Re: [PATCH bpf] bpf: Add explicit cast to 'void *' for __BPF_DISPATCHER_UPDATE()

From: Yonghong Song <yhs@meta.com>
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>,
	"Björn Töpel" <bjorn@kernel.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,
	"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()
Date: Mon, 7 Nov 2022 10:45:34 -0800	[thread overview]
Message-ID: <40da21f7-19d7-142f-0380-41abd0d47d1d@meta.com> (raw)
In-Reply-To: <20221107170711.42409-1-nathan@kernel.org>


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@intel.com>
> Reported-by: "kernelci.org bot" <bot@kernelci.org>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

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