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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Mon, 4 Dec 2023 10:58:48 -0800
Message-ID: <CABCJKudYc5GtO7TLV=d0-ZYFfBxofrV3-q1vpsaT3MD7oTKpnA@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Jiri Olsa <olsajiri@gmail.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Song Liu <song@kernel.org>, Song Liu <songliubraving@meta.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        "David S. Miller" <davem@davemloft.net>,
        David Ahern <dsahern@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Yonghong Song <yonghong.song@linux.dev>,
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On Mon, Dec 4, 2023 at 10:34=E2=80=AFAM Peter Zijlstra <peterz@infradead.or=
g> 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