selftests/exec: Check if the syscall exists and bail if not

On systems which don't implement sys_execveat(), this test produces a
lot of output.

Add a check at the beginning to see if the syscall is present, and if
not just note one error and return.

When we run on a system that doesn't implement the syscall we will get
ENOSYS back from the kernel, so change the logic that handles
__NR_execveat not being defined to also use ENOSYS rather than -ENOSYS.

Signed-off-by: Michael Ellerman <[e-mail]>
Acked-by: David Drysdale <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/exec/execveat.c

···
 #ifdef __NR_execveat
 	return syscall(__NR_execveat, fd, path, argv, envp, flags);
 #else
-	errno = -ENOSYS;
+	errno = ENOSYS;
 	return -1;
 #endif
 }
···
 	int fd_script_ephemeral = open_or_die("script.ephemeral", O_RDONLY);
 	int fd_cloexec = open_or_die("execveat", O_RDONLY|O_CLOEXEC);
 	int fd_script_cloexec = open_or_die("script", O_RDONLY|O_CLOEXEC);
+
+	/* Check if we have execveat at all, and bail early if not */
+	errno = 0;
+	execveat_(-1, NULL, NULL, NULL, 0);
+	if (errno == ENOSYS) {
+		printf("[FAIL] ENOSYS calling execveat - no kernel support?\n");
+		return 1;
+	}
 
 	/* Change file position to confirm it doesn't affect anything */
 	lseek(fd, 10, SEEK_SET);