2011-10-25 15:33:49

by Andreas Dilger

Subject: [PATCH] ext4: avoid setting directory i_nlink to zero

If a directory with more than EXT4_LINK_MAX subdirectories, the nlink
count is set to 1. Subsequently, if any subdirectories are deleted,
ext4_dec_count() decrements the i_nlink count, which may go to 0
temporarily before being incremented back to 1.

While this is done under i_mutex, which prevents races for directory
and inode operations that check i_nlink, the temporary i_nlink == 0
case is exposed to userspace via stat() and similar calls that do not
hold i_mutex.

Instead, change the code to not decrement i_nlink count for any
directories that do not already have i_nlink larger than 2.

Reported-by: Cliff White <[email protected]>
Reviewed-by: Johann Lombardi <[email protected]>
Signed-off-by: Andreas Dilger <[email protected]>
---
fs/ext4/namei.c | 5 ++---
1 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 1c924fa..b13cd39 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1707,9 +1707,8 @@ static void ext4_inc_count(handle_t *handle, struct inode *inode)
*/
static void ext4_dec_count(handle_t *handle, struct inode *inode)
{
- drop_nlink(inode);
- if (S_ISDIR(inode->i_mode) && inode->i_nlink == 0)
- inc_nlink(inode);
+ if (!S_ISDIR(inode->i_mode) || inode->i_nlink > 2)
+ drop_nlink(inode);
}


--
1.7.3.4

2011-10-26 07:30:45

by Theodore Ts'o

Subject: Re: [PATCH] ext4: avoid setting directory i_nlink to zero

On Tue, Oct 25, 2011 at 09:33:05AM -0600, Andreas Dilger wrote:
> If a directory with more than EXT4_LINK_MAX subdirectories, the nlink
> count is set to 1. Subsequently, if any subdirectories are deleted,
> ext4_dec_count() decrements the i_nlink count, which may go to 0
> temporarily before being incremented back to 1.
>
> While this is done under i_mutex, which prevents races for directory
> and inode operations that check i_nlink, the temporary i_nlink == 0
> case is exposed to userspace via stat() and similar calls that do not
> hold i_mutex.
>
> Instead, change the code to not decrement i_nlink count for any
> directories that do not already have i_nlink larger than 2.
>
> Reported-by: Cliff White <[email protected]>
> Reviewed-by: Johann Lombardi <[email protected]>
> Signed-off-by: Andreas Dilger <[email protected]>


Applied, thanks.

- Ted