path: root/pkg/sentry/vfs/filesystem.go

// Copyright 2019 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package vfs

import (
	"sync/atomic"

	"gvisor.dev/gvisor/pkg/abi/linux"
	"gvisor.dev/gvisor/pkg/fspath"
	"gvisor.dev/gvisor/pkg/sentry/context"
)

// A Filesystem is a tree of nodes represented by Dentries, which forms part of
// a VirtualFilesystem.
//
// Filesystems are reference-counted. Unless otherwise specified, all
// Filesystem methods require that a reference is held.
//
// Filesystem is analogous to Linux's struct super_block.
type Filesystem struct {
	// refs is the reference count. refs is accessed using atomic memory
	// operations.
	refs int64

	// vfs is the VirtualFilesystem that uses this Filesystem. vfs is
	// immutable.
	vfs *VirtualFilesystem

	// impl is the FilesystemImpl associated with this Filesystem. impl is
	// immutable. This should be the last field in Dentry.
	impl FilesystemImpl
}

// Init must be called before first use of fs.
func (fs *Filesystem) Init(vfsObj *VirtualFilesystem, impl FilesystemImpl) {
	fs.refs = 1
	fs.vfs = vfsObj
	fs.impl = impl
	vfsObj.filesystemsMu.Lock()
	vfsObj.filesystems[fs] = struct{}{}
	vfsObj.filesystemsMu.Unlock()
}

// VirtualFilesystem returns the containing VirtualFilesystem.
func (fs *Filesystem) VirtualFilesystem() *VirtualFilesystem {
	return fs.vfs
}

// Impl returns the FilesystemImpl associated with fs.
func (fs *Filesystem) Impl() FilesystemImpl {
	return fs.impl
}

// IncRef increments fs' reference count.
func (fs *Filesystem) IncRef() {
	if atomic.AddInt64(&fs.refs, 1) <= 1 {
		panic("Filesystem.IncRef() called without holding a reference")
	}
}

// TryIncRef increments fs' reference count and returns true. If fs' reference
// count is zero, TryIncRef does nothing and returns false.
//
// TryIncRef does not require that a reference is held on fs.
func (fs *Filesystem) TryIncRef() bool {
	for {
		refs := atomic.LoadInt64(&fs.refs)
		if refs <= 0 {
			return false
		}
		if atomic.CompareAndSwapInt64(&fs.refs, refs, refs+1) {
			return true
		}
	}
}

// DecRef decrements fs' reference count.
func (fs *Filesystem) DecRef() {
	if refs := atomic.AddInt64(&fs.refs, -1); refs == 0 {
		fs.vfs.filesystemsMu.Lock()
		delete(fs.vfs.filesystems, fs)
		fs.vfs.filesystemsMu.Unlock()
		fs.impl.Release()
	} else if refs < 0 {
		panic("Filesystem.decRef() called without holding a reference")
	}
}

// FilesystemImpl contains implementation details for a Filesystem.
// Implementations of FilesystemImpl should contain their associated Filesystem
// by value as their first field.
//
// All methods that take a ResolvingPath must resolve the path before
// performing any other checks, including rejection of the operation if not
// supported by the FilesystemImpl. This is because the final FilesystemImpl
// (responsible for actually implementing the operation) isn't known until path
// resolution is complete.
//
// For all methods that take or return linux.Statx, Statx.Uid and Statx.Gid
// should be interpreted as IDs in the root UserNamespace (i.e. as auth.KUID
// and auth.KGID respectively).
//
// FilesystemImpl combines elements of Linux's struct super_operations and
// struct inode_operations, for reasons described in the documentation for
// Dentry.
type FilesystemImpl interface {
	// Release is called when the associated Filesystem reaches zero
	// references.
	Release()

	// Sync "causes all pending modifications to filesystem metadata and cached
	// file data to be written to the underlying [filesystem]", as by syncfs(2).
	Sync(ctx context.Context) error

	// GetDentryAt returns a Dentry representing the file at rp. A reference is
	// taken on the returned Dentry.
	//
	// GetDentryAt does not correspond directly to a Linux syscall; it is used
	// in the implementation of:
	//
	// - Syscalls that need to resolve two paths: rename(), renameat(),
	// renameat2(), link(), linkat().
	//
	// - Syscalls that need to refer to a filesystem position outside the
	// context of a file description: chdir(), fchdir(), chroot(), mount(),
	// umount().
	GetDentryAt(ctx context.Context, rp *ResolvingPath, opts GetDentryOptions) (*Dentry, error)

	// LinkAt creates a hard link at rp representing the same file as vd. It
	// does not take ownership of references on vd.
	//
	// The implementation is responsible for checking that vd.Mount() ==
	// rp.Mount(), and that vd does not represent a directory.
	LinkAt(ctx context.Context, rp *ResolvingPath, vd VirtualDentry) error

	// MkdirAt creates a directory at rp.
	MkdirAt(ctx context.Context, rp *ResolvingPath, opts MkdirOptions) error

	// MknodAt creates a regular file, device special file, or named pipe at
	// rp.
	MknodAt(ctx context.Context, rp *ResolvingPath, opts MknodOptions) error

	// OpenAt returns an FileDescription providing access to the file at rp. A
	// reference is taken on the returned FileDescription.
	OpenAt(ctx context.Context, rp *ResolvingPath, opts OpenOptions) (*FileDescription, error)

	// ReadlinkAt returns the target of the symbolic link at rp.
	ReadlinkAt(ctx context.Context, rp *ResolvingPath) (string, error)

	// RenameAt renames the Dentry represented by vd to rp. It does not take
	// ownership of references on vd.
	//
	// The implementation is responsible for checking that vd.Mount() ==
	// rp.Mount().
	RenameAt(ctx context.Context, rp *ResolvingPath, vd VirtualDentry, opts RenameOptions) error

	// RmdirAt removes the directory at rp.
	RmdirAt(ctx context.Context, rp *ResolvingPath) error

	// SetStatAt updates metadata for the file at the given path.
	SetStatAt(ctx context.Context, rp *ResolvingPath, opts SetStatOptions) error

	// StatAt returns metadata for the file at rp.
	StatAt(ctx context.Context, rp *ResolvingPath, opts StatOptions) (linux.Statx, error)

	// StatFSAt returns metadata for the filesystem containing the file at rp.
	// (This method takes a path because a FilesystemImpl may consist of any
	// number of constituent filesystems.)
	StatFSAt(ctx context.Context, rp *ResolvingPath) (linux.Statfs, error)

	// SymlinkAt creates a symbolic link at rp referring to the given target.
	SymlinkAt(ctx context.Context, rp *ResolvingPath, target string) error

	// UnlinkAt removes the non-directory file at rp.
	UnlinkAt(ctx context.Context, rp *ResolvingPath) error

	// PrependPath prepends a path from vd to vd.Mount().Root() to b.
	//
	// If vfsroot.Ok(), it is the contextual VFS root; if it is encountered
	// before vd.Mount().Root(), PrependPath should stop prepending path
	// components and return a PrependPathAtVFSRootError.
	//
	// If traversal of vd.Dentry()'s ancestors encounters an independent
	// ("root") Dentry that is not vd.Mount().Root() (i.e. vd.Dentry() is not a
	// descendant of vd.Mount().Root()), PrependPath should stop prepending
	// path components and return a PrependPathAtNonMountRootError.
	//
	// Filesystems for which Dentries do not have meaningful paths may prepend
	// an arbitrary descriptive string to b and then return a
	// PrependPathSyntheticError.
	//
	// Most implementations can acquire the appropriate locks to ensure that
	// Dentry.Name() and Dentry.Parent() are fixed for vd.Dentry() and all of
	// its ancestors, then call GenericPrependPath.
	//
	// Preconditions: vd.Mount().Filesystem().Impl() == this FilesystemImpl.
	PrependPath(ctx context.Context, vfsroot, vd VirtualDentry, b *fspath.Builder) error

	// TODO: extended attributes; inotify_add_watch(); bind()
}

// PrependPathAtVFSRootError is returned by implementations of
// FilesystemImpl.PrependPath() when they encounter the contextual VFS root.
type PrependPathAtVFSRootError struct{}

// Error implements error.Error.
func (PrependPathAtVFSRootError) Error() string {
	return "vfs.FilesystemImpl.PrependPath() reached VFS root"
}

// PrependPathAtNonMountRootError is returned by implementations of
// FilesystemImpl.PrependPath() when they encounter an independent ancestor
// Dentry that is not the Mount root.
type PrependPathAtNonMountRootError struct{}

// Error implements error.Error.
func (PrependPathAtNonMountRootError) Error() string {
	return "vfs.FilesystemImpl.PrependPath() reached root other than Mount root"
}

// PrependPathSyntheticError is returned by implementations of
// FilesystemImpl.PrependPath() for which prepended names do not represent real
// paths.
type PrependPathSyntheticError struct{}

// Error implements error.Error.
func (PrependPathSyntheticError) Error() string {
	return "vfs.FilesystemImpl.PrependPath() prepended synthetic name"
}