netdev - Re: [PATCH bpf-next v10 07/12] bpf,x86: add fsession support for x86

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Message-ID: <CAEf4BzYop1GT1v7qxNQi44dsEebjwnLp-sHDp-HPzVrj69WzTw@mail.gmail.com>
Date: Wed, 21 Jan 2026 16:06:55 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: ast@...nel.org, andrii@...nel.org, daniel@...earbox.net, 
	martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org, 
	sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, davem@...emloft.net, 
	dsahern@...nel.org, tglx@...utronix.de, mingo@...hat.com, 
	jiang.biao@...ux.dev, bp@...en8.de, dave.hansen@...ux.intel.com, 
	x86@...nel.org, hpa@...or.com, bpf@...r.kernel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v10 07/12] bpf,x86: add fsession support for x86_64

On Thu, Jan 15, 2026 at 3:24 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> Add BPF_TRACE_FSESSION supporting to x86_64, including:
>
> 1. clear the return value in the stack before fentry to make the fentry
>    of the fsession can only get 0 with bpf_get_func_ret().
>
> 2. clear all the session cookies' value in the stack.
>
> 2. store the index of the cookie to ctx[-1] before the calling to fsession
>
> 3. store the "is_return" flag to ctx[-1] before the calling to fexit of
>    the fsession.
>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> Co-developed-by: Leon Hwang <leon.hwang@...ux.dev>
> Signed-off-by: Leon Hwang <leon.hwang@...ux.dev>
> ---
> v10:
> - use "|" for func_meta instead of "+"
> - pass the "func_meta_off" to invoke_bpf() explicitly, instead of
>   computing it with "stack_size + 8"
> - pass the "cookie_off" to invoke_bpf() instead of computing the current
>   cookie index with "func_meta"
>
> v5:
> - add the variable "func_meta"
> - define cookie_off in a new line
>
> v4:
> - some adjustment to the 1st patch, such as we get the fsession prog from
>   fentry and fexit hlist
> - remove the supporting of skipping fexit with fentry return non-zero
>
> v2:
> - add session cookie support
> - add the session stuff after return value, instead of before nr_args
> ---
>  arch/x86/net/bpf_jit_comp.c | 52 ++++++++++++++++++++++++++++---------
>  1 file changed, 40 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2f31331955b5..16720f2be16c 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -3094,13 +3094,19 @@ static int emit_cond_near_jump(u8 **pprog, void *func, void *ip, u8 jmp_cond)
>
>  static int invoke_bpf(const struct btf_func_model *m, u8 **pprog,
>                       struct bpf_tramp_links *tl, int stack_size,
> -                     int run_ctx_off, bool save_ret,
> -                     void *image, void *rw_image)
> +                     int run_ctx_off, int func_meta_off, bool save_ret,
> +                     void *image, void *rw_image, u64 func_meta,
> +                     int cookie_off)
>  {
> -       int i;
> +       int i, cur_cookie = (cookie_off - stack_size) / 8;

not sure why you went with passing cookie_off and then calculating,
effectively, cookie count out of that?... why not pass cookie count
directly then? it's minor, but just seems like a weird choice here,
tbh



>         u8 *prog = *pprog;
>
>         for (i = 0; i < tl->nr_links; i++) {
> +               if (tl->links[i]->link.prog->call_session_cookie) {
> +                       emit_store_stack_imm64(&prog, BPF_REG_0, -func_meta_off,
> +                                              func_meta | (cur_cookie << BPF_TRAMP_SHIFT_COOKIE));
> +                       cur_cookie--;
> +               }
>                 if (invoke_bpf_prog(m, &prog, tl->links[i], stack_size,
>                                     run_ctx_off, save_ret, image, rw_image))
>                         return -EINVAL;

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