[PATCH] use proper mnt_want_write calls for handle ioctls

[Christoph Hellwig <hch@xxxxxx>]

Since 2.6.26 all writes to filesystems need to be enclosed by a
mnt_want_write / mnt_drop_write pair instead of checking for IS_RDONLY.

XFs was updated for this in mainline but the changes to xfs_ioctl.c
were never megred back into the CVS tree.

The original commit introducing this was:

commit 42a74f206b914db13ee1f5ae932dcd91a77c8579
Author: Dave Hansen <haveblue@xxxxxxxxxx>
Date:   Fri Feb 15 14:37:46 2008 -0800

    [PATCH] r/o bind mounts: elevate write count for ioctls()

Index: linux-2.6-xfs/fs/xfs/linux-2.6/xfs_ioctl.c
===================================================================
--- linux-2.6-xfs.orig/fs/xfs/linux-2.6/xfs_ioctl.c     2008-08-14 
14:54:53.000000000 -0300
+++ linux-2.6-xfs/fs/xfs/linux-2.6/xfs_ioctl.c  2008-08-14 15:15:49.000000000 
-0300
@@ -543,8 +543,6 @@ xfs_attrmulti_attr_set(
        char                    *kbuf;
        int                     error = EFAULT;
 
-       if (IS_RDONLY(inode))
-               return -EROFS;
        if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
                return EPERM;
        if (len > XATTR_SIZE_MAX)
@@ -570,8 +568,6 @@ xfs_attrmulti_attr_remove(
        char                    *name,
        __uint32_t              flags)
 {
-       if (IS_RDONLY(inode))
-               return -EROFS;
        if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
                return EPERM;
        return xfs_attr_remove(XFS_I(inode), name, flags);
@@ -581,6 +577,7 @@ STATIC int
 xfs_attrmulti_by_handle(
        xfs_mount_t             *mp,
        void                    __user *arg,
+       struct file             *parfilp,
        struct inode            *parinode)
 {
        int                     error;
@@ -634,13 +631,21 @@ xfs_attrmulti_by_handle(
                                        &ops[i].am_length, ops[i].am_flags);
                        break;
                case ATTR_OP_SET:
+                       ops[i].am_error = mnt_want_write(parfilp->f_path.mnt);
+                       if (ops[i].am_error)
+                               break;
                        ops[i].am_error = xfs_attrmulti_attr_set(inode,
                                        attr_name, ops[i].am_attrvalue,
                                        ops[i].am_length, ops[i].am_flags);
+                       mnt_drop_write(parfilp->f_path.mnt);
                        break;
                case ATTR_OP_REMOVE:
+                       ops[i].am_error = mnt_want_write(parfilp->f_path.mnt);
+                       if (ops[i].am_error)
+                               break;
                        ops[i].am_error = xfs_attrmulti_attr_remove(inode,
                                        attr_name, ops[i].am_flags);
+                       mnt_drop_write(parfilp->f_path.mnt);
                        break;
                default:
                        ops[i].am_error = EINVAL;
@@ -1431,7 +1436,7 @@ xfs_ioctl(
                return xfs_attrlist_by_handle(mp, arg, inode);
 
        case XFS_IOC_ATTRMULTI_BY_HANDLE:
-               return xfs_attrmulti_by_handle(mp, arg, inode);
+               return xfs_attrmulti_by_handle(mp, arg, filp, inode);
 
        case XFS_IOC_SWAPEXT: {
                error = xfs_swapext((struct xfs_swapext __user *)arg);