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Date:   Mon, 25 Mar 2019 21:14:06 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     syzbot <syzbot+7a8ba368b47fdefca61e@syzkaller.appspotmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: KASAN: use-after-free Read in path_lookupat
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On Mon, Mar 25, 2019 at 12:18:02PM -0700, Linus Torvalds wrote:
> COMPLETELY UNTESTED. And no filesystems converted to actually use the
> new rcu_destroy_inode() thing.
> 
> Hmm?

Maybe, but we really need to come up with sane documentation on the
entire drop_inode/evict_inode/destroy_inode/rcu_destroy_inode
group ;-/  And I want to understand the writeback-related issues
in ocfs2 and f2fs - the current kludges in those smell fishy.

>  	if (unlikely(inode_init_always(sb, inode))) {
> -		if (inode->i_sb->s_op->destroy_inode)
> +		if (inode->i_sb->s_op->destroy_inode) {
>  			inode->i_sb->s_op->destroy_inode(inode);
> -		else
> +			if (!inode->i_sb->s_op->rcu_destroy_inode)
> +				return NULL;
> +		}
> +		if (!inode->i_sb->s_op->rcu_destroy_inode ||
> +		    !inode->i_sb->s_op->rcu_destroy_inode(inode))
>  			kmem_cache_free(inode_cachep, inode);

ITYM  i_callback(inode); here, possibly suitably renamed.