From: Yonghong Song <yhs@fb.com>
Date: Thu, 10 Mar 2022 16:37:21 -0800
Subject: selftests/bpf: Fix a clang compilation error for send_signal.c
Patch-mainline: v5.18-rc1
Git-commit: d3b351f65bf42ccda1f686de3ccb21ea1a0c4f5a
References: jsc#PED-1377

Building selftests/bpf with latest clang compiler (clang15 built
from source), I hit the following compilation error:

  /.../prog_tests/send_signal.c:43:16: error: variable 'j' set but not used [-Werror,-Wunused-but-set-variable]
                  volatile int j = 0;
                               ^
  1 error generated.

The problem also exists with clang13 and clang14. clang12 is okay.

In send_signal.c, we have the following code ...

  volatile int j = 0;
  [...]
  for (int i = 0; i < 100000000 && !sigusr1_received; i++)
    j /= i + 1;

... to burn CPU cycles so bpf_send_signal() helper can be tested
in NMI mode.

Slightly changing 'j /= i + 1' to 'j /= i + j + 1' or 'j++' can
fix the problem. Further investigation indicated this should be
a clang bug ([1]). The upstream fix will be proposed later. But it
is a good idea to workaround the issue to unblock people who build
kernel/selftests with clang.

  [1] https://discourse.llvm.org/t/strange-clang-unused-but-set-variable-error-with-volatile-variables/60841

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220311003721.2177170-1-yhs@fb.com
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
 tools/testing/selftests/bpf/prog_tests/send_signal.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/tools/testing/selftests/bpf/prog_tests/send_signal.c
+++ b/tools/testing/selftests/bpf/prog_tests/send_signal.c
@@ -65,7 +65,7 @@ static void test_send_signal_common(stru
 
 		/* wait a little for signal handler */
 		for (int i = 0; i < 100000000 && !sigusr1_received; i++)
-			j /= i + 1;
+			j /= i + j + 1;
 
 		buf[0] = sigusr1_received ? '2' : '0';
 		ASSERT_EQ(sigusr1_received, 1, "sigusr1_received");