Btrfs: get write access when removing a device

Steps to reproduce:
 # mkfs.btrfs -d single -m single <disk0> <disk1>
 # mount -o ro <disk0> <mnt0>
 # mount -o ro <disk0> <mnt1>
 # mount -o remount,rw <mnt0>
 # umount <mnt0>
 # btrfs device delete <disk1> <mnt1>

We can remove a device from a R/O filesystem. The reason is that we just check
the R/O flag of the super block object. It is not enough, because the kernel
may set the R/O flag only for the mount point. We need invoke

	mnt_want_write_file()

to do a full check.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 2be49b4c82d6..ee36009f8aa1 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2270,20 +2270,23 @@ out:
         return ret;
 }
 
-static long btrfs_ioctl_rm_dev(struct btrfs_root *root, void __user *arg)
+static long btrfs_ioctl_rm_dev(struct file *file, void __user *arg)
 {
+        struct btrfs_root *root = BTRFS_I(fdentry(file)->d_inode)->root;
         struct btrfs_ioctl_vol_args *vol_args;
         int ret;
 
         if (!capable(CAP_SYS_ADMIN))
                 return -EPERM;
 
-        if (root->fs_info->sb->s_flags & MS_RDONLY)
-                return -EROFS;
+        ret = mnt_want_write_file(file);
+        if (ret)
+                return ret;
 
         if (atomic_xchg(&root->fs_info->mutually_exclusive_operation_running,
                         1)) {
                 pr_info("btrfs: dev add/delete/balance/replace/resize operation in progress\n");
+                mnt_drop_write_file(file);
                 return -EINPROGRESS;
         }
 
@@ -2300,6 +2303,7 @@ static long btrfs_ioctl_rm_dev(struct btrfs_root *root, void __user *arg)
         kfree(vol_args);
 out:
         mutex_unlock(&root->fs_info->volume_mutex);
+        mnt_drop_write_file(file);
         atomic_set(&root->fs_info->mutually_exclusive_operation_running, 0);
         return ret;
 }
@@ -3842,7 +3846,7 @@ long btrfs_ioctl(struct file *file, unsigned int
         case BTRFS_IOC_ADD_DEV:
                 return btrfs_ioctl_add_dev(root, argp);
         case BTRFS_IOC_RM_DEV:
-                return btrfs_ioctl_rm_dev(root, argp);
+                return btrfs_ioctl_rm_dev(file, argp);
         case BTRFS_IOC_FS_INFO:
                 return btrfs_ioctl_fs_info(root, argp);
         case BTRFS_IOC_DEV_INFO: