netdev - Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call

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Message-ID: <CAADnVQKsnZfFomQ4wTZz=jMZW5QCV2XiXVsi64bghHkAjJtcmA@mail.gmail.com>
Date: Fri, 8 Dec 2023 11:40:27 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Jiri Olsa <olsajiri@...il.com>, Song Liu <song@...nel.org>, 
	Song Liu <songliubraving@...a.com>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	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>, "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Arnd Bergmann <arnd@...db.de>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Kees Cook <keescook@...omium.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, linux-riscv <linux-riscv@...ts.infradead.org>, 
	LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, 
	bpf <bpf@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, 
	clang-built-linux <llvm@...ts.linux.dev>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Joao Moreira <joao@...rdrivepizza.com>, Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call

On Fri, Dec 8, 2023 at 9:22 AM Peter Zijlstra <peterz@...radead.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 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.
>
> 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 * == struct cgroup * ?
Do we need to change it to 'void *cgrp' ?
What is "sealing" by objtool?