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From:   Daniel Rosenberg <drosen@google.com>
Date:   Thu, 2 Jul 2020 18:01:37 -0700
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Subject: Re: [PATCH v9 2/4] fs: Add standard casefolding support
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     "Theodore Ts'o" <tytso@mit.edu>, linux-ext4@vger.kernel.org,
        Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <chao@kernel.org>,
        linux-f2fs-devel@lists.sourceforge.net,
        linux-fscrypt@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Richard Weinberger <richard@nod.at>,
        linux-mtd@lists.infradead.org,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        kernel-team@android.com
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On Tue, Jun 23, 2020 at 10:57 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> Note that the '!IS_ENCRYPTED(dir) || fscrypt_has_encryption_key(dir)' check can
> be racy, because a process can be looking up a no-key token in a directory while
> concurrently another process initializes the directory's ->i_crypt_info, causing
> fscrypt_has_encryption_key(dir) to suddenly start returning true.
>
> In my rework of filename handling in f2fs, I actually ended up removing all
> calls to needs_casefold(), thus avoiding this race.  f2fs now decides whether
> the name is going to need casefolding early on, in __f2fs_setup_filename(),
> where it knows in a race-free way whether the filename is a no-key token or not.
>
> Perhaps ext4 should work the same way?  It did look like there would be some
> extra complexity due to how the ext4 directory hashing works in comparison to
> f2fs's, but I haven't had a chance to properly investigate it.
>
> - Eric

Hm. I think I should be able to just check for DCACHE_ENCRYPTED_NAME
in the dentry here, right? I'm just trying to avoid casefolding the
no-key token, and that flag should indicate that.
I'll see if I can rework the ext4 patches to not need needs_casefold
as well, since then there'd be no need to export it.
-Daniel