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

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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.