[PATCH] autofs4: add ioctl to query unmountability

From: Ian Kent <raven@themaw.net>

Add ioctl to find out if autofs mount can be umounted.  When the daemon
discovers this it's past the point of no return.

fs/autofs4/root.c

@@ -673,6 +673,23 @@ static inline int autofs4_get_protosubver(struct autofs_sb_info *sbi, int *p)
 	return put_user(sbi->sub_version, p);
 }
 
+/*
+* Tells the daemon whether it can umount the autofs mount.
+*/
+static inline int autofs4_ask_umount(struct vfsmount *mnt, int *p)
+{
+	int status = 0;
+
+	if (may_umount(mnt) == 0)
+		status = 1;
+
+	DPRINTK(("autofs_ask_umount: returning %d\n", status));
+
+	status = put_user(status, p);
+
+	return status;
+}
+
 /* Identify autofs4_dentries - this is so we can tell if there's
    an extra dentry refcount or not.  We only hold a refcount on the
    dentry if its non-negative (ie, d_inode != NULL)
@@ -719,6 +736,9 @@ static int autofs4_root_ioctl(struct inode *inode, struct file *filp,
 	case AUTOFS_IOC_SETTIMEOUT:
 		return autofs4_get_set_timeout(sbi,(unsigned long *)arg);
 
+	case AUTOFS_IOC_ASKUMOUNT:
+		return autofs4_ask_umount(filp->f_vfsmnt, (int *) arg);
+
 	/* return a single thing to expire */
 	case AUTOFS_IOC_EXPIRE:
 		return autofs4_expire_run(inode->i_sb,filp->f_vfsmnt,sbi,

include/linux/auto_fs4.h

@@ -49,6 +49,7 @@ union autofs_packet_union {
 
 #define AUTOFS_IOC_EXPIRE_MULTI _IOW(0x93,0x66,int)
 #define AUTOFS_IOC_PROTOSUBVER  _IOR(0x93,0x67,int)
+#define AUTOFS_IOC_ASKUMOUNT	_IOR(0x93,0x70,int)
 
 
 #endif /* _LINUX_AUTO_FS4_H */