Inodes

[pwmarcz]

Parent issue: #24

This is an issue to track the inodes rewrite project, mostly for myself to keep my thoughts in order.

Motivation

Currently Gramine uses shim_dentry as both "directory entry" and "file metadata" object. This is inconvenient for several reasons:

• It's hard or even impossible to implement unlink and rename correctly: if we we unlink/rename a file that is open, things often break. Old implementation had a hacky workaround that essentially copied file metadata to a handle, but it worked only partially.
• Locking is weird and broken. There's a global lock (g_dcache_lock) but also a per-object lock (shim_dentry.lock), and many operations do not use locks at all (FS-related operations are racy and need proper locking graphene#944).
• The shim_dentry structure is complicated. I think splitting it into dentry and inode will make both structures much simpler, and the interactions more manageable.

Goal

• shim_dentry is a mapping from name to inode. It doesn't contain file information or filesystem-specific data.
• All access to shim_dentry is guarded by a global lock (g_dcache_lock). Operations on a dentry (e.g. lookup, open, unlink) all take g_dcache_lock.
• shim_inode holds file metadata (enough to answer stat()). It has a per-object lock for mutable data.

Plan

Rough draft, later steps might evolve.

• ✔️ Introduce shim_inode, convert chroot fs (PR [LibOS] Introduce inodes, rewrite chroot filesystem #5)
• ✔️ Define semantics for d_ops callbacks, make sure they take the global lock (g_dcache_lock) (PR Take g_dcache_lock in dentry-related functions #282)
• ✔️ Get rid of shim_dentry.lock (make g_dcache_lock the global lock for dentry access)
  • (I want to simplify before converting more code to inodes)
  • ✔️ Don't use shim_dentry.lock in the fs_lock module (POSIX locks) (PR [LibOS] fs_lock: Use a separate lock for all operations #283)
  • ✔️ Don't use shim_dentry.lock in filesystem callbacks (PR Take g_dcache_lock in dentry-related functions #282)
  • ✔️ Remove shim_dentry.data and shim_dentry.lock fields (PR Convert fifo and socket filesystems to use inodes  #414)
• ✔️ Convert individual filesystems to use inodes
  • ✔️ : tmpfs (PR Convert tmpfs to use inodes #297)
  • ✔️ synthetic nodes (PR [LibOS] Replace DENTRY_SYNTHETIC with a separate filesystem #383)
  • ✔️ pseudofs (PR [LibOS] Convert pseudo filesystem to use inodes #402)
  • ✔️ : socket, pipe (PR Convert fifo and socket filesystems to use inodes  #414)
• ✔️ Convert all FS code to use inodes, remove redundant fields from shim_dentry (PR [LibOS] Move fully to inodes #428)

[pwmarcz]

This is now done.