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References: <20240311093526.1010158-1-dongmenglong.8@bytedance.com> <20240311093526.1010158-6-dongmenglong.8@bytedance.com>
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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Mon, 11 Mar 2024 18:51:30 -0700
Message-ID: <CAADnVQKw4HUbwvivysVBQPpA2MC2e56MwrvJy89qs8rx_ixOnw@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2 5/9] bpf: verifier: add btf to the function
 args of bpf_check_attach_target
To: Menglong Dong <dongmenglong.8@bytedance.com>
Cc: Andrii Nakryiko <andrii@kernel.org>, Alexei Starovoitov <ast@kernel.org>, 
	Daniel Borkmann <daniel@iogearbox.net>, Martin KaFai Lau <martin.lau@linux.dev>, Eddy Z <eddyz87@gmail.com>, 
	Song Liu <song@kernel.org>, Yonghong Song <yonghong.song@linux.dev>, 
	John Fastabend <john.fastabend@gmail.com>, KP Singh <kpsingh@kernel.org>, 
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>, 
	Alexander Gordeev <agordeev@linux.ibm.com>, Christian Borntraeger <borntraeger@linux.ibm.com>, 
	Sven Schnelle <svens@linux.ibm.com>, "David S. Miller" <davem@davemloft.net>, 
	David Ahern <dsahern@kernel.org>, Dave Hansen <dave.hansen@linux.intel.com>, 
	X86 ML <x86@kernel.org>, Steven Rostedt <rostedt@goodmis.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Quentin Monnet <quentin@isovalent.com>, 
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	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>, LKML <linux-kernel@vger.kernel.org>, 
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On Mon, Mar 11, 2024 at 2:35=E2=80=AFAM 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 =3D {};
> +               struct btf *btf;
>
> -               err =3D bpf_check_attach_target(NULL, prog, tgt_prog, btf=
_id,
> -                                             &tgt_info);
> +               btf =3D 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 =3D bpf_check_attach_target(NULL, prog, tgt_prog, btf=
,
> +                                             btf_id, &tgt_info);