linux-ext4 - Re: [PATCH] ext4: fix reading new encrypted symlinks on no-journal filesystems

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Message-ID: <20161121231924.GG30672@google.com>
Date:   Mon, 21 Nov 2016 15:19:24 -0800
From:   Eric Biggers <ebiggers@...gle.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     linux-ext4 <linux-ext4@...r.kernel.org>,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH] ext4: fix reading new encrypted symlinks on no-journal
 filesystems

On Fri, Nov 18, 2016 at 02:52:22PM -0700, Andreas Dilger wrote:
> 
> > Yes, this would be a much nicer way to detect fast symlinks.
> > 
> > The only thing I'd be concerned about is the possibility of pre-existing
> > "slow" symlinks that actually have targets short enough to be "fast"
> > symlinks, perhaps in filesystems created by old drivers or by external
> > tools.  If such links happened to work before, then a change to check
> > i_size would break them.
> > 
> > This may not be an issue in practice.  I checked some old ext4 versions,
> > ext2 from Linux 0.99.7, e2fsprogs, Android's ext4_utils, and FreeBSD's
> > ext2 driver.
> > They all create "fast" symlinks if the length of the symlink target length
> > excluding the terminating null (i_size) is < 60.
> 
> I did a similar analysis with similar results.
> 

Ted, what would you say about Andreas' suggestion to use i_size to distinguish
fast symlinks from slow symlinks?

It looks like this was discussed some years ago but the discussion died out and
no change was made: see https://www.spinics.net/lists/linux-ext4/msg05693.html

Given the investigation I did it seems it would very likely be safe, but we can
never be 100% sure it won't break some obscure tool or (version of a tool) to
create symlinks on ext2/ext3/ext4 filesystems we don't know about.

Eric
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