Re: [RFC PATCH] ext4: increase the protection of drop nlink and ext4 inode destroy

[zhangyi (F)]

on 2017/1/5 7:35, Theodore Ts'o wrote:
> On Wed, Jan 04, 2017 at 01:54:24PM -0800, Darrick J. Wong wrote:
>>
>> if (inode->i_nlink == 0) {
>> 	ext4_warning_inode(inode, "nlink is already 0");
>> 	return;
>> }
> 
> We can't do that because the place where Zhangyi is proposing to
> change is in fs/inode.c:drop_nlink(), so we can't add a call to
> ext4_error() or ext4_warning().
> 
> So how exactly how did we get into this state?  When we read the inode
> into memory, if i_nlink is zero, we declare the file system as
> corrupted immediately.
> 
> So I assume this is happening the on-disk i_links_count (which is read
> into inode->i_nlink) was too low.  So I think the way we should be
> handling this is in unlink and rename, before we let i_nlink drop to
> zero, we need to check to see if there are other dcache entries
> pointing at the inode.  If so, we need to call ext4_error(), and in
> the errors=continue case, return EFSCORRUPTED (aka EUCLEAN).
> 
>     		    	  	 	      - Ted
> 

Hi Theodore:

The i_nlink underflow and memory corruption problem on ext4fs remains inconclusive.

You suggest we can check dcache entries when i_nlink drop to zero in unlink and
rename. But I think it may still have some problems, assume the following situation:

(1) The file we want to unlink have many hard links, but only one dcache entry in memory.
(2) open this file, but it's inode->i_nlink read from disk was 1 (too low).
(3) some one call rename and drop it's i_nlink to zero.
(4) it's inode is still in use and do not destroy (not closed), at the same time,
    some others open it's hard link and create a dcache entry.
(5) call rename again and it's i_nlink will still underflow and cause memory corruption.

For simplicity, I think we can add underflow protection in ext4_rename or
drop_nlink as V2 and V3 patch wrote. What do you think?

yi zhang