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

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Date: Fri, 8 Dec 2023 12:58:01 -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 12:52 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Dec 08, 2023 at 12:41:03PM -0800, Alexei Starovoitov wrote:
> > On Fri, Dec 8, 2023 at 12:35 PM Peter Zijlstra <peterz@...radead.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 checks,
and eventually:
addr = kallsyms_lookup_name(dtor_func_name);
field->kptr.dtor = (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.