linux-kernel - Re: [Ecryptfs-devel] [PATCH] ecryptfs: some inode attrs, and a question

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Date:	Fri, 16 Jan 2009 16:36:06 +0900
From:	hooanon05@...oo.co.jp
To:	Tyler Hicks <tyhicks@...ux.vnet.ibm.com>
Cc:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org, ecryptfs-devel@...ts.launchpad.net
Subject: Re: [Ecryptfs-devel] [PATCH] ecryptfs: some inode attrs, and a question 

Tyler Hicks:
> >> - The ecryptfs inode holds a reference to the lower inode, but doesn't=
> 
> >>   increment the reference counter. When a user sets inotify to the
> >>   ecryptfs inode, it may live without the corresponding dentry. In thi=
> s
> >>   case the referecen to the lower inode may be broken.
> >>   This patch maintains the reference of the lower inode.
> 
> Is this a problem that you've experienced or something you found during
> a code review?  How can it be reproduced?  We get a reference to the
> lower inode with the igrab() in ecryptfs_interpose() and put it back in
> ecryptfs_clear_inode().  This part of the patch seems to just increment
> the ref counts again.

I could confirm the igrab() in ecryptfs_interpose(), so touching i_count
in my patch are unnecessary.
Although I don't remember the detail, I have experienced a trouble (and
I wrote the patch). If I can recall the problem and find the
reproducible way, I will write again.
One thing I remember is, vfs_unlink() in ecryptfs_unlink() made
lower_dentry->d_inode NULL unexpectedly.

I guess the problem was related to the i_count including my fixes for
ecryptfs_unlink() and ecryptfs_link(). Current ecryptfs_link() calls
ecryptfs_interpose(), but obviously the inode is not I_NEW and the
incremented i_count is decremented again (finally unchanged).
The current unnecessary d_drop()s may help hiding the problem, but I am
not sure.

> I see that do_unlinkat does this, but since eCryptfs already holds these
> references, I don't think that we need to do it here.

Whether the counter is one or more is more important than to hold or not
to hold simply.

> This part of the patch is valid, nice catch!

I am happy that my patch was not totally useless. :-)

And one more suggestion. It is better to set f_type in
ecryptfs_statfs(), and move ECRYPTFS_SUPER_MAGIC under include/linux/,
in order to be usable from userspace (or other modules).

J. R. Okajima
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