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

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Message-ID: <20170104233550.oy7nzc3rxppmejbk@thunk.org>
Date:   Wed, 4 Jan 2017 18:35:50 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     "zhangyi (F)" <yi.zhang@...wei.com>, Valdis.Kletnieks@...edu,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, adilger.kernel@...ger.ca
Subject: Re: [RFC PATCH] ext4: increase the protection of drop nlink and ext4
 inode destroy

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