Re: [PATCH bpf-next v2 5/9] bpf: verifier: add btf to the function args of bpf_check_attach_target

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Mar 11, 2024 at 2:35 AM Menglong Dong
<dongmenglong.8@bytedance.com> wrote:
>
> Add target btf to the function args of bpf_check_attach_target(), then
> the caller can specify the btf to check.
>
> Signed-off-by: Menglong Dong <dongmenglong.8@bytedance.com>
> ---
>  include/linux/bpf_verifier.h | 1 +
>  kernel/bpf/syscall.c         | 6 ++++--
>  kernel/bpf/trampoline.c      | 1 +
>  kernel/bpf/verifier.c        | 8 +++++---
>  4 files changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 4b0f6600e499..6cb20efcfac3 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -811,6 +811,7 @@ static inline void bpf_trampoline_unpack_key(u64 key, u32 *obj_id, u32 *btf_id)
>  int bpf_check_attach_target(struct bpf_verifier_log *log,
>                             const struct bpf_prog *prog,
>                             const struct bpf_prog *tgt_prog,
> +                           struct btf *btf,
>                             u32 btf_id,
>                             struct bpf_attach_target_info *tgt_info);
>  void bpf_free_kfunc_btf_tab(struct bpf_kfunc_btf_tab *tab);
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index d1cd645ef9ac..6128c3131141 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3401,9 +3401,11 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
>                  * need a new trampoline and a check for compatibility
>                  */
>                 struct bpf_attach_target_info tgt_info = {};
> +               struct btf *btf;
>
> -               err = bpf_check_attach_target(NULL, prog, tgt_prog, btf_id,
> -                                             &tgt_info);
> +               btf = tgt_prog ? tgt_prog->aux->btf : prog->aux->attach_btf;

I think it's better to keep this bit inside bpf_check_attach_target(),
since a lot of other code in there is working with if (tgt_prog) ...
so if the caller messes up passing tgt_prog->aux->btf with tgt_prog
the bug will be difficult to debug.

> +               err = bpf_check_attach_target(NULL, prog, tgt_prog, btf,
> +                                             btf_id, &tgt_info);