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

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Message-ID: <87wn86d567.fsf@all.your.base.are.belong.to.us>
Date:   Mon, 07 Nov 2022 18:51:12 +0100
From:   Björn Töpel <bjorn@...nel.org>
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>,
        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, Nathan Chancellor <nathan@...nel.org>,
        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()

Nathan Chancellor <nathan@...nel.org> writes:

> 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: Björn Töpel <bjorn@...nel.org>