Check in gVisor.

PiperOrigin-RevId: 194583126
Change-Id: Ica1d8821a90f74e7e745962d71801c598c652463

1 files changed, 172 insertions, 0 deletions

diff --git a/pkg/sentry/kernel/fs_context.go b/pkg/sentry/kernel/fs_context.go
new file mode 100644
index 000000000..9aa6fa951
--- /dev/null
+++ b/pkg/sentry/kernel/fs_context.go
@@ -0,0 +1,172 @@
+// Copyright 2018 Google Inc.
+//
+// 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 kernel
+
+import (
+	"fmt"
+	"sync"
+
+	"gvisor.googlesource.com/gvisor/pkg/refs"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/fs"
+)
+
+// FSContext contains filesystem context.
+//
+// This includes umask and working directory.
+type FSContext struct {
+	refs.AtomicRefCount
+
+	// mu protects below.
+	mu sync.Mutex `state:"nosave"`
+
+	// root is the filesystem root. Will be nil iff the FSContext has been
+	// destroyed.
+	root *fs.Dirent
+
+	// cwd is the current working directory. Will be nil iff the FSContext
+	// has been destroyed.
+	cwd *fs.Dirent
+
+	// umask is the current file mode creation mask. When a thread using this
+	// context invokes a syscall that creates a file, bits set in umask are
+	// removed from the permissions that the file is created with.
+	umask uint
+}
+
+// newFSContext returns a new filesystem context.
+func newFSContext(root, cwd *fs.Dirent, umask uint) *FSContext {
+	root.IncRef()
+	cwd.IncRef()
+	return &FSContext{
+		root:  root,
+		cwd:   cwd,
+		umask: umask,
+	}
+}
+
+// destroy is the destructor for an FSContext.
+//
+// This will call DecRef on both root and cwd Dirents.  If either call to
+// DecRef returns an error, then it will be propigated.  If both calls to
+// DecRef return an error, then the one from root.DecRef will be propigated.
+//
+// Note that there may still be calls to WorkingDirectory() or RootDirectory()
+// (that return nil).  This is because valid references may still be held via
+// proc files or other mechanisms.
+func (f *FSContext) destroy() {
+	f.root.DecRef()
+	f.root = nil
+
+	f.cwd.DecRef()
+	f.cwd = nil
+}
+
+// DecRef implements RefCounter.DecRef with destructor f.destroy.
+func (f *FSContext) DecRef() {
+	f.DecRefWithDestructor(f.destroy)
+}
+
+// Fork forks this FSContext.
+//
+// This is not a valid call after destroy.
+func (f *FSContext) Fork() *FSContext {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.cwd.IncRef()
+	f.root.IncRef()
+	return &FSContext{
+		cwd:   f.cwd,
+		root:  f.root,
+		umask: f.umask,
+	}
+}
+
+// WorkingDirectory returns the current working directory.
+// You should call DecRef on the returned Dirent when finished.
+//
+// This will return nil if called after destroy().
+func (f *FSContext) WorkingDirectory() *fs.Dirent {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if f.cwd != nil {
+		f.cwd.IncRef()
+	}
+	return f.cwd
+}
+
+// SetWorkingDirectory sets the current working directory.
+// This will take an extra reference on the Dirent.
+//
+// This is not a valid call after destroy.
+func (f *FSContext) SetWorkingDirectory(d *fs.Dirent) {
+	if d == nil {
+		panic("FSContext.SetWorkingDirectory called with nil dirent")
+	}
+	if f.cwd == nil {
+		panic(fmt.Sprintf("FSContext.SetWorkingDirectory(%v)) called after destroy", d))
+	}
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	old := f.cwd
+	f.cwd = d
+	d.IncRef()
+	old.DecRef()
+}
+
+// RootDirectory returns the current filesystem root.
+// You should call DecRef on the returned Dirent when finished.
+//
+// This will return nil if called after destroy().
+func (f *FSContext) RootDirectory() *fs.Dirent {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.root.IncRef()
+	return f.root
+}
+
+// SetRootDirectory sets the root directory.
+// This will take an extra reference on the Dirent.
+//
+// This is not a valid call after free.
+func (f *FSContext) SetRootDirectory(d *fs.Dirent) {
+	if d == nil {
+		panic("FSContext.SetRootDirectory called with nil dirent")
+	}
+	if f.root == nil {
+		panic(fmt.Sprintf("FSContext.SetRootDirectory(%v)) called after destroy", d))
+	}
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	old := f.root
+	f.root = d
+	d.IncRef()
+	old.DecRef()
+}
+
+// Umask returns the current umask.
+func (f *FSContext) Umask() uint {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return f.umask
+}
+
+// SwapUmask atomically sets the current umask and returns the old umask.
+func (f *FSContext) SwapUmask(mask uint) uint {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	old := f.umask
+	f.umask = mask
+	return old
+}