VFS and the filesystem family

The VFS contract

fs/fs_vfs.{c,h}: one API - open/read/write/seek/close/stat/mkdir/ rmdir/readdir/lookup - over registered filesystem types. A type registers mount/unmount hooks plus file_ops and dir_ops. The global node tree resolves paths and crosses mount points transparently.

Real behaviours: per-fs case sensitivity in the node cache, enforced permission bits (mode 0000 refuses), bitwise write-access checks, i64 seek, umount / refused, mount shadowing without duplicate children, bounded list_mounts.

FAT32 (fs/fs_fat32.c)

LFN with alias generation + collision scan; checksum and sequence validation on read; cross-cluster LFN deletion with backtrack; dual- FAT mirror maintained; FSINFO read + maintained + persisted with the next-free hint.

exFAT (fs/fs_exfat.c)

Active-FAT tracking, upcase table handling, cluster chains.

ext4 (fs/fs_ext4.c)

Extent-based reads and writes, block groups, inode tables.

ramfs (fs/fs_ramfs.c)

The volatile root filesystem; sparse holes read back as zeros.

The /etc volume

A 32 MiB FAT32 image mounted at /etc (config, SSH keys, logs, crash dumps) - built by the Makefile from etc/opencube.conf. It is the persistence story for an otherwise in-memory system.

Testing

fstest (the dual-sided assertion suite), fsck, fatstat, fs_mount_test, plus the host-side harnesses in tests/ (FAT structure parsing with ASAN). The regression battery runs fstest on every boot.