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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Fri, 8 Dec 2023 12:45:51 -0800
Message-ID: <CAADnVQ+1nVBuKkjdvh0eu19p+J0UqbO9mcCf3yzVeQtALxzQ+Q@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Jiri Olsa <olsajiri@gmail.com>, Song Liu <song@kernel.org>,
        Song Liu <songliubraving@meta.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        "David S. Miller" <davem@davemloft.net>,
        David Ahern <dsahern@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Yonghong Song <yonghong.song@linux.dev>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Arnd Bergmann <arnd@arndb.de>,
        Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>,
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        Josh Poimboeuf <jpoimboe@kernel.org>,
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        Mark Rutland <mark.rutland@arm.com>
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On Fri, Dec 8, 2023 at 12:18=E2=80=AFPM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
> On Fri, Dec 08, 2023 at 11:32:07AM -0800, Alexei Starovoitov wrote:
> > On Fri, Dec 8, 2023 at 5:41=E2=80=AFAM Peter Zijlstra <peterz@infradead=
.org> wrote:
> > >
> > > On Fri, Dec 08, 2023 at 11:29:40AM +0100, Peter Zijlstra wrote:
> > > > The only problem I now have is the one XXX, I'm not entirely sure w=
hat
> > > > signature to use there.
> > >
> > > > @@ -119,6 +119,7 @@ int bpf_struct_ops_test_run(struct bpf_p
> > > >       op_idx =3D prog->expected_attach_type;
> > > >       err =3D bpf_struct_ops_prepare_trampoline(tlinks, link,
> > > >                                               &st_ops->func_models[=
op_idx],
> > > > +                                             /* XXX */ NULL,
> > > >                                               image, image + PAGE_S=
IZE);
> > > >       if (err < 0)
> > > >               goto out;
> > >
> > > Duh, that should ofcourse be something of dummy_ops_test_ret_fn type.
> > > Let me go fix that.
> >
> > Right. That should work.
> > A bit wasteful to generate real code just to read hash from it
> > via cfi_get_func_hash(), but it's a neat idea.
>
> Right, bit wasteful. But the advantage is that I get a structure with
> pointers that exactly mirrors the structure we're writing.
>
> > I guess it's hard to get kcfi from __ADDRESSABLE in plain C
> > and sprinkling asm("cfi_xxx: .long   __kcfi_typeid..."); is worse?
> > Even if it's a macro ?
>
> I can try this, but I'm not sure it'll be pretty. Even if I wrap it in a
> decent macro, I still get to define a ton of variables and then wrap the
> lot into a structure -- one that expects function pointers.
>
> I'll see how horrible it will become.

I mean we don't need to store a pointer to a func in stubs.
Can it be, roughly:

extern void bpf_tcp_ca_cong_avoid(struct sock *sk, u32 ack, u32 acked);
KCFI_MACRO(hash_of_cong_avoid, bpf_tcp_ca_cong_avoid);
u32 __array_of_kcfi_hash[] =3D {hash_of_cong_avoid, hash_of_set_state,...};
      .bpf_ops_stubs =3D __array_of_kcfi_hash,