On Tue, Dec 03, 2019 at 04:21:02PM -0500, Gabriel Krisman Bertazi wrote: > Eric Biggers <ebiggers@xxxxxxxxxx> writes: > > > On Tue, Dec 03, 2019 at 02:42:10PM -0500, Gabriel Krisman Bertazi wrote: > >> Gao Xiang <gaoxiang25@xxxxxxxxxx> writes: > > >> I think Daniel's approach of moving this into VFS is the simplest way to > >> actually solve the issue, instead of extending and duplicating a lot of > >> functionality into filesystem hooks to support the possible mixes of > >> case-insensitive, overlayfs and fscrypt. > >> > > > > I think we can actually get everything we want using dentry_operations only, > > since the filesystem can set ->d_op during ->lookup() (like what is done for > > encrypted filenames now) rather than at dentry allocation time. And fs/crypto/ > > can export fscrypt_d_revalidate() rather than setting ->d_op itself. > > Problem is, differently from fscrypt, case-insensitive uses the d_hash() > hook and for a lookup, we actually use > dentry->d_parent->d_ops->d_hash(). Which works well, until you are flipping the > casefold flag. Then the dentry already exists and you need to modify > the d_ops on the fly, which I couldn't find precedent anywhere. I tried > invalidating the dentry whenever we flip the flag, but then if it has > negative dentries as children,I wasn't able to reliably invalidate it, > and that's when I reached the limit of my knowledge in VFS. In > particular, in every attempt I made to implement it like this, I was > able to race and do a case-insensitive lookup on a directory that was > just made case sensitive. > > I'm not saying there isn't a way. But it is a bit harder than this > proposal. I tried it already and still didn't manage to make it work. > Maybe someone who better understands vfs. Yes you're right, I forgot that for ->d_hash() and ->d_compare() it's actually the parent's directory dentry_operations that are used. > > > It's definitely ugly to have to handle the 3 cases of encrypt, casefold, and > > encrypt+casefold separately -- and this will need to be duplicated for each > > filesystem. But we do have to weigh that against adding additional complexity > > and overhead to the VFS for everyone. If we do go with the VFS changes, please > > try to make them as simple and unobtrusive as possible. > > Well, it is just not case-insensitive+fscrypt. Also overlayfs > there. Probably more. So we have much more cases. I understand the VFS > changes need to be very well thought, but when I worked on this it > started to look a more correct solution than using the hooks. Well the point of my proof-of-concept patch having separate ext4_ci_dentry_ops, ext4_encrypted_dentry_ops, and ext4_encrypted_ci_dentry_ops is supposed to be for overlayfs support -- since overlayfs requires that some operations are not present. If we didn't need overlayfs support, we could just use a single ext4_dentry_ops for all dentries instead. I think we could still support fscrypt, casefold, fscrypt+casefold, and fscrypt+overlayfs with dentry_operations only. It's casefold+overlayfs that's the biggest problem, due to the possibility of the casefold flag being set on a directory later as you pointed out. - Eric
