martin.lau@linux.dev Cc: Song Liu song@kernel.org Cc: Yonghong Song yhs@fb.com Cc: John Fastabend john.fastabend@gmail.com Cc: KP Singh kpsingh@kernel.org Cc: Hao Luo haoluo@google.com Cc: Jiri Olsa jolsa@kernel.org Cc: Mykola Lysenko mykolal@fb.com Cc: Shuah Khan shuah@kernel.org Cc: Haowen Bai baihaowen@meizu.com Cc: bpf@vger.kernel.org Cc: linux-kselftest@vger.kernel.org Signed-off-by: Kees Cook keescook@chromium.org
include/uapi/linux/bpf.h | 15 +++++++++++++-- kernel/bpf/lpm_trie.c | 16 +++++++++------- samples/bpf/map_perf_test_user.c | 2 +- samples/bpf/xdp_router_ipv4_user.c | 2 +- tools/testing/selftests/bpf/test_lpm_map.c | 14 +++++++------- 5 files changed, 31 insertions(+), 18 deletions(-)
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index ba0f0cfb5e42..f843a7582456 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -76,10 +76,21 @@ struct bpf_insn { __s32 imm; /* signed immediate constant */ };
-/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */ +/* Header for a key of a BPF_MAP_TYPE_LPM_TRIE entry */ struct bpf_lpm_trie_key { __u32 prefixlen; /* up to 32 for AF_INET, 128 for AF_INET6 */
__u8 data[0]; /* Arbitrary size */
__u8 data[0]; /* Deprecated field: use struct bpf_lpm_trie_key_u8 */+};
+/* Raw (u8 byte array) key of a BPF_MAP_TYPE_LPM_TRIE entry */ +struct bpf_lpm_trie_key_u8 {
union {struct bpf_lpm_trie_key hdr;struct {__u32 prefixlen;__u8 data[];};};};
Do we need to add a new type to UAPI at all here? We can make this new struct internal to kernel code (e.g. struct bpf_lpm_trie_key_kern) and point out that it should match the layout of struct bpf_lpm_trie_key. User-space can decide whether to use bpf_lpm_trie_key as-is, or if just to ensure their custom struct has the same layout (I see some internal users at Meta do just this, just make sure that they have __u32 prefixlen as first member).
This whole union work-around seems like just extra cruft that we don't really need in UAPI.
Or did I miss anything?
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