Commit d59729f4 authored Jul 30, 2011 by Theodore Ts'o

ext4: fix races in ext4_sync_parent()



Fix problems if fsync() races against a rename of a parent directory
as pointed out by Al Viro in his own inimitable way:

>While we are at it, could somebody please explain what the hell is ext4
>doing in
>static int ext4_sync_parent(struct inode *inode)
>{
>        struct writeback_control wbc;
>        struct dentry *dentry = NULL;
>        int ret = 0;
>
>        while (inode && ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
>                ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
>                dentry = list_entry(inode->i_dentry.next,
>                                    struct dentry, d_alias);
>                if (!dentry || !dentry->d_parent || !dentry->d_parent->d_inode)
>                        break;
>                inode = dentry->d_parent->d_inode;
>                ret = sync_mapping_buffers(inode->i_mapping);
>                ...
>Note that dentry obviously can't be NULL there.  dentry->d_parent is never
>NULL.  And dentry->d_parent would better not be negative, for crying out
>loud!  What's worse, there's no guarantees that dentry->d_parent will
>remain our parent over that sync_mapping_buffers() *and* that inode won't
>just be freed under us (after rename() and memory pressure leading to
>eviction of what used to be our dentry->d_parent)......

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

parent 29ae07b7

fs/ext4/fsync.c

+21 −5

Original line number	Diff line number	Diff line
		@@ -129,15 +129,30 @@ static int ext4_sync_parent(struct inode *inode)
		{
		struct writeback_control wbc;
		struct dentry *dentry = NULL;
		struct inode *next;
		int ret = 0;

		while (inode && ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
		if (!ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY))
		return 0;
		inode = igrab(inode);
		while (ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
		ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
		dentry = list_entry(inode->i_dentry.next,
		dentry = NULL;
		spin_lock(&inode->i_lock);
		if (!list_empty(&inode->i_dentry)) {
		dentry = list_first_entry(&inode->i_dentry,
		struct dentry, d_alias);
		if (!dentry \|\| !dentry->d_parent \|\| !dentry->d_parent->d_inode)
		dget(dentry);
		}
		spin_unlock(&inode->i_lock);
		if (!dentry)
		break;
		next = igrab(dentry->d_parent->d_inode);
		dput(dentry);
		if (!next)
		break;
		inode = dentry->d_parent->d_inode;
		iput(inode);
		inode = next;
		ret = sync_mapping_buffers(inode->i_mapping);
		if (ret)
		break;
		@@ -148,6 +163,7 @@ static int ext4_sync_parent(struct inode *inode)
		if (ret)
		break;
		}
		iput(inode);
		return ret;
		}