linux-kernel - Re: [PATCH RFC bpf-next 4/7] bpf,x86: adjust the "jmp" mode for bpf trampoline

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Message-ID: <CAADnVQJU39q9amZMuVLzsg7CK5MLT_xFr0K4Bx9zp7P5C6MCRw@mail.gmail.com>
Date: Fri, 14 Nov 2025 10:22:13 -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 4/7] bpf,x86: adjust the "jmp" mode for bpf trampoline

On Fri, Nov 14, 2025 at 1:25 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> In the origin call case, if BPF_TRAMP_F_SKIP_FRAME is not set, it means
> that the trampoline is not called, but "jmp".
>
> Introduce the function bpf_trampoline_need_jmp() to check if the
> trampoline is in "jmp" mode.
>
> Do some adjustment on the "jmp" mode for the x86_64. The main adjustment
> that we make is for the stack parameter passing case, as the stack
> alignment logic changes in the "jmp" mode without the "rip". What's more,
> the location of the parameters on the stack also changes.
>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> ---
>  arch/x86/net/bpf_jit_comp.c | 15 ++++++++++-----
>  include/linux/bpf.h         | 12 ++++++++++++
>  2 files changed, 22 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2d300ab37cdd..21ce2b8457ec 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -2830,7 +2830,7 @@ static int get_nr_used_regs(const struct btf_func_model *m)
>  }
>
>  static void save_args(const struct btf_func_model *m, u8 **prog,
> -                     int stack_size, bool for_call_origin)
> +                     int stack_size, bool for_call_origin, bool jmp)

I have an allergy to bool args.

Please pass flags and do
boll jmp_based_tramp = bpf_trampoline_uses_jmp(flags);

I think bpf_trampoline_uses_jmp() is more descriptive than
bpf_trampoline_need_jmp().

The actual math lgtm.