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Date:   Fri, 8 Dec 2023 21:18:19 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
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>,
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        Kees Cook <keescook@chromium.org>,
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        Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call
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On Fri, Dec 08, 2023 at 11:32:07AM -0800, Alexei Starovoitov wrote:
> On Fri, Dec 8, 2023 at 5:41 AM 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 what
> > > signature to use there.
> >
> > > @@ -119,6 +119,7 @@ int bpf_struct_ops_test_run(struct bpf_p
> > >       op_idx = prog->expected_attach_type;
> > >       err = bpf_struct_ops_prepare_trampoline(tlinks, link,
> > >                                               &st_ops->func_models[op_idx],
> > > +                                             /* XXX */ NULL,
> > >                                               image, image + PAGE_SIZE);
> > >       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.