Re: [PATCH] bpf: Deprecate "data" member of bpf_lpm_trie_key

[Andrii Nakryiko]

On Thu, Feb 9, 2023 at 11:23 AM Kees Cook <keescook@xxxxxxxxxxxx> 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:
>
> 1) 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.
>
> 2) 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
>
> 3) 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&perpkg=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-arrays
>     https://people.kernel.org/kees/bounded-flexible-arrays-in-c
>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> 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: 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
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  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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