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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Fri, 8 Dec 2023 12:58:01 -0800
Message-ID: <CAADnVQL3KsJONShsstDq5jrpbc_4FOU-VQPJgDCt50N9asoFzA@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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        LKML <linux-kernel@vger.kernel.org>,
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        bpf <bpf@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        clang-built-linux <llvm@lists.linux.dev>,
        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 12:52=E2=80=AFPM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
> On Fri, Dec 08, 2023 at 12:41:03PM -0800, Alexei Starovoitov wrote:
> > On Fri, Dec 8, 2023 at 12:35=E2=80=AFPM Peter Zijlstra <peterz@infradea=
d.org> wrote:
>
> > > -__bpf_kfunc void bpf_task_release(struct task_struct *p)
> > > +__bpf_kfunc void bpf_task_release(void *p)
> >
> > Yeah. That won't work. We need a wrapper.
> > Since bpf prog is also calling it directly.
> > In progs/task_kfunc_common.h
> > void bpf_task_release(struct task_struct *p) __ksym;
> >
> > than later both libbpf and the verifier check that
> > what bpf prog is calling actually matches the proto
> > of what is in the kernel.
> > Effectively we're doing strong prototype check at load time.
>
> I'm still somewhat confused on how this works, where does BPF get the
> address of the function from? and what should I call the wrapper?

It starts with
register_btf_id_dtor_kfuncs() that takes a set of btf_ids:
{btf_id_of_type, btf_id_of_dtor_function}, ...

Then based on btf_id_of_dtor_function we find its type proto, name, do chec=
ks,
and eventually:
addr =3D kallsyms_lookup_name(dtor_func_name);
field->kptr.dtor =3D (void *)addr;

bpf_task_release(struct task_struct *p) would need to stay as-is,
but we can have a wrapper
void bpf_task_release_dtor(void *p)
{
  bpf_task_release(p);
}

And adjust the above lookup with extra "_dtor" suffix.

> > btw instead of EXPORT_SYMBOL_GPL(bpf_task_release)
> > can __ADDRESSABLE be used ?
> > Since it's not an export symbol.
>
> No __ADDRESSABLE() is expressly ignored, but we have IBT_NOSEAL() that
> should do it. I'll rename the thing and lift it out of x86 to avoid
> breaking all other arch builds.

Makes sense.