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From: =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
To: Nathan Chancellor <nathan@kernel.org>, Alexei Starovoitov
 <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, Andrii Nakryiko
 <andrii@kernel.org>
Cc: John Fastabend <john.fastabend@gmail.com>, Jiri Olsa <jolsa@kernel.org>,
 Peter Zijlstra <peterz@infradead.org>, Nick Desaulniers
 <ndesaulniers@google.com>, Tom Rix <trix@redhat.com>, bpf@vger.kernel.org,
 linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
 patches@lists.linux.dev, Nathan Chancellor <nathan@kernel.org>, kernel
 test robot <lkp@intel.com>, "kernelci.org bot" <bot@kernelci.org>
Subject: Re: [PATCH bpf] bpf: Add explicit cast to 'void *' for
 __BPF_DISPATCHER_UPDATE()
In-Reply-To: <20221107170711.42409-1-nathan@kernel.org>
References: <20221107170711.42409-1-nathan@kernel.org>
Date: Mon, 07 Nov 2022 18:51:12 +0100
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Nathan Chancellor <nathan@kernel.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-misma=
tch]
>           __BPF_DISPATCHER_UPDATE(d, new ?: &bpf_dispatcher_nop_func);
>                                      ~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~
>   ./include/linux/bpf.h:1045:54: note: expanded from macro '__BPF_DISPATC=
HER_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() (n=
ot ftrace)")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1755
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: "kernelci.org bot" <bot@kernelci.org>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Acked-by: Bj=C3=B6rn T=C3=B6pel <bjorn@kernel.org>