On Fri, Apr 30, 2021 at 6:45 AM Jeff Layton <jlayton@redhat.com> wrote:
The client can stuff that into the xattr blob when creating a new inode, and the MDS can scrape it out of that and move the data into the correct field in the inode. A setxattr on this field would update the new field too. It's an ugly interface, but shouldn't be too bad to handle and we have some precedent for this sort of thing.
The rules for handling the new field in the client would be a bit weird though. We'll need to allow it to reading the fscrypt_ctx part without any caps (since that should be static once it's set), but the size handling needs to be under the same caps as the traditional size field (Is that Fsx? The rules for this are never quite clear to me.)
Would it be better to have two different fields here -- fscrypt_auth and fscrypt_file? Or maybe, fscrypt_static/_dynamic? We don't necessarily need to keep all of this info together, but it seemed neater that way.
I'm not seeing a reason to split the struct.
What caps should this live under? We have different requirements for different parts of the struct.
1) fscrypt context: needs to be always available, especially when an inode is initially instantiated, though it should almost always be static once it's set. The exception is that an empty directory can grow a new context when it's first encrypted, and we'll want other clients to pick up on this change when it occurs.
Do clients need to see this when not reading/writing to the file?
2) "real" size: needs to be under Fwx, I think (though I need to look more closely at the truncation path to be sure).
Frs would need the size as well.
...and that's not even considering what rules we might want in the future for other info we stuff into here. Given that the MDS needs to treat this as opaque, what locks/caps should cover this new field?
I think because the encryption context is used for reads/writes, it can fall under the same lock domain as the file size. I don't see a need (yet) for accessing e.g. the encrypted version/blocksize outside of the Fsx cap. It's good to think about though and I wonder if anyone else has thoughts on it. -- Patrick Donnelly, Ph.D. He / Him / His Principal Software Engineer Red Hat Sunnyvale, CA GPG: 19F28A586F808C2402351B93C3301A3E258DD79D