On Fri, Dec 21, 2018 at 11:28:13AM -0500, Theodore Y. Ts'o wrote: > On Fri, Dec 21, 2018 at 07:53:54AM -0800, Matthew Wilcox wrote: > > In contrast to "we'll just fix it up later" (which usually applies > > to in-kernel interfaces), we have a policy of not breaking userspace, > > so accepting this interface means setting it in stone. We should get > > it right. > > I'm not convinced it's a "fix", but my point is that if later on you > want to add extra complexity transforming > > ioctl(fd, FS_IOC_ENABLE_VERITY); > > so it does the equivalent of > > ioctl(fd, FS_IOC_ENABLE_VERITY_NOW_WITH_EXTRA_USELESS_COMPLEXITY, > fd, sizeof_data, sizeof_verity_data); I disagree with your EXTRA_USELESS_COMPLEXITY appendage. The interface you designed reflects the implementation you did in ext4, so I understand why it seems simple from your point of view. From the user point of view, it looks completely weird. You write a file, being a series of bytes, then all of a sudden have to know that it's composed of blocks, seek to the next block, write a header, then this Merkle data structure, then write a footer which isn't allowed to cross a block boundary for some unknowable reason. It seems much more logical to have the header+Merkle+footer as a separate data stream which the filesystem can then layout according to its own rules.
