Re: [PATCH] f2fs: reserve bits for fs-verity

[Eric Biggers via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

On Tue, Apr 03, 2018 at 07:49:59AM +1000, Dave Chinner wrote:
> On Mon, Apr 02, 2018 at 11:48:37AM -0700, Eric Biggers wrote:
> > [+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
> 
> Use FS_IOC_{GS}ETXATTR instead?. I'ts not tied to an on-disk format.
> 

Yes, that API is better -- though, *setting* the fs-verity bit will likely be
done through its own ioctl (tentatively called FS_IOC_ENABLE_VERITY) as it will
involve more work than simply flipping the bit.  So for *getting* it we maybe
should just add it as a statx attribute, and not use the GETFLAGS or GETXATTR
ioctls at all.

But right now the real problem is that f2fs is using the FS_* namespace for its
on-disk ->i_flags (other than a few in the f2fs-specific ->i_advise), which
gives the impression that all new f2fs on-disk flags need to be exposed through
FS_IOC_GETFLAGS/SETFLAGS, and also reserved for ext4 as well...

Eric

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