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

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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@xxxxxxxxxx>
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@xxxxxxxxxx>
CC: Eric Sandeen<sandeen@xxxxxxxxxx>
CC: Ted Ts'o tytso@xxxxxxx>
CC: Dave Chinner<david@xxxxxxxxxxxxx>
CC: Lukas Czerner<lczerner@xxxxxxxxxx>
CC: Andreas Dilger<adilger@xxxxxxxxxxxxx>
CC: Szabolcs Szakacsits<szaka@xxxxxxxxxx>
Signed-off-by: Zheng Liu<wenqing.lz@xxxxxxxxxx>
---
  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;
  }

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