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diff --git a/pkg/sentry/fs/README.md b/pkg/sentry/fs/README.md deleted file mode 100644 index c4e8faa3c..000000000 --- a/pkg/sentry/fs/README.md +++ /dev/null @@ -1,229 +0,0 @@ -This package provides an implementation of the Linux virtual filesystem. - -[TOC] - -## Overview - -- An `fs.Dirent` caches an `fs.Inode` in memory at a path in the VFS, giving - the `fs.Inode` a relative position with respect to other `fs.Inode`s. - -- If an `fs.Dirent` is referenced by two file descriptors, then those file - descriptors are coherent with each other: they depend on the same - `fs.Inode`. - -- A mount point is an `fs.Dirent` for which `fs.Dirent.mounted` is true. It - exposes the root of a mounted filesystem. - -- The `fs.Inode` produced by a registered filesystem on mount(2) owns an - `fs.MountedFilesystem` from which other `fs.Inode`s will be looked up. For a - remote filesystem, the `fs.MountedFilesystem` owns the connection to that - remote filesystem. - -- In general: - -``` -fs.Inode <------------------------------ -| | -| | -produced by | -exactly one | -| responsible for the -| virtual identity of -v | -fs.MountedFilesystem ------------------- -``` - -Glossary: - -- VFS: virtual filesystem. - -- inode: a virtual file object holding a cached view of a file on a backing - filesystem (includes metadata and page caches). - -- superblock: the virtual state of a mounted filesystem (e.g. the virtual - inode number set). - -- mount namespace: a view of the mounts under a root (during path traversal, - the VFS makes visible/follows the mount point that is in the current task's - mount namespace). - -## Save and restore - -An application's hard dependencies on filesystem state can be broken down into -two categories: - -- The state necessary to execute a traversal on or view the *virtual* - filesystem hierarchy, regardless of what files an application has open. - -- The state necessary to represent open files. - -The first is always necessary to save and restore. An application may never have -any open file descriptors, but across save and restore it should see a coherent -view of any mount namespace. NOTE(b/63601033): Currently only one "initial" -mount namespace is supported. - -The second is so that system calls across save and restore are coherent with -each other (e.g. so that unintended re-reads or overwrites do not occur). - -Specifically this state is: - -- An `fs.MountManager` containing mount points. - -- A `kernel.FDMap` containing pointers to open files. - -Anything else managed by the VFS that can be easily loaded into memory from a -filesystem is synced back to those filesystems and is not saved. Examples are -pages in page caches used for optimizations (i.e. readahead and writeback), and -directory entries used to accelerate path lookups. - -### Mount points - -Saving and restoring a mount point means saving and restoring: - -- The root of the mounted filesystem. - -- Mount flags, which control how the VFS interacts with the mounted - filesystem. - -- Any relevant metadata about the mounted filesystem. - -- All `fs.Inode`s referenced by the application that reside under the mount - point. - -`fs.MountedFilesystem` is metadata about a filesystem that is mounted. It is -referenced by every `fs.Inode` loaded into memory under the mount point -including the `fs.Inode` of the mount point itself. The `fs.MountedFilesystem` -maps file objects on the filesystem to a virtualized `fs.Inode` number and vice -versa. - -To restore all `fs.Inode`s under a given mount point, each `fs.Inode` leverages -its dependency on an `fs.MountedFilesystem`. Since the `fs.MountedFilesystem` -knows how an `fs.Inode` maps to a file object on a backing filesystem, this -mapping can be trivially consulted by each `fs.Inode` when the `fs.Inode` is -restored. - -In detail, a mount point is saved in two steps: - -- First, after the kernel is paused but before state.Save, we walk all mount - namespaces and install a mapping from `fs.Inode` numbers to file paths - relative to the root of the mounted filesystem in each - `fs.MountedFilesystem`. This is subsequently called the set of `fs.Inode` - mappings. - -- Second, during state.Save, each `fs.MountedFilesystem` decides whether to - save the set of `fs.Inode` mappings. In-memory filesystems, like tmpfs, have - no need to save a set of `fs.Inode` mappings, since the `fs.Inode`s can be - entirely encoded in state file. Each `fs.MountedFilesystem` also optionally - saves the device name from when the filesystem was originally mounted. Each - `fs.Inode` saves its virtual identifier and a reference to a - `fs.MountedFilesystem`. - -A mount point is restored in two steps: - -- First, before state.Load, all mount configurations are stored in a global - `fs.RestoreEnvironment`. This tells us what mount points the user wants to - restore and how to re-establish pointers to backing filesystems. - -- Second, during state.Load, each `fs.MountedFilesystem` optionally searches - for a mount in the `fs.RestoreEnvironment` that matches its saved device - name. The `fs.MountedFilesystem` then reestablishes a pointer to the root of - the mounted filesystem. For example, the mount specification provides the - network connection for a mounted remote filesystem client to communicate - with its remote file server. The `fs.MountedFilesystem` also trivially loads - its set of `fs.Inode` mappings. When an `fs.Inode` is encountered, the - `fs.Inode` loads its virtual identifier and its reference a - `fs.MountedFilesystem`. It uses the `fs.MountedFilesystem` to obtain the - root of the mounted filesystem and the `fs.Inode` mappings to obtain the - relative file path to its data. With these, the `fs.Inode` re-establishes a - pointer to its file object. - -A mount point can trivially restore its `fs.Inode`s in parallel since -`fs.Inode`s have a restore dependency on their `fs.MountedFilesystem` and not on -each other. - -### Open files - -An `fs.File` references the following filesystem objects: - -```go -fs.File -> fs.Dirent -> fs.Inode -> fs.MountedFilesystem -``` - -The `fs.Inode` is restored using its `fs.MountedFilesystem`. The -[Mount points](#mount-points) section above describes how this happens in -detail. The `fs.Dirent` restores its pointer to an `fs.Inode`, pointers to -parent and children `fs.Dirents`, and the basename of the file. - -Otherwise an `fs.File` restores flags, an offset, and a unique identifier (only -used internally). - -It may use the `fs.Inode`, which it indirectly holds a reference on through the -`fs.Dirent`, to reestablish an open file handle on the backing filesystem (e.g. -to continue reading and writing). - -## Overlay - -The overlay implementation in the fs package takes Linux overlayfs as a frame of -reference but corrects for several POSIX consistency errors. - -In Linux overlayfs, the `struct inode` used for reading and writing to the same -file may be different. This is because the `struct inode` is dissociated with -the process of copying up the file from the upper to the lower directory. Since -flock(2) and fcntl(2) locks, inotify(7) watches, page caches, and a file's -identity are all stored directly or indirectly off the `struct inode`, these -properties of the `struct inode` may be stale after the first modification. This -can lead to file locking bugs, missed inotify events, and inconsistent data in -shared memory mappings of files, to name a few problems. - -The fs package maintains a single `fs.Inode` to represent a directory entry in -an overlay and defines operations on this `fs.Inode` which synchronize with the -copy up process. This achieves several things: - -+ File locks, inotify watches, and the identity of the file need not be copied - at all. - -+ Memory mappings of files coordinate with the copy up process so that if a - file in the lower directory is memory mapped, all references to it are - invalidated, forcing the application to re-fault on memory mappings of the - file under the upper directory. - -The `fs.Inode` holds metadata about files in the upper and/or lower directories -via an `fs.overlayEntry`. The `fs.overlayEntry` implements the `fs.Mappable` -interface. It multiplexes between upper and lower directory memory mappings and -stores a copy of memory references so they can be transferred to the upper -directory `fs.Mappable` when the file is copied up. - -The lower filesystem in an overlay may contain another (nested) overlay, but the -upper filesystem may not contain another overlay. In other words, nested -overlays form a tree structure that only allows branching in the lower -filesystem. - -Caching decisions in the overlay are delegated to the upper filesystem, meaning -that the Keep and Revalidate methods on the overlay return the same values as -the upper filesystem. A small wrinkle is that the lower filesystem is not -allowed to return `true` from Revalidate, as the overlay can not reload inodes -from the lower filesystem. A lower filesystem that does return `true` from -Revalidate will trigger a panic. - -The `fs.Inode` also holds a reference to a `fs.MountedFilesystem` that -normalizes across the mounted filesystem state of the upper and lower -directories. - -When a file is copied from the lower to the upper directory, attempts to -interact with the file block until the copy completes. All copying synchronizes -with rename(2). - -## Future Work - -### Overlay - -When a file is copied from a lower directory to an upper directory, several -locks are taken: the global renamuMu and the copyMu of the `fs.Inode` being -copied. This blocks operations on the file, including fault handling of memory -mappings. Performance could be improved by copying files into a temporary -directory that resides on the same filesystem as the upper directory and doing -an atomic rename, holding locks only during the rename operation. - -Additionally files are copied up synchronously. For large files, this causes a -noticeable latency. Performance could be improved by pipelining copies at -non-overlapping file offsets. |