Received-SPF: pass (google.com: domain of linux-ext4-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 18 May 2022 20:43:07 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Gabriel Krisman Bertazi <krisman@collabora.com>
Cc:     tytso@mit.edu, adilger.kernel@dilger.ca, jaegeuk@kernel.org,
        linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net,
        kernel@collabora.com
Subject: Re: [PATCH v6 4/8] ext4: Reuse generic_ci_match for ci comparisons
Message-ID: <YoW8yx9Fw9Rwiaja@sol.localdomain>
References: <20220519014044.508099-1-krisman@collabora.com>
 <20220519014044.508099-5-krisman@collabora.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220519014044.508099-5-krisman@collabora.com>
Precedence: bulk

On Wed, May 18, 2022 at 09:40:40PM -0400, Gabriel Krisman Bertazi wrote:
> Instead of reimplementing ext4_match_ci, use the new libfs helper.
> 
> It should be fine to drop the fname->cf_name in the encrypted directory
> case for the hash verification optimization because the only two ways
> for fname->cf_name to be NULL on a case-insensitive lookup is
> 
>  (1) if name under lookup has an invalid encoding and the FS is not in
>  strict mode; or
> 
>  (2) if the directory is encrypted and we don't have the
>  key.
> 
> For case (1), it doesn't matter, because the lookup hash will be
> generated with fname->usr_name, the same as the disk (fallback to
> invalid encoding behavior on !strict mode).  Case (2) is caught by the
> previous check (!IS_ENCRYPTED(parent) ||
> fscrypt_has_encryption_key(parent)), so we never reach this code.

The code actually can be reached in case (2), because the key could have been
added between ext4_fname_setup_ci_filename() and ext4_match().

I *think* your change doesn't make it any worse, since in such a case the name
comparison is going to be comparing a no-key name to a regular one, which will
very likely fail.  So adding an additional way for the match to fail seems fine.

It's hard to reason about, though.  f2fs does things in a much cleaner way, as
I've mentioned before, since it decides which type of match it wants at the
beginning, when initializing struct f2fs_filename.

- Eric