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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Mon, 26 Oct 2020 15:26:11 -0700
Message-ID: <CAEf4BzaKSzEbPStm8EXk4RH417jHieSF3LCazQPXZ6qGskqB6Q@mail.gmail.com>
Subject: Re: [PATCH] bpf: suppress -Wcast-function-type warning
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Arnd Bergmann <arnd@arndb.de>, Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        David Miller <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jakub Sitnicki <jakub@cloudflare.com>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@intel.com>,
        =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
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On Mon, Oct 26, 2020 at 2:03 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> Building with -Wextra shows lots of warnings in the bpf
> code such as
>
> kernel/bpf/verifier.c: In function =E2=80=98jit_subprogs=E2=80=99:
> include/linux/filter.h:345:4: warning: cast between incompatible function=
 types from =E2=80=98unsigned int (*)(const void *, const struct bpf_insn *=
)=E2=80=99 to =E2=80=98u64 (*)(u64,  u64,  u64,  u64,  u64)=E2=80=99 {aka =
=E2=80=98long long unsigned int (*)(long long unsigned int,  long long unsi=
gned int,  long long unsigned int,  long long unsigned int,  long long unsi=
gned int)=E2=80=99} [-Wcast-function-type]
>   345 |   ((u64 (*)(u64, u64, u64, u64, u64))(x))
>       |    ^
> kernel/bpf/verifier.c:10706:16: note: in expansion of macro =E2=80=98BPF_=
CAST_CALL=E2=80=99
> 10706 |    insn->imm =3D BPF_CAST_CALL(func[subprog]->bpf_func) -
>       |                ^~~~~~~~~~~~~
>
> This appears to be intentional, so change the cast in a way that
> suppresses the warning.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---

LGTM.

Acked-by: Andrii Nakryiko <andrii@kernel.org>

>  include/linux/filter.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 1b62397bd124..20ba04583eaa 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -342,7 +342,7 @@ static inline bool insn_is_zext(const struct bpf_insn=
 *insn)
>  /* Function call */
>
>  #define BPF_CAST_CALL(x)                                       \
> -               ((u64 (*)(u64, u64, u64, u64, u64))(x))
> +               ((u64 (*)(u64, u64, u64, u64, u64))(uintptr_t)(x))
>
>  #define BPF_EMIT_CALL(FUNC)                                    \
>         ((struct bpf_insn) {                                    \
> --
> 2.27.0
>