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

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Message-ID: <CABCJKudYc5GtO7TLV=d0-ZYFfBxofrV3-q1vpsaT3MD7oTKpnA@mail.gmail.com>
Date:   Mon, 4 Dec 2023 10:58:48 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Olsa <olsajiri@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...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>,
        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 Mon, Dec 4, 2023 at 10:34 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> So afaict this is used through bpf_for_each_map_elem(), where the
> argument still is properly callback_fn. However, in the desriptor
> bpf_for_each_map_elem_proto the argument gets described as:
> ARG_PTR_TO_FUNC, which in turn has a comment like:
>
>   ARG_PTR_TO_FUNC,        /* pointer to a bpf program function */
>
> Which to me sounds like there is definite type punning involved. The
> call in bpf_for_each_array_elem() is a regular C indirect call, which
> gets adorned with the kCFI magic.
>
> But I doubt the BPF function that gets used gets the correct matching
> bits on.
>
> TL;DR, I think this is a pre-existing problem with kCFI + eBPF and not
> caused by my patches.

It is a pre-existing problem, I ran into the same failures when I
looked into this briefly last year:

https://github.com/ClangBuiltLinux/linux/issues/1727

In addition to bpf_for_each_array_elem, a few other callers also use
the same function pointer type that doesn't match cfi_bpf_hash.

Sami