linux-kernel - Re: [PATCH bpf-next v8 04/11] bpf: support fsession for bpf_session_is

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Message-ID: <5075208.31r3eYUQgx@7950hx>
Date: Sat, 10 Jan 2026 11:37:36 +0800
From: Menglong Dong <menglong.dong@...ux.dev>
To: Menglong Dong <menglong8.dong@...il.com>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, "David S. Miller" <davem@...emloft.net>,
 David Ahern <dsahern@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, jiang.biao@...ux.dev,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
 bpf <bpf@...r.kernel.org>, Network Development <netdev@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>
Subject:
 Re: [PATCH bpf-next v8 04/11] bpf: support fsession for bpf_session_is_return

On 2026/1/10 10:40, Alexei Starovoitov wrote:
> On Wed, Jan 7, 2026 at 6:25 PM Menglong Dong <menglong8.dong@...il.com> wrote:
> >
> > +       } else if (func_id == special_kfunc_list[KF_bpf_session_is_return]) {
> > +               if (prog->expected_attach_type == BPF_TRACE_FSESSION)
> > +                       addr = (unsigned long)bpf_fsession_is_return;
> 
> ...
> 
> > +bool bpf_fsession_is_return(void *ctx)
> > +{
> > +       /* This helper call is inlined by verifier. */
> > +       return !!(((u64 *)ctx)[-1] & (1 << BPF_TRAMP_M_IS_RETURN));
> > +}
> > +
> 
> Why do this specialization and introduce a global function
> that will never be called, since it will be inlined anyway?

Ah, the specialization and the definition of the global function
is not unnecessary. I thought that it's kinda fallback solution
that we define the function even if it is inlined by the verifier.

> 
> Remove the first hunk and make the 2nd a comment instead of a real function?

Agree. So it will be:

+static bool bpf_fsession_is_return(void *ctx)
+{
+       /* This helper call is implemented and inlined by the verifier, and the logic is:
+         *   return !!(((u64 *)ctx)[-1] & (1 << BPF_TRAMP_M_IS_RETURN));
+         */
+        return false;
+}

> 
>