linux-kernel - Re: [PATCH bpf-next] bpf: remove is_return in struct bpf_session_run

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Message-ID: <CAADnVQLZMwNUF0PwoCyLUC6tWVuyx80qJF692VgnGoJVm_M=eQ@mail.gmail.com>
Date: Tue, 23 Sep 2025 12:23:44 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: Song Liu <song@...nel.org>, Jiri Olsa <jolsa@...nel.org>, KP Singh <kpsingh@...nel.org>, 
	Matt Bobrowski <mattbobrowski@...gle.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	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 bpf-next] bpf: remove is_return in struct bpf_session_run_ctx

On Mon, Sep 22, 2025 at 7:11 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> On Mon, Sep 22, 2025 at 10:08 PM Song Liu <song@...nel.org> wrote:
> >
> > On Mon, Sep 22, 2025 at 11:57 AM Menglong Dong <menglong8.dong@...il.com> wrote:
> > >
> > > The "data" in struct bpf_session_run_ctx is always 8-bytes aligned.
> > > Therefore, we can store the "is_return" to the last bit of the "data",
> > > which can make bpf_session_run_ctx 8-bytes aligned and save memory.
> >
> > Does this really save anything? AFAICT, bpf_session_run_ctx is
> > only allocated on the stack. Therefore, we don't save any memory
> > unless there is potential risk of stack overflow.
>
> Hi, Song. My original intention is to save the usage of the
> stack to prevent potential stack overflow,

8 bytes won't matter, but wasting 8 bytes for 1 bit is indeed annoying.

> especially when we
> trace all the kernel functions with kprobe-multi.

What do you mean? kprobe-multi won't recurse,
so tracing all or a few functions is the same concern
from stack overflow pov, no ?

> The most thing for me is that the unaligned field in the struct
> looks very awkward, and it consumes 8-bytes only for a bit.

let's keep it as-is. If stack overflow is indeed an issue we need
a generic way to detect it and prevent it.
We've been thinking whether vmap stack guard pages
can become JIT's extable-like things, so when stack overflow
happens we unwind stack and stop bpf prog instead of panicing.