netdev - Re: [PATCH bpf-next v6 1/2] bpf, x86: inline bpf_get_current_task() for x86

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAADnVQ+Om14RF1GvA0vkhKdsCtVGzbacftEZarLJ-LKdYXUZ+g@mail.gmail.com>
Date: Tue, 20 Jan 2026 19:10:26 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Menglong Dong <menglong.dong@...ux.dev>
Cc: Menglong Dong <menglong8.dong@...il.com>, Andrii Nakryiko <andrii.nakryiko@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Eduard <eddyz87@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, 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>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v6 1/2] bpf, x86: inline bpf_get_current_task()
 for x86_64

On Tue, Jan 20, 2026 at 5:58 PM Menglong Dong <menglong.dong@...ux.dev> wrote:
>
> On 2026/1/21 09:23 Andrii Nakryiko <andrii.nakryiko@...il.com> write:
> > On Mon, Jan 19, 2026 at 11:06 PM Menglong Dong <menglong8.dong@...il.com> wrote:
> > >
> > > Inline bpf_get_current_task() and bpf_get_current_task_btf() for x86_64
> > > to obtain better performance.
> > >
> > > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > > Acked-by: Eduard Zingerman <eddyz87@...il.com>
> > > ---
> > > v5:
> > > - don't support the !CONFIG_SMP case
> > >
> > > v4:
> > > - handle the !CONFIG_SMP case
> > >
> > > v3:
> > > - implement it in the verifier with BPF_MOV64_PERCPU_REG() instead of in
> > >   x86_64 JIT.
> > > ---
> > >  kernel/bpf/verifier.c | 22 ++++++++++++++++++++++
> > >  1 file changed, 22 insertions(+)
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 9de0ec0c3ed9..c4e2ffadfb1f 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -17739,6 +17739,10 @@ static bool verifier_inlines_helper_call(struct bpf_verifier_env *env, s32 imm)
> > >         switch (imm) {
> > >  #ifdef CONFIG_X86_64
> > >         case BPF_FUNC_get_smp_processor_id:
> > > +#ifdef CONFIG_SMP
> > > +       case BPF_FUNC_get_current_task_btf:
> > > +       case BPF_FUNC_get_current_task:
> > > +#endif
> >
> > Does this have to be x86-64 specific inlining? With verifier inlining
> > and per_cpu instruction support it should theoretically work across
> > all architectures that do support per-cpu instruction, no?
> >
> > Eduard pointed out [0] to me for why we have that x86-64 specific
> > check. But looking at do_misc_fixups(), we have that early
> > bpf_jit_inlines_helper_call(insn->imm)) check, so if some JIT has more
> > performant inlining implementation, we will just do that.
> >
> > So it seems like we can just drop all that x86-64 specific logic and
> > claim all three of these functions as inlinable, no?
> >
> > And even more. We can drop rather confusing
> > verifier_inlines_helper_call() that duplicates the decision of which
> > helpers can be inlined or not, and have:
>
> The verifier_inlines_helper_call() is confusing, but I think we can't
> remove the x86-64 checking. For example, some architecture
> don't support BPF_FUNC_get_current_task both in
> bpf_jit_inlines_helper_call() and verifier_inlines_helper_call(), which
> means it can't be inline.
>
> >
> > if (env->prog->jit_requested && bpf_jit_supports_percpu_insn() {
> >     switch (insn->imm) {
> >     case BPF_FUNC_get_smp_processor_id:
> >         ...
> >         break;
> >     case BPF_FUNC_get_current_task_btf:
> >     case BPF_FUNC_get_current_task_btf:
> >         ...
> >         break;
> >     default:
> > }
> >
> > And the decision about inlining will live in one place.
> >
> > Or am I missing some complications?
>
> As Alexei said, the implement of "current" is architecture specific,
> and the per-cpu variable "current_task" only exist on x86_64.
>
> >
> > And with all that, should we mark get_current_task and
> > get_current_task_btf as __bpf_fastcall?
>
> I think it make sense, and the I saw bpf_get_smp_processor_id does
> such operation:
>
> const struct bpf_func_proto bpf_get_smp_processor_id_proto = {
>         [...]
>         .allow_fastcall = true,
> };
>
> PS: I'm a little confused about the fast call. We inline many helper,
> but it seems that bpf_get_smp_processor_id is the only one that
> use the "allow_fastcall". Why? I'd better study harder.

It's
static __bpf_fastcall __u32 (* const bpf_get_smp_processor_id)(void) =
(void *) 8;

and
#define __bpf_fastcall __attribute__((bpf_fastcall))

which makes LLVM use more registers at the callsite (less spill/fill).

Looking at the patch again. I think it's fine as-is.
fastcall can be a follow up.