c/r: fcntl: add F_GETOWNER_UIDS option

When we restore file descriptors we would like them to look exactly as
they were at dumping time.

With help of fcntl it's almost possible, the missing snippet is file
owners UIDs.

To be able to read their values the F_GETOWNER_UIDS is introduced.

This option is valid iif CONFIG_CHECKPOINT_RESTORE is turned on, otherwise
returning -EINVAL.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fcntl.c

@@ -20,6 +20,7 @@
 #include <linux/signal.h>
 #include <linux/rcupdate.h>
 #include <linux/pid_namespace.h>
+#include <linux/user_namespace.h>
 
 #include <asm/poll.h>
 #include <asm/siginfo.h>
@@ -340,6 +341,31 @@ static int f_getown_ex(struct file *filp, unsigned long arg)
 	return ret;
 }
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+static int f_getowner_uids(struct file *filp, unsigned long arg)
+{
+	struct user_namespace *user_ns = current_user_ns();
+	uid_t * __user dst = (void * __user)arg;
+	uid_t src[2];
+	int err;
+
+	read_lock(&filp->f_owner.lock);
+	src[0] = from_kuid(user_ns, filp->f_owner.uid);
+	src[1] = from_kuid(user_ns, filp->f_owner.euid);
+	read_unlock(&filp->f_owner.lock);
+
+	err  = put_user(src[0], &dst[0]);
+	err |= put_user(src[1], &dst[1]);
+
+	return err;
+}
+#else
+static int f_getowner_uids(struct file *filp, unsigned long arg)
+{
+	return -EINVAL;
+}
+#endif
+
 static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 		struct file *filp)
 {
@@ -396,6 +422,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	case F_SETOWN_EX:
 		err = f_setown_ex(filp, arg);
 		break;
+	case F_GETOWNER_UIDS:
+		err = f_getowner_uids(filp, arg);
+		break;
 	case F_GETSIG:
 		err = filp->f_owner.signum;
 		break;