linux-kernel - Re: [PATCH bpf-next v3 4/7] bpf,x86: add tracing session supporting for x86

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Message-ID: <CAADnVQLfxjOUqbbexFvvVJ4JTUQ2TKL0wvUn3iHv6vXvGfitoQ@mail.gmail.com>
Date: Thu, 30 Oct 2025 18:42:15 -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>, Leon Hwang <leon.hwang@...ux.dev>, jiang.biao@...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 bpf-next v3 4/7] bpf,x86: add tracing session supporting
 for x86_64

On Sat, Oct 25, 2025 at 8:02 PM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> Add BPF_TRACE_SESSION supporting to x86_64. invoke_bpf_session_entry and
> invoke_bpf_session_exit is introduced for this purpose.
>
> In invoke_bpf_session_entry(), we will check if the return value of the
> fentry is 0, and set the corresponding session flag if not. And in
> invoke_bpf_session_exit(), we will check if the corresponding flag is
> set. If set, the fexit will be skipped.
>
> As designed, the session flags and session cookie address is stored after
> the return value, and the stack look like this:
>
>   cookie ptr    -> 8 bytes
>   session flags -> 8 bytes
>   return value  -> 8 bytes
>   argN          -> 8 bytes
>   ...
>   arg1          -> 8 bytes
>   nr_args       -> 8 bytes
>   ...
>   cookieN       -> 8 bytes
>   cookie1       -> 8 bytes
>
> In the entry of the session, we will clear the return value, so the fentry
> will always get 0 with ctx[nr_args] or bpf_get_func_ret().
>
> Before the execution of the BPF prog, the "cookie ptr" will be filled with
> the corresponding cookie address, which is done in
> invoke_bpf_session_entry() and invoke_bpf_session_exit().

...

> +       if (session->nr_links) {
> +               for (i = 0; i < session->nr_links; i++) {
> +                       if (session->links[i]->link.prog->call_session_cookie)
> +                               stack_size += 8;
> +               }
> +       }
> +       cookies_off = stack_size;

This is not great. It's all root and such,
but if somebody attaches 64 progs that use session cookies
then the trampoline will consume 64*8 of stack space just for
these cookies. Plus more for args, cookie, ptr, session_flag, etc.
Sigh.
I understand that cookie from one session shouldn't interfere
with another, but it's all getting quite complex
especially when everything is in assembly.
And this is just x86 JIT. Other JITs would need to copy
this complex logic :(
At this point I'm not sure that "symmetry with kprobe_multi_session"
is justified as a reason to add all that.
We don't have a kprobe_session for individual kprobes after all.

I think we better spend the energy designing a mechanism to
connect existing fentry prog with fexit prog without hacking
it through a stack in the bpf trampoline.

Sorry.

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