tools/testing/selftests/proc/proc-pid-vm.c: hide "segfault at ffffffffff600000" dmesg spam

Test tries to access vsyscall page and if it doesn't exist gets SIGSEGV
which can spam into dmesg. However the segfault happens by design.
Handle it and carry information via exit code to parent.

Link: http://lkml.kernel.org/r/20190524181256.GA2260@avx2
Signed-off-by: Alexey Dobriyan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/proc/proc-pid-vm.c

···
 "ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]\n";
 
 #ifdef __x86_64__
+static void sigaction_SIGSEGV(int _, siginfo_t *__, void *___)
+{
+	_exit(1);
+}
+
 /*
  * vsyscall page can't be unmapped, probe it with memory load.
  */
···
 	if (pid == 0) {
 		struct rlimit rlim = {0, 0};
 		(void)setrlimit(RLIMIT_CORE, &rlim);
+
+		/* Hide "segfault at ffffffffff600000" messages. */
+		struct sigaction act;
+		memset(&act, 0, sizeof(struct sigaction));
+		act.sa_flags = SA_SIGINFO;
+		act.sa_sigaction = sigaction_SIGSEGV;
+		(void)sigaction(SIGSEGV, &act, NULL);
+
 		*(volatile int *)0xffffffffff600000UL;
 		exit(0);
 	}
-	wait(&wstatus);
-	if (WIFEXITED(wstatus)) {
+	waitpid(pid, &wstatus, 0);
+	if (WIFEXITED(wstatus) && WEXITSTATUS(wstatus) == 0) {
 		g_vsyscall = true;
 	}
 }