selftests/x86/vdso: Fix no-vDSO segfaults

test_vdso would try to call a NULL pointer if the vDSO was missing.

vdso_restorer_32 hit a genuine failure: trying to use the
kernel-provided signal restorer doesn't work if the vDSO is missing.
Skip the test if the vDSO is missing, since the test adds no particular
value in that case.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/618ea7b8c55b10d08b1cb139e9a3a957934b8647.1584653439.git.luto@kernel.org

tools/testing/selftests/x86/test_vdso.c

@@ -259,6 +259,11 @@ static void test_one_clock_gettime(int clock, const char *name)
 
 static void test_clock_gettime(void)
 {
+	if (!vdso_clock_gettime) {
+		printf("[SKIP]\tNo vDSO, so skipping clock_gettime() tests\n");
+		return;
+	}
+
 	for (int clock = 0; clock < sizeof(clocknames) / sizeof(clocknames[0]);
 	     clock++) {
 		test_one_clock_gettime(clock, clocknames[clock]);

tools/testing/selftests/x86/vdso_restorer.c

@@ -15,6 +15,7 @@
 
 #include <err.h>
 #include <stdio.h>
+#include <dlfcn.h>
 #include <string.h>
 #include <signal.h>
 #include <unistd.h>
@@ -46,11 +47,23 @@ int main()
 	int nerrs = 0;
 	struct real_sigaction sa;
 
+	void *vdso = dlopen("linux-vdso.so.1",
+			    RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);
+	if (!vdso)
+		vdso = dlopen("linux-gate.so.1",
+			      RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);
+	if (!vdso) {
+		printf("[SKIP]\tFailed to find vDSO.  Tests are not expected to work.\n");
+		return 0;
+	}
+
 	memset(&sa, 0, sizeof(sa));
 	sa.handler = handler_with_siginfo;
 	sa.flags = SA_SIGINFO;
 	sa.restorer = NULL;	/* request kernel-provided restorer */
 
+	printf("[RUN]\tRaise a signal, SA_SIGINFO, sa.restorer == NULL\n");
+
 	if (syscall(SYS_rt_sigaction, SIGUSR1, &sa, NULL, 8) != 0)
 		err(1, "raw rt_sigaction syscall");
 
@@ -63,6 +76,8 @@ int main()
 		nerrs++;
 	}
 
+	printf("[RUN]\tRaise a signal, !SA_SIGINFO, sa.restorer == NULL\n");
+
 	sa.flags = 0;
 	sa.handler = handler_without_siginfo;
 	if (syscall(SYS_sigaction, SIGUSR1, &sa, 0) != 0)