proc: save LOC in vsyscall test

Do one fork in vsyscall detection code and let SIGSEGV handler exit and
carry information to the parent saving LOC.

[[e-mail]: redo original patch, delete unnecessary variables, minimise code changes]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Alexey Dobriyan <[e-mail]>
Signed-off-by: Brian Foster <[e-mail]>
Reviewed-by: Brian Foster <[e-mail]>
Tested-by: Brian Foster <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

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

···
 
 /*
  * 0: vsyscall VMA doesn't exist	vsyscall=none
- * 1: vsyscall VMA is r-xp		vsyscall=emulate
- * 2: vsyscall VMA is --xp		vsyscall=xonly
+ * 1: vsyscall VMA is --xp		vsyscall=xonly
+ * 2: vsyscall VMA is r-xp		vsyscall=emulate
  */
-static int g_vsyscall;
+static volatile int g_vsyscall;
 static const char *str_vsyscall;
 
 static const char str_vsyscall_0[] = "";
 static const char str_vsyscall_1[] =
-"ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]\n";
-static const char str_vsyscall_2[] =
 "ffffffffff600000-ffffffffff601000 --xp 00000000 00:00 0                  [vsyscall]\n";
+static const char str_vsyscall_2[] =
+"ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]\n";
 
 #ifdef __x86_64__
 static void sigaction_SIGSEGV(int _, siginfo_t *__, void *___)
 {
-	_exit(1);
+	_exit(g_vsyscall);
 }
 
 /*
···
 		act.sa_sigaction = sigaction_SIGSEGV;
 		(void)sigaction(SIGSEGV, &act, NULL);
 
+		g_vsyscall = 0;
 		/* gettimeofday(NULL, NULL); */
 		asm volatile (
 			"call %P0"
···
 			: "i" (0xffffffffff600000), "D" (NULL), "S" (NULL)
 			: "rax", "rcx", "r11"
 		);
-		exit(0);
+
+		g_vsyscall = 1;
+		*(volatile int *)0xffffffffff600000UL;
+
+		g_vsyscall = 2;
+		exit(g_vsyscall);
 	}
 	waitpid(pid, &wstatus, 0);
-	if (WIFEXITED(wstatus) && WEXITSTATUS(wstatus) == 0) {
-		/* vsyscall page exists and is executable. */
+	if (WIFEXITED(wstatus)) {
+		g_vsyscall = WEXITSTATUS(wstatus);
 	} else {
-		/* vsyscall page doesn't exist. */
-		g_vsyscall = 0;
-		return;
-	}
-
-	pid = fork();
-	if (pid < 0) {
-		fprintf(stderr, "fork, errno %d\n", errno);
+		fprintf(stderr, "error: wstatus %08x\n", wstatus);
 		exit(1);
 	}
-	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);
-	}
-	waitpid(pid, &wstatus, 0);
-	if (WIFEXITED(wstatus) && WEXITSTATUS(wstatus) == 0) {
-		/* vsyscall page is readable and executable. */
-		g_vsyscall = 1;
-		return;
-	}
-
-	/* vsyscall page is executable but unreadable. */
-	g_vsyscall = 2;
 }
 
 int main(void)