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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Fri, 8 Dec 2023 11:40:27 -0800
Message-ID: <CAADnVQKsnZfFomQ4wTZz=jMZW5QCV2XiXVsi64bghHkAjJtcmA@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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        Mark Rutland <mark.rutland@arm.com>
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On Fri, Dec 8, 2023 at 9:22=E2=80=AFAM Peter Zijlstra <peterz@infradead.org=
> wrote:
>
> On Fri, Dec 08, 2023 at 02:40:41PM +0100, Peter Zijlstra 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 wha=
t
> > > 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_i=
dx],
> > > +                                           /* 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.
>
> Next one.. bpf_obj_free_fields: field->kptr.dtor(xchg_field);
>
> The one that trips is bpf_cgroup_release().
>
> objtool doesn't think the address of that function 'escapes' and
> 'helpfully' seals that function, and then BPF thinks it does escape and
> manages the above indirect call and *boom*.
>
> How can I tell which functions escape according to BPF such that I might
> teach objtool this?

I'm not following.
Are you asking to annotate
__bpf_kfunc void bpf_cgroup_release(struct cgroup *cgrp)
somehow so that objtool knows that it will be called indirectly?
typedef void (*btf_dtor_kfunc_t)(void *);
        btf_dtor_kfunc_t dtor;
but the bpf_cgroup_release takes 'struct cgroup*'.
From kcfi pov void * =3D=3D struct cgroup * ?
Do we need to change it to 'void *cgrp' ?
What is "sealing" by objtool?