ea_inode

extended attributes and grub

intro

I've been working on new a project that utilizes extended attributes. In the past, I always used XFS on my systems, mainly because of its extended attribute support. On my more recent systems, though, I've been going with ext4 because it's come so far, and on paper supports everything I need. During this project though, I started running into limitations. Some filesystem related, but the VFS itself is also limiting.

ea_inode

By default on ext, extended attributes must fit within the file's inode, or a single dedicated fs block attached to it. (There's also a VFS limit of 64 KiB for the total extended attribute table, but that's a whole other issue.) IMO, it's a real "640K should be enough..." moment. It mostly works for the current common use cases, mostly around security features like ACLs, but not much else. Ext4 does support storing larger attributes, but you must enable the "ea_inode" feature on the filesystem to enable it. At first, it's a super simple fix: "sudo tune2fs -O ea_inode /dev/sda2". Large attributes supported! But...

grub2

Grub refuses to access ext4 filesystems with the "ea_inode" feature set. That tune2fs call just rendered our system unbootable... Grub actually rejects any filesystem with features it doesn't recognize. I wouldn't argue that specific decision, but making that decision and then not keeping up with features added is, not great. And it isn't just ea_inode; features like large directories and metadata checksums had the same issue. At one point, mkfs.ext4 was setting a feature by default that grub wasn't updated for, making unbootable systems by default.

patch

Patches for some of the features eventually landed in grub2 (example), but so far, no ea_inode. Fortunately, the patches for those features are super easy to duplicate for our case. We only have to define the flag and add it to the list of features ignored by grub's ext2 driver (my diff). I don't know anywhere near enough about the driver to say whether this change could, somehow, result in issues; but it's working perfectly for me.

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2025/NOV/27