ext4: no need to remove extent if len is 0 in ext4_es_remove_extent()

len is 0 means no extent needs to be removed, so return immediately.
Otherwise it could trigger the following BUG_ON()

	436	end = offset + len - 1;
	437	BUG_ON(end < offset);

This could be reproduced by a simple truncate(1) command by an
unprivileged user

	truncate -s $(($((2**32 - 1)) * 4096)) /mnt/ext4/testfile

The same is true for __es_insert_extent().

Patched kernel passed xfstests regression test.

Also remove comments about EXT4_I(inode)->i_es_lock, this rwlock
isn't hold by callers.

Cc: Zheng Liu <wenqing.lz@taobao.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
---
 fs/ext4/extents_status.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

On Fri, Feb 22, 2013 at 01:34:03PM +0800, Eryu Guan wrote:
> len is 0 means no extent needs to be removed, so return immediately.
> Otherwise it could trigger the following BUG_ON()
> 
> 	436	end = offset + len - 1;
> 	437	BUG_ON(end < offset);
> 
> This could be reproduced by a simple truncate(1) command by an
> unprivileged user
> 
> 	truncate -s $(($((2**32 - 1)) * 4096)) /mnt/ext4/testfile
> 
> The same is true for __es_insert_extent().
> 
> Patched kernel passed xfstests regression test.
> 
> Also remove comments about EXT4_I(inode)->i_es_lock, this rwlock
> isn't hold by callers.
> 
> Cc: Zheng Liu <wenqing.lz@taobao.com>
> Cc: "Theodore Ts'o" <tytso@mit.edu>
> Signed-off-by: Eryu Guan <guaneryu@gmail.com>

Thanks for fixing it.

Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>

Regards,
                                                - Zheng

> ---
>  fs/ext4/extents_status.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
> index 564d981..3ac09ca 100644
> --- a/fs/ext4/extents_status.c
> +++ b/fs/ext4/extents_status.c
> @@ -328,6 +328,9 @@ static int __es_insert_extent(struct ext4_es_tree *tree, ext4_lblk_t offset,
>  	struct extent_status *es;
>  	ext4_lblk_t end = offset + len - 1;
>  
> +	if (!len)
> +		return 0;
> +
>  	BUG_ON(end < offset);
>  	es = tree->cache_es;
>  	if (es && offset == (extent_status_end(es) + 1)) {
> @@ -386,7 +389,6 @@ out:
>  
>  /*
>   * ext4_es_insert_extent() adds a space to a delayed extent tree.
> - * Caller holds inode->i_es_lock.
>   *
>   * ext4_es_insert_extent is called by ext4_da_write_begin and
>   * ext4_es_remove_extent.
> @@ -415,7 +417,6 @@ int ext4_es_insert_extent(struct inode *inode, ext4_lblk_t offset,
>  
>  /*
>   * ext4_es_remove_extent() removes a space from a delayed extent tree.
> - * Caller holds inode->i_es_lock.
>   *
>   * Return 0 on success, error code on failure.
>   */
> @@ -433,6 +434,9 @@ int ext4_es_remove_extent(struct inode *inode, ext4_lblk_t offset,
>  	es_debug("remove [%u/%u) from extent status tree of inode %lu\n",
>  		 offset, len, inode->i_ino);
>  
> +	if (!len)
> +		return err;
> +
>  	end = offset + len - 1;
>  	BUG_ON(end < offset);
>  	write_lock(&EXT4_I(inode)->i_es_lock);
> -- 
> 1.8.1.2
> 
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