linux-ext4 - Re: [PATCH v7 2/8] fs: Add standard casefolding support

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Date:   Mon, 10 Feb 2020 23:42:07 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Daniel Rosenberg <drosen@...gle.com>
Cc:     Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        Eric Biggers <ebiggers@...nel.org>,
        linux-fscrypt@...r.kernel.org, Richard Weinberger <richard@....at>,
        linux-mtd@...ts.infradead.org,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 2/8] fs: Add standard casefolding support

On Mon, Feb 10, 2020 at 03:11:13PM -0800, Daniel Rosenberg wrote:
> On Fri, Feb 7, 2020 at 6:12 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Fri, Feb 07, 2020 at 05:35:46PM -0800, Daniel Rosenberg wrote:
> >
> >
> > Again, is that safe in case when the contents of the string str points to
> > keeps changing under you?
> 
> I'm not sure what you mean. I thought it was safe to use the str and
> len passed into d_compare. Even if it gets changed under RCU
> conditions I thought there was some code to ensure that the name/len
> pair passed in is consistent, and any other inconsistencies would get
> caught by d_seq later. Are there unsafe code paths that can follow?

If you ever fetch the same byte twice, you might see different values.
You need a fairly careful use of READ_ONCE() or equivalents to make
sure that you don't get screwed over by that.

Sure, ->d_seq mismatch will throw the result out, but you need to make
sure you won't oops/step on uninitialized memory/etc. in process.

It's not impossible to get right, but it's not trivial and you need all
code working with that much more careful than normal for string handling.