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

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Message-ID: <20231208205241.GK28727@noisy.programming.kicks-ass.net>
Date: Fri, 8 Dec 2023 21:52:41 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
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 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?

> 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.