On Wed, Feb 17, 2021 at 2:48 PM Andreas Dilger <adilger@xxxxxxxxx> wrote: > > On Feb 17, 2021, at 9:08 AM, Theodore Ts'o <tytso@xxxxxxx> wrote: > > > > On Tue, Feb 16, 2021 at 08:01:11PM -0800, Daniel Rosenberg wrote: > >> I'm not sure what the conflict is, at least format-wise. Naturally, > >> there would need to be some work to reconcile the two patches, but my > >> patch only alters the format for directories which are encrypted and > >> casefolded, which always must have the additional hash field. In the > >> case of dirdata along with encryption and casefolding, couldn't we > >> have the dirdata simply follow after the existing data? Since we > >> always already know the length, it'd be unambiguous where that would > >> start. Casefolding can only be altered on an empty directory, and you > >> can only enable encryption for an empty directory, so I'm not too > >> concerned there. I feel like having it swapping between the different > >> methods makes it more prone to bugs, although it would be doable. I've > >> started rebasing the dirdata patch on my end to see how easy it is to > >> mix the two. At a glance, they touch a lot of the same areas in > >> similar ways, so it shouldn't be too hard. It's more of a question of > >> which way we want to resolve that, and which patch goes first. > >> > >> I've been trying to figure out how many devices in the field are using > >> casefolded encryption, but haven't found out yet. The code is > >> definitely available though, so I would not be surprised if it's being > >> used, or is about to be. > > > > The problem is in how the space after the filename in a directory is > > encoded. The dirdata format is (mildly) expandable, supporting up to > > 4 different metadata chunks after the filename, using a very > > compatctly encoded TLV (or moral equivalent) scheme. For directory > > inodes that have both the encyption and compression flags set, we have > > a single blob which gets used as the IV for the crypto. > > > > So it's the difference between a simple blob that is only used for one > > thing in this particular case, and something which is the moral > > equivalent of simple ASN.1 or protobuf encoding. > > > > Currently, datadata has defined uses for 2 of the 4 "chunks", which is > > used in Lustre servers. The proposal which Andreas has suggested is > > if the dirdata feature is supported, then the 3rd dirdata chunk would > > be used for the case where we currently used by the > > encrypted-casefolded extension, and the 4th would get reserved for a > > to-be-defined extension mechanism. > > > > If there ext4 encrypted/casefold is not yet in use, and we can get the > > changes out to all potential users before they release products out > > into the field, then one approach would be to only support > > encrypted/casefold when dirdata is also enabled. > > > > If ext4 encrypted/casefold is in use, my suggestion is that we support > > both encrypted/casefold && !dirdata as you have currently implemented > > it, and encrypted/casefold && dirdata as Andreas has proposed. > > > > IIRC, supporting that Andreas's scheme essentially means that we use > > the top four bits in the rec_len field to indicate which chunks are > > present, and then for each chunk which is present, there is a 1 byte > > length followed by payload. So that means in the case where it's > > encrypted/casefold && dirdata, the required storage of the directory > > entry would take one additional byte, plus setting a bit indicating > > that the encrypted/casefold dirdata chunk was present. > > I think your email already covers pretty much all of the points. > > One small difference between current "raw" encrypted/casefold hash vs. > dirdata is that the former is 4-byte aligned within the dirent, while > dirdata is packed. So in 3/4 cases dirdata would take the same amount > of space (the 1-byte length would use one of the 1-3 bytes of padding > vs. the raw format), since the next dirent needs to be aligned anyway. > > The other implication here is that the 8-byte hash may need to be > copied out of the dirent into a local variable before use, due to > alignment issues, but I'm not sure if that is actually needed or not. > > > So, no, they aren't incompatible ultimatly, but it might require a > > tiny bit more work to integrate the combined support for dirdata plus > > encrypted/casefold. One way we can do this, if we have to support the > > current encrypted/casefold format because it's out there in deployed > > implementations already, is to integrate encrypted/casefold && > > !dirdata first upstream, and then when we integrate dirdata into > > upstream, we'll have to add support for the encrypted/casefold && > > dirdata case. This means that we'll have two variants of the on-disk > > format to test and support, but I don't think it's the going to be > > that difficult. > > It would be possible to detect if the encrypted/casefold+dirdata > variant is in use, because the dirdata variant would have the 0x40 > bit set in the file_type byte. It isn't possible to positively > identify the "raw" non-dirdata variant, but the assumption would be > if (rec_len >= round_up(name_len, 4) + 8) in an encrypted+casefold > directory that the "raw" hash must be present in the dirent. > > Cheers, Andreas > > > > > So sounds like we're going with the combined version. Andreas, do you have any suggestions for changes to the casefolding patch to ease the eventual merging with dirdata? A bunch of the changes are already pretty similar, so some of it is just calling essentially the same functions different things. -Daniel
