> Filesystems have metadata: which is where a MAC can go, next to the filename.
File-level MACs would require reading/writing the entire file in one go, andpreclude efficient in-place modification of files. The alternative, lots of per-block MACs stored in extended attributes, wouldn't work for ext4 because of the 4 kb extended attribute size limitation, and would also have problems that the grandparent listed (lots of seeks).
A fair point. btrfs would have a much easier time of it. We then have the interesting consequence that we can observe times and sizes of COW updates, but in truth SSDs have that anyway.
File-level MACs would require reading/writing the entire file in one go, andpreclude efficient in-place modification of files. The alternative, lots of per-block MACs stored in extended attributes, wouldn't work for ext4 because of the 4 kb extended attribute size limitation, and would also have problems that the grandparent listed (lots of seeks).