Re: [RFC][PATCH v2] ext4: add expose_stale_data flag in fallocate

[Ric Wheeler <rwheeler@redhat.com>]

On 05/03/2012 12:14 AM, Zheng Liu wrote:
> Hi all,
>
> Here is the v2 of FALLOC_FL_NO_HIDE_STALE in fallocate.  Now no new flag is
> added into vfs in order to reduce the impacts and avoid a huge security hole.
> The application cannot call fallocate with a new flag to create an unwritten
> extent.  It needs to call ioctl to enable/disable this feature.  Meanwhile, in
> ioctl, filesystem will check CAP_SYS_RAWIO to ensure that the user has a
> privilege to switch on/off it.  Currently, I only implement it in ext4.

Hi Zheng,

I thought that we had pretty much decided to try and fix the ext4 performance 
impact, not pursue this?

Thanks!

Ric

>
> Even though I try to reduce its impact, this feature still brings a security
> hole.  So the application must ensure that it initializes an unwritten extent
> by itself before reading it, and it is used in a limited environment.
>
> v1 ->  v2:
> * remove FALLOC_FL_NO_HIDE_STALE flag in vfs
> * add 'i_expose_stale_data' in ext4 to enable/disable it
>
> Regards,
> Zheng
>
>  From 530045b4a1f75df5afd40c0e20c89917f97d7d5a Mon Sep 17 00:00:00 2001
> From: Zheng Liu<wenqing.lz@taobao.com>
> Date: Thu, 3 May 2012 11:21:44 +0800
> Subject: [RFC][PATCH v2] ext4: add expose_stale_data flag in fallocate
>
> We can use fallocate to preallocate sequential blocks.  But these extents are
> uninitialized.  So when the application does a write, filesystem will
> initialized these unwritten extents and it brings a huge overhead in some
> cases.
>
> This patch adds a new flag in inode to indicate whether initialize an unwritten
> extent or not.  This flag is enable/disable within ioctl that switch on/off this
> feature.  The application must call ioctl to enable this feature before it tries
> to preallocate some blocks.
>
> Obviously, this feature brings a huge security hole.  The application must
> guarantee to initialize this file by itself before reading it at the same
> offset.  So the application *MUST* use it carefully.
>
> CC: Ric Wheeler<rwheeler@redhat.com>
> CC: Eric Sandeen<sandeen@redhat.com>
> CC: Ted Ts'o tytso@mit.edu>
> CC: Dave Chinner<david@fromorbit.com>
> CC: Lukas Czerner<lczerner@redhat.com>
> CC: Andreas Dilger<adilger@whamcloud.com>
> CC: Szabolcs Szakacsits<szaka@tuxera.com>
> Signed-off-by: Zheng Liu<wenqing.lz@taobao.com>
> ---
>   fs/ext4/ext4.h    |    5 +++++
>   fs/ext4/extents.c |    6 +++++-
>   fs/ext4/ioctl.c   |   43 +++++++++++++++++++++++++++++++++++++++++++
>   fs/ext4/super.c   |    1 +
>   4 files changed, 54 insertions(+), 1 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 0e01e90..f56caf0 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -593,6 +593,8 @@ enum {
>   #define EXT4_IOC_ALLOC_DA_BLKS		_IO('f', 12)
>   #define EXT4_IOC_MOVE_EXT		_IOWR('f', 15, struct move_extent)
>   #define EXT4_IOC_RESIZE_FS		_IOW('f', 16, __u64)
> +#define EXT4_IOC_GET_EXPOSE_STALE	_IOR('f', 17, int)
> +#define EXT4_IOC_SET_EXPOSE_STALE	_IOW('f', 18, int)
>
>   #if defined(__KERNEL__)&&  defined(CONFIG_COMPAT)
>   /*
> @@ -908,6 +910,9 @@ struct ext4_inode_info {
>   	 */
>   	tid_t i_sync_tid;
>   	tid_t i_datasync_tid;
> +
> +	/* expose stale data in creating a new extent */
> +	int i_expose_stale_data;
>   };
>
>   /*
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index abcdeab..14f54f1 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -4269,6 +4269,7 @@ static void ext4_falloc_update_inode(struct inode *inode,
>   long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
>   {
>   	struct inode *inode = file->f_path.dentry->d_inode;
> +	struct ext4_inode_info *ei = EXT4_I(inode);
>   	handle_t *handle;
>   	loff_t new_size;
>   	unsigned int max_blocks;
> @@ -4312,7 +4313,10 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
>   		trace_ext4_fallocate_exit(inode, offset, max_blocks, ret);
>   		return ret;
>   	}
> -	flags = EXT4_GET_BLOCKS_CREATE_UNINIT_EXT;
> +	if (ei->i_expose_stale_data)
> +		flags = EXT4_GET_BLOCKS_CREATE;
> +	else
> +		flags = EXT4_GET_BLOCKS_CREATE_UNINIT_EXT;
>   	if (mode&  FALLOC_FL_KEEP_SIZE)
>   		flags |= EXT4_GET_BLOCKS_KEEP_SIZE;
>   	/*
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index 6eee255..a37db8e 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -432,6 +432,47 @@ resizefs_out:
>   		return 0;
>   	}
>
> +	case EXT4_IOC_GET_EXPOSE_STALE: {
> +		int enable;
> +
> +		/* security check */
> +		if (!capable(CAP_SYS_RAWIO))
> +			return -EPERM;
> +
> +		/*
> +		 * currently only extent-based files support (pre)allocate with
> +		 * EXPOSE_STALE_DATA flag
> +		 */
> +		if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
> +			return -EOPNOTSUPP;
> +
> +		enable = ei->i_expose_stale_data;
> +
> +		return put_user(enable, (int __user *) arg);
> +	}
> +
> +	case EXT4_IOC_SET_EXPOSE_STALE: {
> +		int enable;
> +
> +		/* security check */
> +		if (!capable(CAP_SYS_RAWIO))
> +			return -EPERM;
> +
> +		/*
> +		 * currently only extent-based files support (pre)allocate with
> +		 * EXPOSE_STALE_DATA flag
> +		 */
> +		if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
> +			return -EOPNOTSUPP;
> +
> +		if (get_user(enable, (int __user *) arg))
> +			return -EFAULT;
> +
> +		ei->i_expose_stale_data = enable;
> +
> +		return 0;
> +	}
> +
>   	default:
>   		return -ENOTTY;
>   	}
> @@ -495,6 +536,8 @@ long ext4_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
>   	case EXT4_IOC_MOVE_EXT:
>   	case FITRIM:
>   	case EXT4_IOC_RESIZE_FS:
> +	case EXT4_IOC_GET_EXPOSE_STALE:
> +	case EXT4_IOC_SET_EXPOSE_STALE:
>   		break;
>   	default:
>   		return -ENOIOCTLCMD;
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index e1fb1d5..6de2db0 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -943,6 +943,7 @@ static struct inode *ext4_alloc_inode(struct super_block *sb)
>   	ei->i_datasync_tid = 0;
>   	atomic_set(&ei->i_ioend_count, 0);
>   	atomic_set(&ei->i_aiodio_unwritten, 0);
> +	ei->i_expose_stale_data = 0;
>
>   	return&ei->vfs_inode;
>   }