Re: [PATCH v2 4/7] libfs: Support revalidation of encrypted case-insensitive dentries

Eric Biggers
2023-07-18T22:10:48Z

On Tue, Jul 18, 2023 at 03:34:13PM -0400, Gabriel Krisman Bertazi wrote:
> Eric Biggers <ebiggers@kernel.org> writes:
>
> > On Fri, Apr 21, 2023 at 08:03:07PM -0400, Gabriel Krisman Bertazi wrote:
> >> From: Gabriel Krisman Bertazi <krisman@collabora.com>
> >>
> >> Preserve the existing behavior for encrypted directories, by rejecting
> >> negative dentries of encrypted+casefolded directories. This allows
> >> generic_ci_d_revalidate to be used by filesystems with both features
> >> enabled, as long as the directory is either casefolded or encrypted, but
> >> not both at the same time.
> >>
> >> Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
> >> ---
> >> fs/libfs.c | 8 ++++++--
> >> 1 file changed, 6 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/fs/libfs.c b/fs/libfs.c
> >> index f8881e29c5d5..0886044db593 100644
> >> --- a/fs/libfs.c
> >> +++ b/fs/libfs.c
> >> @@ -1478,6 +1478,9 @@ static inline int generic_ci_d_revalidate(struct dentry *dentry,
> >> const struct inode *dir = READ_ONCE(parent->d_inode);
> >>
> >> if (dir && needs_casefold(dir)) {
> >> + if (IS_ENCRYPTED(dir))
> >> + return 0;
> >> +
> >
> > Why not allow negative dentries in case-insensitive encrypted directories?
> > I can't think any reason why it wouldn't just work.
>
> TBH, I'm not familiar with the details of combined encrypted+casefold
> support to be confident it works.This patch preserves the current
> behavior of disabling them for encrypted+casefold directories.

Not allowing that combination reduces the usefulness of this patchset.
Note that Android's use of casefold is always combined with encryption.

> I suspect it might require extra work that I'm not focusing on this
> patchset. For instance, what should be the order of
> fscrypt_d_revalidate and the checks I'm adding here?

Why would order matter? If either "feature" wants the dentry to be invalidated,
then the dentry gets invalidated.

> Note we will start creating negative dentries in casefold directories after
> patch 6/7, so unless we disable it here, we will start calling
> fscrypt_d_revalidate for negative+casefold.

fscrypt_d_revalidate() only cares about the DCACHE_NOKEY_NAME flag, so that's
not a problem.

>
> Should I just drop this hunk? Unless you are confident it works as is, I
> prefer to add this support in stages and keep negative dentries of
> encrypted+casefold directories disabled for now.

Unless I'm missing something, I think you're overcomplicating it. It should
just work if you don't go out of your way to prohibit this case. I.e., just
don't add the IS_ENCRYPTED(dir) check to generic_ci_d_revalidate().

- Eric