linux-kernel - Re: [PATCH RFC bpf-next 3/5] bpf,x86: add tracing session supporting for x86

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Message-ID: <CAADnVQLN96WZd0eWWb=__62g49y_wPfjTPKXaB_=o5jdVE7uKQ@mail.gmail.com>
Date: Tue, 21 Oct 2025 11:16:53 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Jiri Olsa <jolsa@...nel.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>, 
	Matt Bobrowski <mattbobrowski@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Leon Hwang <leon.hwang@...ux.dev>, 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 3/5] bpf,x86: add tracing session supporting
 for x86_64

On Sat, Oct 18, 2025 at 7:21 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>  /* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
>  #define LOAD_TRAMP_TAIL_CALL_CNT_PTR(stack)    \
>         __LOAD_TCC_PTR(-round_up(stack, 8) - 8)
> @@ -3179,8 +3270,10 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
>                                          void *func_addr)
>  {
>         int i, ret, nr_regs = m->nr_args, stack_size = 0;
> -       int regs_off, nregs_off, ip_off, run_ctx_off, arg_stack_off, rbx_off;
> +       int regs_off, nregs_off, session_off, ip_off, run_ctx_off,
> +           arg_stack_off, rbx_off;
>         struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
> +       struct bpf_tramp_links *session = &tlinks[BPF_TRAMP_SESSION];
>         struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
>         struct bpf_tramp_links *fmod_ret = &tlinks[BPF_TRAMP_MODIFY_RETURN];
>         void *orig_call = func_addr;
> @@ -3222,6 +3315,8 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
>          *
>          * RBP - nregs_off [ regs count      ]  always
>          *
> +        * RBP - session_off [ session flags ] tracing session
> +        *
>          * RBP - ip_off    [ traced function ]  BPF_TRAMP_F_IP_ARG flag
>          *
>          * RBP - rbx_off   [ rbx value       ]  always
> @@ -3246,6 +3341,8 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
>         /* regs count  */
>         stack_size += 8;
>         nregs_off = stack_size;
> +       stack_size += 8;
> +       session_off = stack_size;

Unconditional stack increase? :(