linux-kernel - Re: [PATCH] bpf: suppress -Wcast-function-type warning

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Message-ID: <CAEf4BzaKSzEbPStm8EXk4RH417jHieSF3LCazQPXZ6qGskqB6Q@mail.gmail.com>
Date:   Mon, 26 Oct 2020 15:26:11 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Arnd Bergmann <arnd@...db.de>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andrii@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        David Miller <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@...el.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: suppress -Wcast-function-type warning

On Mon, Oct 26, 2020 at 2:03 PM Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> Building with -Wextra shows lots of warnings in the bpf
> code such as
>
> kernel/bpf/verifier.c: In function ‘jit_subprogs’:
> include/linux/filter.h:345:4: warning: cast between incompatible function types from ‘unsigned int (*)(const void *, const struct bpf_insn *)’ to ‘u64 (*)(u64,  u64,  u64,  u64,  u64)’ {aka ‘long long unsigned int (*)(long long unsigned int,  long long unsigned int,  long long unsigned int,  long long unsigned int,  long long unsigned int)’} [-Wcast-function-type]
>   345 |   ((u64 (*)(u64, u64, u64, u64, u64))(x))
>       |    ^
> kernel/bpf/verifier.c:10706:16: note: in expansion of macro ‘BPF_CAST_CALL’
> 10706 |    insn->imm = 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@...db.de>
> ---

LGTM.

Acked-by: Andrii Nakryiko <andrii@...nel.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
>