linux-kernel - Re: [PATCH bpf] bpf: Add explicit cast to 'void *' for __BPF_DISPATCHER

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Date:   Mon, 7 Nov 2022 10:45:34 -0800
From:   Yonghong Song <yhs@...a.com>
To:     Nathan Chancellor <nathan@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Björn Töpel <bjorn@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        patches@...ts.linux.dev, kernel test robot <lkp@...el.com>,
        "kernelci.org bot" <bot@...nelci.org>
Subject: Re: [PATCH bpf] bpf: Add explicit cast to 'void *' for
 __BPF_DISPATCHER_UPDATE()



On 11/7/22 9:07 AM, Nathan Chancellor wrote:
> When building with clang:
> 
>    kernel/bpf/dispatcher.c:126:33: error: pointer type mismatch ('void *' and 'unsigned int (*)(const void *, const struct bpf_insn *, bpf_func_t)' (aka 'unsigned int (*)(const void *, const struct bpf_insn *, unsigned int (*)(const void *, const struct bpf_insn *))')) [-Werror,-Wpointer-type-mismatch]
>            __BPF_DISPATCHER_UPDATE(d, new ?: &bpf_dispatcher_nop_func);
>                                       ~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~
>    ./include/linux/bpf.h:1045:54: note: expanded from macro '__BPF_DISPATCHER_UPDATE'
>            __static_call_update((_d)->sc_key, (_d)->sc_tramp, (_new))
>                                                                ^~~~
>    1 error generated.
> 
> The warning is pointing out that the type of new ('void *') and
> &bpf_dispatcher_nop_func are not compatible, which could have side
> effects coming out of a conditional operator due to promotion rules.
> 
> Add the explicit cast to 'void *' to make it clear that this is
> expected, as __BPF_DISPATCHER_UPDATE() expands to a call to
> __static_call_update(), which expects a 'void *' as its final argument.
> 
> Fixes: c86df29d11df ("bpf: Convert BPF_DISPATCHER to use static_call() (not ftrace)")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1755
> Reported-by: kernel test robot <lkp@...el.com>
> Reported-by: "kernelci.org bot" <bot@...nelci.org>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

Acked-by: Yonghong Song <yhs@...com>