Hi Ted, Today ngkaho1234 pointed out on IRC that if you do the following: $ mkfs.ext2 /dev/sda -F $ mount /dev/sda /mnt -t ext4 $ touch /mnt/x $ setfattr -n user.name1 -v moo /mnt/x $ umount /mnt $ mount /dev/sda /mnt -t ext2 $ setfattr -n user.name1 -v urk /mnt/x $ umount /mnt Then you get this broken looking result: $ mount /dev/sda /mnt -e ext4 $ getfattr /mnt/x getfattr: Removing leading '/' from absolute path names # file: mnt/x user.name1 user.name1 Looking through debugfs, it seems that ext4 wrote user.name1=moo as an ibody xattr, then ext2 wrote user.name1=urk into an external xattr block and set i_file_acl. ext4 sees the attr it wrote, ext2 sees the attr it wrote, and neither can see the attr the other wrote. $ debugfs -R 'features' /dev/sda Filesystem features: ext_attr resize_inode dir_index filetype sparse_super large_file $ debugfs -R 'stat /x' /dev/sda Inode: 12 Type: regular Mode: 0644 Flags: 0x0 Generation: 2341792653 Version: 0x00000000:00000001 User: 0 Group: 0 Project: 0 Size: 0 File ACL: 617 Directory ACL: 0 Links: 1 Blockcount: 8 Fragment: Address: 0 Number: 0 Size: 0 ctime: 0x58b45e2b:926e39a8 -- Mon Feb 27 09:13:15 2017 atime: 0x58b45e21:c4f666c0 -- Mon Feb 27 09:13:05 2017 mtime: 0x58b45e21:c4f666c0 -- Mon Feb 27 09:13:05 2017 crtime: 0x58b45e21:c4f666c0 -- Mon Feb 27 09:13:05 2017 Size of extra inode fields: 32 Extended attributes: user.name1 (3) = "moo" user.name1 (3) = "urk" BLOCKS: This is wrong -- we don't have RO_COMPAT_EXTRA_ISIZE set, so ext4 should /not/ be setting i_extra_isize=32. Unfortunately, it does set i_extra_isize, which enables ext4_xattr_ibody_set to write xattrs into the inode body. That's a problem, because ext2 doesn't know about inline attrs or i_extra_isize. It occurred to me to check the s_inode_size, which is 256 on this supposedly ext2 filesystem. I'd have thought _fill_super would check this value, but apparently its only criteria are that s_inode_size is at least 128, a power of 2, and no larger than a block. But AFAIK ext2 has no ability to use any inode space beyond the first 128 bytes, so what good are large inodes? Oh, and e2fsck apparently doesn't notice if there are inodes with i_extra_isize set even if ro_compat_extra_isize is not set. I could see a bunch of fixes to resolve this problem: 1) Teach ext4 not to set i_extra_isize unless the feature bit is set. 2) ext2_iget grows the ability to return -EFSCORRUPTED for inodes that have big inodes and i_extra_isize set, to encourage people to run e2fsck. 3) e2fsck will move attrs and zap i_extra_isize if i_extra_isize is set on a filesystem that doesn't support it. 4) mke2fs probably should stop allocating 256 byte inodes with the ext2 and ext3 profiles, though it's not clear to me why the ext2 driver even allows this -- maybe there's some context here I don't know? So ... /me isn't sure how to deal with this. List? :) --D
