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

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Message-Id: <20221107170711.42409-1-nathan@kernel.org>
Date:   Mon,  7 Nov 2022 10:07:11 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     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, Nathan Chancellor <nathan@...nel.org>,
        kernel test robot <lkp@...el.com>,
        "kernelci.org bot" <bot@...nelci.org>
Subject: [PATCH bpf] bpf: Add explicit cast to 'void *' for __BPF_DISPATCHER_UPDATE()

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>
---
 kernel/bpf/dispatcher.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/dispatcher.c b/kernel/bpf/dispatcher.c
index 7dfb8d0d5202..c19719f48ce0 100644
--- a/kernel/bpf/dispatcher.c
+++ b/kernel/bpf/dispatcher.c
@@ -123,7 +123,7 @@ static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
 			return;
 	}
 
-	__BPF_DISPATCHER_UPDATE(d, new ?: &bpf_dispatcher_nop_func);
+	__BPF_DISPATCHER_UPDATE(d, new ?: (void *)&bpf_dispatcher_nop_func);
 
 	if (new)
 		d->image_off = noff;

base-commit: c86df29d11dfba27c0a1f5039cd6fe387fbf4239
-- 
2.38.1