linux-kernel - Re: [PATCH RFC bpf-next 7/7] bpf: implement "jmp" mode for trampoline

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Message-ID: <CAADnVQKw9PtRYooO+qKQ70xgNusEn8qusBFfzU+bZ7WXRg3-3A@mail.gmail.com>
Date: Fri, 14 Nov 2025 10:50:25 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>, 
	Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC bpf-next 7/7] bpf: implement "jmp" mode for trampoline

On Fri, Nov 14, 2025 at 1:25 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> Implement the "jmp" mode for the bpf trampoline. For the ftrace_managed
> case, we need only to set the FTRACE_OPS_FL_JMP on the tr->fops if "jmp"
> is needed.
>
> For the bpf poke case, the new flag BPF_TRAMP_F_JMPED is introduced to
> store and check if the trampoline is in the "jmp" mode.
>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> ---
>  include/linux/bpf.h     |  6 +++++
>  kernel/bpf/trampoline.c | 53 ++++++++++++++++++++++++++++++++++-------
>  2 files changed, 50 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index aec7c65539f5..3598785ac8d1 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1201,6 +1201,12 @@ struct btf_func_model {
>   */
>  #define BPF_TRAMP_F_INDIRECT           BIT(8)
>
> +/*
> + * Indicate that the trampoline is using "jmp" instead of "call". This flag
> + * is only used in the !ftrace_managed case.
> + */
> +#define BPF_TRAMP_F_JMPED              BIT(9)
> +
>  /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
>   * bytes on x86.
>   */
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 5949095e51c3..02a9f33d8f6c 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -175,15 +175,37 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
>         return tr;
>  }
>
> -static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
> +static int bpf_text_poke(struct bpf_trampoline *tr, void *old_addr,
> +                        void *new_addr)

The bpf_text_poke is a generic name. It really doesn't fit here.
Use bpf_trampoline_update_fentry() or something along those lines.

>  {
> +       enum bpf_text_poke_type new_t = BPF_MOD_CALL, old_t = BPF_MOD_CALL;
>         void *ip = tr->func.addr;
>         int ret;
>
> +       if (bpf_trampoline_need_jmp(tr->flags))
> +               new_t = BPF_MOD_JUMP;
> +       if (tr->flags & BPF_TRAMP_F_JMPED)
> +               old_t = BPF_MOD_JUMP;

Now I see why you picked _need_jmp().. to alternate with F_JMPED ?
_uses_jmp() suggestions isn't quite right.

How about bpf_trampoline_must_jmp() ?
and drop if (!ret) fallback and BPF_TRAMP_F_JMPED bit.
It doesn't look to be necessary.