[+Cc linux-ext4, linux-fsdevel] On Mon, Apr 02, 2018 at 06:07:10PM +0800, Chao Yu wrote: > Hi Eric and Jaegeuk, > > On 2018/3/31 2:34, Eric Biggers wrote: > > Hi Chao and Jaegeuk, > > > > On Fri, Mar 30, 2018 at 09:41:36AM -0700, Jaegeuk Kim wrote: > >> On 03/30, Chao Yu wrote: > >>> Hi Eric, > >>> > >>> On 2018/3/29 2:15, Eric Biggers wrote: > >>>> Reserve an F2FS feature flag and inode flag for fs-verity. This is an > >>>> in-development feature that is planned be discussed at LSF/MM 2018 [1]. > >>>> It will provide file-based integrity and authenticity for read-only > >>>> files. Most code will be in a filesystem-independent module, with > >>>> smaller changes needed to individual filesystems that opt-in to > >>>> supporting the feature. An early prototype supporting F2FS is available > >>>> [2]. Reserving the F2FS on-disk bits for fs-verity will prevent users > >>>> of the prototype from conflicting with other new F2FS features. > >>>> > >>>> Note that we're reserving the inode flag in f2fs_inode.i_advise, which > >>>> isn't really appropriate since it's not a hint or advice. But > >>>> ->i_advise is already being used to hold the 'encrypt' flag; and F2FS's > >>>> ->i_flags uses the generic FS_* values, so it seems ->i_flags can't be > >>>> used for an F2FS-specific flag without additional work to remove the > >>>> assumption that ->i_flags uses the generic flags namespace. > >>> > >>> At a glance, this is a VFS feature, can we search free slot, and define > >>> FS_VERITY_FL like other generic flags, so we can intergrate this flag into > >>> f2fs_inode::i_flags? > >> > >> Do we need to get/set this bit of i_flags to user? And, f2fs doesn't synchronize > >> it with inode block update. I think this should be set by internal f2fs > >> operations likewise fscrypt. > >> > > > > The fs-verity inode flag won't be modifiable using FS_IOC_SETFLAGS. Like > > Verity flag can also be wrapped by FS_FL_USER_MODIFIABLE like for FS_ENCRYPT_FL? > > > fscrypt, it will only be possible to set it using a dedicated ioctl (tentatively > > called FS_IOC_ENABLE_VERITY), and it won't be supported to clear the bit once > > set, short of deleting and re-creating the file. So it doesn't really matter > > where the bit goes in the on-disk inode, it just needs to go somewhere. I'm > > just hesitant to reserve a flag in the UAPI flags namespace which is really more > > "owned" by ext4 than by f2fs, so has more implications than just f2fs as we > > would effectively be reserving the flag for ext4's on-disk format too. > > IMO, because this is a VFS feature, it will be better that we can put it in more > generic place, also user can check this bit in generic way (via > FS_IOC_GETFLAGS), and then for other filesystem who wants add this feature, that > will be simple to place this bit. > > What I can see is, for encryption feature: > > vfs::i_flags > #define FS_ENCRYPT_FL 0x00000800 /* Encrypted file */ > > ext4:i_flags > #define EXT4_ENCRYPT_FL 0x00000800 /* encrypted file */ > > f2fs::i_advice > #define FADVISE_ENCRYPT_BIT 0x04 > > It's very wired that f2fs didn't use well defined FS_ENCRYPT_FL bit position, > result in that we leave a hole in on-disk i_flags, and if we want to show the > same 'encrypted' flags status in FS_IOC_GETFLAGS, we need to change more codes. > > Anyway, I just ask why not let generic status goes into i_flags, and private > status goes into i_advices? Ted and others, what would you say about allocating FS_VERITY_FL as one of the unused ext4 / "VFS" inode flags like 0x01000000, or maybe 0x00000400 if the compression flags aren't being used anymore? I do wish that we separated the on-disk flags namespaces from the FS_IOC_GETFLAGS/FS_IOC_SETFLAGS namespace though... Then adding the flag to each namespace could be done separately which would make more sense. As-is, the flags are all being conflated, so by allocating a flag in f2fs ->i_flags we would effectively also be allocating it for ext4 and for the UAPI, which we don't necessarily need to do yet. > > > > > I do think the flag *should* go in i_flags rather than i_advise, but I think the > > assumption that f2fs's inode flags namespace matches ext4's would first need to > > be removed so as to not tie the on-disk formats of different filesystems > > together. > > > >>> > >>> And how about applying this patch inside the patchset of new fsverity feature? > >>> Since once fsverity feature has some design modification, I worry about that may > >>> be we need to change this bit? result in disk layout incompatibility. > >> > >> The whole body is not fully mergeable, so once reserving the bits first, we can > >> support it f2fs-tools and prepare the feature in advance. Based on previous > >> fscrypt experience, I don't expect we need to modify or drop these dramatically > >> later. > >> > >> Any thoughts? > > Since I don't know current progress of this feature development, I hope this > feature will not be against by vfs developers or suffer design change after we > reserved bit for it. :) > > >> > > > > Yep, the full patchset isn't ready to be merged upstream yet. Other parts of > > the API/ABI such as the fs-verity metadata format, the ioctl numbers, and the > > semantics of accessing such files is subject to change. We know we'll need a > > superblock feature flag and a per-inode bit in any case, though. Personally I'd > > prefer to wait for the full patchset too, but we have people who want to start > > using the prototype of the feature already, so having f2fs-tools support the > > feature flag and having the bits not conflict with other f2fs features will be > > helpful. > > Oh, so we want a stable on-disk layout, so image for experience will contain > fsverity bit in stable position, after formal android released, image with > fsverity feature can still be valid, right? > The fs-verity file format is not finalized, but in any case there will need to be a superblock flag and a per-inode flag that indicates whether it is enabled. There will also be a version number built into the fs-verity metadata, so the file format can be updated without having to change to using a different per-inode flag. Eric
