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

[Stanislav Fomichev]

On Sat, Feb 4, 2023 at 10:32 AM Kees Cook <keescook@xxxxxxxxxxxx> 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 */
>       |                 ^~~~
>
> This includes fixing the selftest which was incorrectly using a
> variable length struct as a header, identified earlier[1]. Avoid this
> by just explicitly including the prefixlen member instead of struct
> bpf_lpm_trie_key.
>
> [1] https://lore.kernel.org/all/202206281009.4332AA33@keescook/
>
> Cc: Alexei Starovoitov <ast@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: 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
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  include/uapi/linux/bpf.h                         | 2 +-
>  tools/testing/selftests/bpf/progs/map_ptr_kern.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index ba0f0cfb5e42..5930bc5c7e2c 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -79,7 +79,7 @@ struct bpf_insn {
>  /* Key of an 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[];         /* Arbitrary size */
>  };

That's a UAPI change, can we do it? The safest option is probably just
to remove this field if it's causing any problems (and not do the
map_ptr_kern.c change below).
The usual use-case (at least that's what we do) is to define some new
struct over it:

struct my_key {
  struct bpf_lpm_trie_key prefix;
  int a, b, c;
};

So I really doubt that the 'data' is ever touched by any programs at all..

>
>  struct bpf_cgroup_storage_key {
> diff --git a/tools/testing/selftests/bpf/progs/map_ptr_kern.c b/tools/testing/selftests/bpf/progs/map_ptr_kern.c
> index db388f593d0a..543012deb349 100644
> --- a/tools/testing/selftests/bpf/progs/map_ptr_kern.c
> +++ b/tools/testing/selftests/bpf/progs/map_ptr_kern.c
> @@ -311,7 +311,7 @@ struct lpm_trie {
>  } __attribute__((preserve_access_index));
>
>  struct lpm_key {
> -       struct bpf_lpm_trie_key trie_key;
> +       __u32 prefixlen;
>         __u32 data;
>  };
> --
> 2.34.1
>