add a static usermode helper

The binary is used in tandem with CONFIG_STATIC_USERMODEHELPER=y in 4.11+,
see the big comment in the binary for the current whitelist of binaries.

Signed-off-by: Tycho Andersen <tycho@docker.com>

pkg/init/usermode-helper.c

@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+int main(int argc, char *argv[])
+{
+	int i;
+
+	/* TODO: this doesn't go anywhere useful right now. It would be nice to
+	 * switch this to syslog() (or some other mechanism) so that we can
+	 * actually read the contents.
+	 */
+	fprintf(stderr, "usermodehelper: ");
+	for (i = 0; i < argc; i++) {
+		fprintf(stderr, "%s ", argv[i]);
+	}
+	fprintf(stderr, "\n");
+
+	if (!strcmp(argv[0], "/sbin/mdev")) {
+		/* busybox uses /sbin/mdev for early uevent bootstrapping */
+		execv(argv[0], argv);
+	} else if (!strcmp(argv[0], "/sbin/modprobe")) {
+		/* allow modprobe */
+		execv(argv[0], argv);
+	} else if (!strcmp(argv[0], "/sbin/poweroff") ||
+			!strcmp(argv[0], "/sbin/reboot")) {
+		/* poweroff and reboot are allowed */
+		execv(argv[0], argv);
+	} else {
+		/* This means either we got an unexpected call from the kernel
+		 * or someone is doing something nefarious. Some other possible
+		 * expected callers are:
+		 *  - for core dumps. we don't have a "core" binary, and don't
+		 *    set this by default to anything. when we do, we need to
+		 *    whitelist it here
+		 *  - /linuxrc: we're not doing legacy root setup, so we don't
+		 *     need this
+		 *  - a few drivers and filesystems (drbd, nfs, nfsd, ocfs2)
+		 *  - cgroup notify_on_release handlers, which we do not set
+		 *    (but e.g. systemd needs, if anyone ever tries to boot
+		 *    that on linuxkit)
+		 *  - /sbin/request-key, which we don't provide
+		 *  - on x86, machine check
+		 *
+		 * Today we only call mdev and modprobe, but as we add more
+		 * features to linuxkit this whitelist may need changing (or a
+		 * policy, like always allow stuff in /sbin).
+		 */
+		exit(2);
+	}
+
+	perror("exec failed");
+	return 1;
+}