[PATCH v9 2/4] fs: Add standard casefolding support

[Eric Biggers]

On Thu, Jul 02, 2020 at 06:01:37PM -0700, Daniel Rosenberg wrote:
> On Tue, Jun 23, 2020 at 10:57 PM Eric Biggers <ebiggers at 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.

Ideally yes, but currently the 'struct dentry' isn't always available.  See how
fscrypt_setup_filename(), f2fs_setup_filename(), f2fs_find_entry(),
ext4_find_entry(), etc. take a 'struct qstr', not a 'struct dentry'.

At some point we should fix that by passing down the dentry whenever it's
available, so that we reliably know whether the name is a no-key name or not.

So even my new f2fs code is still racy.  But it at least handles each filename
in a consistent way within each directory operation.  In comparison, your
proposed ext4 code can treat a filename as a no-key name while matching one
dir_entry and then as a regular filename while matching the next.  I think the
f2fs way is more on the right track, both correctness-wise and efficiency-wise.

- Eric