Convert memfs into proto-tmpfs.

- Renamed memfs to tmpfs.
- Copied fileRangeSet bits from fs/fsutil/ to fsimpl/tmpfs/
- Changed tmpfs to be backed by filemem instead of byte slice.
- regularFileReadWriter uses a sync.Pool, similar to gofer client.

PiperOrigin-RevId: 288356380

pkg/sentry/fsimpl/tmpfs/BUILD

@@ -0,0 +1,92 @@
+load("//tools/go_stateify:defs.bzl", "go_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_test")
+load("//tools/go_generics:defs.bzl", "go_template_instance")
+
+package(licenses = ["notice"])
+
+go_template_instance(
+    name = "dentry_list",
+    out = "dentry_list.go",
+    package = "tmpfs",
+    prefix = "dentry",
+    template = "//pkg/ilist:generic_list",
+    types = {
+        "Element": "*dentry",
+        "Linker": "*dentry",
+    },
+)
+
+go_library(
+    name = "tmpfs",
+    srcs = [
+        "dentry_list.go",
+        "directory.go",
+        "filesystem.go",
+        "named_pipe.go",
+        "regular_file.go",
+        "symlink.go",
+        "tmpfs.go",
+    ],
+    importpath = "gvisor.dev/gvisor/pkg/sentry/fsimpl/tmpfs",
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/amutex",
+        "//pkg/fspath",
+        "//pkg/log",
+        "//pkg/sentry/arch",
+        "//pkg/sentry/context",
+        "//pkg/sentry/fs",
+        "//pkg/sentry/fs/fsutil",
+        "//pkg/sentry/kernel",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/kernel/pipe",
+        "//pkg/sentry/memmap",
+        "//pkg/sentry/pgalloc",
+        "//pkg/sentry/platform",
+        "//pkg/sentry/safemem",
+        "//pkg/sentry/usage",
+        "//pkg/sentry/usermem",
+        "//pkg/sentry/vfs",
+        "//pkg/syserror",
+    ],
+)
+
+go_test(
+    name = "benchmark_test",
+    size = "small",
+    srcs = ["benchmark_test.go"],
+    deps = [
+        ":tmpfs",
+        "//pkg/abi/linux",
+        "//pkg/fspath",
+        "//pkg/refs",
+        "//pkg/sentry/context",
+        "//pkg/sentry/context/contexttest",
+        "//pkg/sentry/fs",
+        "//pkg/sentry/fs/tmpfs",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/vfs",
+        "//pkg/syserror",
+    ],
+)
+
+go_test(
+    name = "tmpfs_test",
+    size = "small",
+    srcs = [
+        "pipe_test.go",
+        "regular_file_test.go",
+    ],
+    embed = [":tmpfs"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/fspath",
+        "//pkg/sentry/context",
+        "//pkg/sentry/context/contexttest",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/kernel/contexttest",
+        "//pkg/sentry/usermem",
+        "//pkg/sentry/vfs",
+        "//pkg/syserror",
+    ],
+)

pkg/sentry/fsimpl/tmpfs/benchmark_test.go

@@ -0,0 +1,487 @@
+// 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 benchmark_test
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/refs"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/context/contexttest"
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+	_ "gvisor.dev/gvisor/pkg/sentry/fs/tmpfs"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/tmpfs"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// Differences from stat_benchmark:
+//
+// - Syscall interception, CopyInPath, copyOutStat, and overlayfs overheads are
+// not included.
+//
+// - *MountStat benchmarks use a tmpfs root mount and a tmpfs submount at /tmp.
+// Non-MountStat benchmarks use a tmpfs root mount and no submounts.
+// stat_benchmark uses a varying root mount, a tmpfs submount at /tmp, and a
+// subdirectory /tmp/<top_dir> (assuming TEST_TMPDIR == "/tmp"). Thus
+// stat_benchmark at depth 1 does a comparable amount of work to *MountStat
+// benchmarks at depth 2, and non-MountStat benchmarks at depth 3.
+var depths = []int{1, 2, 3, 8, 64, 100}
+
+const (
+	mountPointName = "tmp"
+	filename       = "gvisor_test_temp_0_1557494568"
+)
+
+// This is copied from syscalls/linux/sys_file.go, with the dependency on
+// kernel.Task stripped out.
+func fileOpOn(ctx context.Context, mntns *fs.MountNamespace, root, wd *fs.Dirent, dirFD int32, path string, resolve bool, fn func(root *fs.Dirent, d *fs.Dirent) error) error {
+	var (
+		d   *fs.Dirent // The file.
+		rel *fs.Dirent // The relative directory for search (if required.)
+		err error
+	)
+
+	// Extract the working directory (maybe).
+	if len(path) > 0 && path[0] == '/' {
+		// Absolute path; rel can be nil.
+	} else if dirFD == linux.AT_FDCWD {
+		// Need to reference the working directory.
+		rel = wd
+	} else {
+		// Need to extract the given FD.
+		return syserror.EBADF
+	}
+
+	// Lookup the node.
+	remainingTraversals := uint(linux.MaxSymlinkTraversals)
+	if resolve {
+		d, err = mntns.FindInode(ctx, root, rel, path, &remainingTraversals)
+	} else {
+		d, err = mntns.FindLink(ctx, root, rel, path, &remainingTraversals)
+	}
+	if err != nil {
+		return err
+	}
+
+	err = fn(root, d)
+	d.DecRef()
+	return err
+}
+
+func BenchmarkVFS1TmpfsStat(b *testing.B) {
+	for _, depth := range depths {
+		b.Run(fmt.Sprintf("%d", depth), func(b *testing.B) {
+			ctx := contexttest.Context(b)
+
+			// Create VFS.
+			tmpfsFS, ok := fs.FindFilesystem("tmpfs")
+			if !ok {
+				b.Fatalf("failed to find tmpfs filesystem type")
+			}
+			rootInode, err := tmpfsFS.Mount(ctx, "tmpfs", fs.MountSourceFlags{}, "", nil)
+			if err != nil {
+				b.Fatalf("failed to create tmpfs root mount: %v", err)
+			}
+			mntns, err := fs.NewMountNamespace(ctx, rootInode)
+			if err != nil {
+				b.Fatalf("failed to create mount namespace: %v", err)
+			}
+			defer mntns.DecRef()
+
+			var filePathBuilder strings.Builder
+			filePathBuilder.WriteByte('/')
+
+			// Create nested directories with given depth.
+			root := mntns.Root()
+			defer root.DecRef()
+			d := root
+			d.IncRef()
+			defer d.DecRef()
+			for i := depth; i > 0; i-- {
+				name := fmt.Sprintf("%d", i)
+				if err := d.Inode.CreateDirectory(ctx, d, name, fs.FilePermsFromMode(0755)); err != nil {
+					b.Fatalf("failed to create directory %q: %v", name, err)
+				}
+				next, err := d.Walk(ctx, root, name)
+				if err != nil {
+					b.Fatalf("failed to walk to directory %q: %v", name, err)
+				}
+				d.DecRef()
+				d = next
+				filePathBuilder.WriteString(name)
+				filePathBuilder.WriteByte('/')
+			}
+
+			// Create the file that will be stat'd.
+			file, err := d.Inode.Create(ctx, d, filename, fs.FileFlags{Read: true, Write: true}, fs.FilePermsFromMode(0644))
+			if err != nil {
+				b.Fatalf("failed to create file %q: %v", filename, err)
+			}
+			file.DecRef()
+			filePathBuilder.WriteString(filename)
+			filePath := filePathBuilder.String()
+
+			dirPath := false
+			runtime.GC()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				err := fileOpOn(ctx, mntns, root, root, linux.AT_FDCWD, filePath, true /* resolve */, func(root *fs.Dirent, d *fs.Dirent) error {
+					if dirPath && !fs.IsDir(d.Inode.StableAttr) {
+						return syserror.ENOTDIR
+					}
+					uattr, err := d.Inode.UnstableAttr(ctx)
+					if err != nil {
+						return err
+					}
+					// Sanity check.
+					if uattr.Perms.User.Execute {
+						b.Fatalf("got wrong permissions (%0o)", uattr.Perms.LinuxMode())
+					}
+					return nil
+				})
+				if err != nil {
+					b.Fatalf("stat(%q) failed: %v", filePath, err)
+				}
+			}
+			// Don't include deferred cleanup in benchmark time.
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkVFS2MemfsStat(b *testing.B) {
+	for _, depth := range depths {
+		b.Run(fmt.Sprintf("%d", depth), func(b *testing.B) {
+			ctx := contexttest.Context(b)
+			creds := auth.CredentialsFromContext(ctx)
+
+			// Create VFS.
+			vfsObj := vfs.New()
+			vfsObj.MustRegisterFilesystemType("tmpfs", tmpfs.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+				AllowUserMount: true,
+			})
+			mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "tmpfs", &vfs.GetFilesystemOptions{})
+			if err != nil {
+				b.Fatalf("failed to create tmpfs root mount: %v", err)
+			}
+			defer mntns.DecRef(vfsObj)
+
+			var filePathBuilder strings.Builder
+			filePathBuilder.WriteByte('/')
+
+			// Create nested directories with given depth.
+			root := mntns.Root()
+			defer root.DecRef()
+			vd := root
+			vd.IncRef()
+			for i := depth; i > 0; i-- {
+				name := fmt.Sprintf("%d", i)
+				pop := vfs.PathOperation{
+					Root:  root,
+					Start: vd,
+					Path:  fspath.Parse(name),
+				}
+				if err := vfsObj.MkdirAt(ctx, creds, &pop, &vfs.MkdirOptions{
+					Mode: 0755,
+				}); err != nil {
+					b.Fatalf("failed to create directory %q: %v", name, err)
+				}
+				nextVD, err := vfsObj.GetDentryAt(ctx, creds, &pop, &vfs.GetDentryOptions{})
+				if err != nil {
+					b.Fatalf("failed to walk to directory %q: %v", name, err)
+				}
+				vd.DecRef()
+				vd = nextVD
+				filePathBuilder.WriteString(name)
+				filePathBuilder.WriteByte('/')
+			}
+
+			// Create the file that will be stat'd.
+			fd, err := vfsObj.OpenAt(ctx, creds, &vfs.PathOperation{
+				Root:               root,
+				Start:              vd,
+				Path:               fspath.Parse(filename),
+				FollowFinalSymlink: true,
+			}, &vfs.OpenOptions{
+				Flags: linux.O_RDWR | linux.O_CREAT | linux.O_EXCL,
+				Mode:  0644,
+			})
+			vd.DecRef()
+			vd = vfs.VirtualDentry{}
+			if err != nil {
+				b.Fatalf("failed to create file %q: %v", filename, err)
+			}
+			defer fd.DecRef()
+			filePathBuilder.WriteString(filename)
+			filePath := filePathBuilder.String()
+
+			runtime.GC()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				stat, err := vfsObj.StatAt(ctx, creds, &vfs.PathOperation{
+					Root:               root,
+					Start:              root,
+					Path:               fspath.Parse(filePath),
+					FollowFinalSymlink: true,
+				}, &vfs.StatOptions{})
+				if err != nil {
+					b.Fatalf("stat(%q) failed: %v", filePath, err)
+				}
+				// Sanity check.
+				if stat.Mode&^linux.S_IFMT != 0644 {
+					b.Fatalf("got wrong permissions (%0o)", stat.Mode)
+				}
+			}
+			// Don't include deferred cleanup in benchmark time.
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkVFS1TmpfsMountStat(b *testing.B) {
+	for _, depth := range depths {
+		b.Run(fmt.Sprintf("%d", depth), func(b *testing.B) {
+			ctx := contexttest.Context(b)
+
+			// Create VFS.
+			tmpfsFS, ok := fs.FindFilesystem("tmpfs")
+			if !ok {
+				b.Fatalf("failed to find tmpfs filesystem type")
+			}
+			rootInode, err := tmpfsFS.Mount(ctx, "tmpfs", fs.MountSourceFlags{}, "", nil)
+			if err != nil {
+				b.Fatalf("failed to create tmpfs root mount: %v", err)
+			}
+			mntns, err := fs.NewMountNamespace(ctx, rootInode)
+			if err != nil {
+				b.Fatalf("failed to create mount namespace: %v", err)
+			}
+			defer mntns.DecRef()
+
+			var filePathBuilder strings.Builder
+			filePathBuilder.WriteByte('/')
+
+			// Create and mount the submount.
+			root := mntns.Root()
+			defer root.DecRef()
+			if err := root.Inode.CreateDirectory(ctx, root, mountPointName, fs.FilePermsFromMode(0755)); err != nil {
+				b.Fatalf("failed to create mount point: %v", err)
+			}
+			mountPoint, err := root.Walk(ctx, root, mountPointName)
+			if err != nil {
+				b.Fatalf("failed to walk to mount point: %v", err)
+			}
+			defer mountPoint.DecRef()
+			submountInode, err := tmpfsFS.Mount(ctx, "tmpfs", fs.MountSourceFlags{}, "", nil)
+			if err != nil {
+				b.Fatalf("failed to create tmpfs submount: %v", err)
+			}
+			if err := mntns.Mount(ctx, mountPoint, submountInode); err != nil {
+				b.Fatalf("failed to mount tmpfs submount: %v", err)
+			}
+			filePathBuilder.WriteString(mountPointName)
+			filePathBuilder.WriteByte('/')
+
+			// Create nested directories with given depth.
+			d, err := root.Walk(ctx, root, mountPointName)
+			if err != nil {
+				b.Fatalf("failed to walk to mount root: %v", err)
+			}
+			defer d.DecRef()
+			for i := depth; i > 0; i-- {
+				name := fmt.Sprintf("%d", i)
+				if err := d.Inode.CreateDirectory(ctx, d, name, fs.FilePermsFromMode(0755)); err != nil {
+					b.Fatalf("failed to create directory %q: %v", name, err)
+				}
+				next, err := d.Walk(ctx, root, name)
+				if err != nil {
+					b.Fatalf("failed to walk to directory %q: %v", name, err)
+				}
+				d.DecRef()
+				d = next
+				filePathBuilder.WriteString(name)
+				filePathBuilder.WriteByte('/')
+			}
+
+			// Create the file that will be stat'd.
+			file, err := d.Inode.Create(ctx, d, filename, fs.FileFlags{Read: true, Write: true}, fs.FilePermsFromMode(0644))
+			if err != nil {
+				b.Fatalf("failed to create file %q: %v", filename, err)
+			}
+			file.DecRef()
+			filePathBuilder.WriteString(filename)
+			filePath := filePathBuilder.String()
+
+			dirPath := false
+			runtime.GC()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				err := fileOpOn(ctx, mntns, root, root, linux.AT_FDCWD, filePath, true /* resolve */, func(root *fs.Dirent, d *fs.Dirent) error {
+					if dirPath && !fs.IsDir(d.Inode.StableAttr) {
+						return syserror.ENOTDIR
+					}
+					uattr, err := d.Inode.UnstableAttr(ctx)
+					if err != nil {
+						return err
+					}
+					// Sanity check.
+					if uattr.Perms.User.Execute {
+						b.Fatalf("got wrong permissions (%0o)", uattr.Perms.LinuxMode())
+					}
+					return nil
+				})
+				if err != nil {
+					b.Fatalf("stat(%q) failed: %v", filePath, err)
+				}
+			}
+			// Don't include deferred cleanup in benchmark time.
+			b.StopTimer()
+		})
+	}
+}
+
+func BenchmarkVFS2MemfsMountStat(b *testing.B) {
+	for _, depth := range depths {
+		b.Run(fmt.Sprintf("%d", depth), func(b *testing.B) {
+			ctx := contexttest.Context(b)
+			creds := auth.CredentialsFromContext(ctx)
+
+			// Create VFS.
+			vfsObj := vfs.New()
+			vfsObj.MustRegisterFilesystemType("tmpfs", tmpfs.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+				AllowUserMount: true,
+			})
+			mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "tmpfs", &vfs.GetFilesystemOptions{})
+			if err != nil {
+				b.Fatalf("failed to create tmpfs root mount: %v", err)
+			}
+			defer mntns.DecRef(vfsObj)
+
+			var filePathBuilder strings.Builder
+			filePathBuilder.WriteByte('/')
+
+			// Create the mount point.
+			root := mntns.Root()
+			defer root.DecRef()
+			pop := vfs.PathOperation{
+				Root:  root,
+				Start: root,
+				Path:  fspath.Parse(mountPointName),
+			}
+			if err := vfsObj.MkdirAt(ctx, creds, &pop, &vfs.MkdirOptions{
+				Mode: 0755,
+			}); err != nil {
+				b.Fatalf("failed to create mount point: %v", err)
+			}
+			// Save the mount point for later use.
+			mountPoint, err := vfsObj.GetDentryAt(ctx, creds, &pop, &vfs.GetDentryOptions{})
+			if err != nil {
+				b.Fatalf("failed to walk to mount point: %v", err)
+			}
+			defer mountPoint.DecRef()
+			// Create and mount the submount.
+			if err := vfsObj.MountAt(ctx, creds, "", &pop, "tmpfs", &vfs.MountOptions{}); err != nil {
+				b.Fatalf("failed to mount tmpfs submount: %v", err)
+			}
+			filePathBuilder.WriteString(mountPointName)
+			filePathBuilder.WriteByte('/')
+
+			// Create nested directories with given depth.
+			vd, err := vfsObj.GetDentryAt(ctx, creds, &pop, &vfs.GetDentryOptions{})
+			if err != nil {
+				b.Fatalf("failed to walk to mount root: %v", err)
+			}
+			for i := depth; i > 0; i-- {
+				name := fmt.Sprintf("%d", i)
+				pop := vfs.PathOperation{
+					Root:  root,
+					Start: vd,
+					Path:  fspath.Parse(name),
+				}
+				if err := vfsObj.MkdirAt(ctx, creds, &pop, &vfs.MkdirOptions{
+					Mode: 0755,
+				}); err != nil {
+					b.Fatalf("failed to create directory %q: %v", name, err)
+				}
+				nextVD, err := vfsObj.GetDentryAt(ctx, creds, &pop, &vfs.GetDentryOptions{})
+				if err != nil {
+					b.Fatalf("failed to walk to directory %q: %v", name, err)
+				}
+				vd.DecRef()
+				vd = nextVD
+				filePathBuilder.WriteString(name)
+				filePathBuilder.WriteByte('/')
+			}
+
+			// Verify that we didn't create any directories under the mount
+			// point (i.e. they were all created on the submount).
+			firstDirName := fmt.Sprintf("%d", depth)
+			if child := mountPoint.Dentry().Child(firstDirName); child != nil {
+				b.Fatalf("created directory %q under root mount, not submount", firstDirName)
+			}
+
+			// Create the file that will be stat'd.
+			fd, err := vfsObj.OpenAt(ctx, creds, &vfs.PathOperation{
+				Root:               root,
+				Start:              vd,
+				Path:               fspath.Parse(filename),
+				FollowFinalSymlink: true,
+			}, &vfs.OpenOptions{
+				Flags: linux.O_RDWR | linux.O_CREAT | linux.O_EXCL,
+				Mode:  0644,
+			})
+			vd.DecRef()
+			if err != nil {
+				b.Fatalf("failed to create file %q: %v", filename, err)
+			}
+			fd.DecRef()
+			filePathBuilder.WriteString(filename)
+			filePath := filePathBuilder.String()
+
+			runtime.GC()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				stat, err := vfsObj.StatAt(ctx, creds, &vfs.PathOperation{
+					Root:               root,
+					Start:              root,
+					Path:               fspath.Parse(filePath),
+					FollowFinalSymlink: true,
+				}, &vfs.StatOptions{})
+				if err != nil {
+					b.Fatalf("stat(%q) failed: %v", filePath, err)
+				}
+				// Sanity check.
+				if stat.Mode&^linux.S_IFMT != 0644 {
+					b.Fatalf("got wrong permissions (%0o)", stat.Mode)
+				}
+			}
+			// Don't include deferred cleanup in benchmark time.
+			b.StopTimer()
+		})
+	}
+}
+
+func init() {
+	// Turn off reference leak checking for a fair comparison between vfs1 and
+	// vfs2.
+	refs.SetLeakMode(refs.NoLeakChecking)
+}

pkg/sentry/fsimpl/tmpfs/directory.go

@@ -0,0 +1,187 @@
+// 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 tmpfs
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+type directory struct {
+	inode inode
+
+	// childList is a list containing (1) child Dentries and (2) fake Dentries
+	// (with inode == nil) that represent the iteration position of
+	// directoryFDs. childList is used to support directoryFD.IterDirents()
+	// efficiently. childList is protected by filesystem.mu.
+	childList dentryList
+}
+
+func (fs *filesystem) newDirectory(creds *auth.Credentials, mode linux.FileMode) *inode {
+	dir := &directory{}
+	dir.inode.init(dir, fs, creds, mode)
+	dir.inode.nlink = 2 // from "." and parent directory or ".." for root
+	return &dir.inode
+}
+
+func (i *inode) isDir() bool {
+	_, ok := i.impl.(*directory)
+	return ok
+}
+
+type directoryFD struct {
+	fileDescription
+	vfs.DirectoryFileDescriptionDefaultImpl
+
+	// Protected by filesystem.mu.
+	iter *dentry
+	off  int64
+}
+
+// Release implements vfs.FileDescriptionImpl.Release.
+func (fd *directoryFD) Release() {
+	if fd.iter != nil {
+		fs := fd.filesystem()
+		dir := fd.inode().impl.(*directory)
+		fs.mu.Lock()
+		dir.childList.Remove(fd.iter)
+		fs.mu.Unlock()
+		fd.iter = nil
+	}
+}
+
+// IterDirents implements vfs.FileDescriptionImpl.IterDirents.
+func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback) error {
+	fs := fd.filesystem()
+	vfsd := fd.vfsfd.VirtualDentry().Dentry()
+
+	fs.mu.Lock()
+	defer fs.mu.Unlock()
+
+	if fd.off == 0 {
+		if !cb.Handle(vfs.Dirent{
+			Name:    ".",
+			Type:    linux.DT_DIR,
+			Ino:     vfsd.Impl().(*dentry).inode.ino,
+			NextOff: 1,
+		}) {
+			return nil
+		}
+		fd.off++
+	}
+	if fd.off == 1 {
+		parentInode := vfsd.ParentOrSelf().Impl().(*dentry).inode
+		if !cb.Handle(vfs.Dirent{
+			Name:    "..",
+			Type:    parentInode.direntType(),
+			Ino:     parentInode.ino,
+			NextOff: 2,
+		}) {
+			return nil
+		}
+		fd.off++
+	}
+
+	dir := vfsd.Impl().(*dentry).inode.impl.(*directory)
+	var child *dentry
+	if fd.iter == nil {
+		// Start iteration at the beginning of dir.
+		child = dir.childList.Front()
+		fd.iter = &dentry{}
+	} else {
+		// Continue iteration from where we left off.
+		child = fd.iter.Next()
+		dir.childList.Remove(fd.iter)
+	}
+	for child != nil {
+		// Skip other directoryFD iterators.
+		if child.inode != nil {
+			if !cb.Handle(vfs.Dirent{
+				Name:    child.vfsd.Name(),
+				Type:    child.inode.direntType(),
+				Ino:     child.inode.ino,
+				NextOff: fd.off + 1,
+			}) {
+				dir.childList.InsertBefore(child, fd.iter)
+				return nil
+			}
+			fd.off++
+		}
+		child = child.Next()
+	}
+	dir.childList.PushBack(fd.iter)
+	return nil
+}
+
+// Seek implements vfs.FileDescriptionImpl.Seek.
+func (fd *directoryFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	fs := fd.filesystem()
+	fs.mu.Lock()
+	defer fs.mu.Unlock()
+
+	switch whence {
+	case linux.SEEK_SET:
+		// Use offset as given.
+	case linux.SEEK_CUR:
+		offset += fd.off
+	default:
+		return 0, syserror.EINVAL
+	}
+	if offset < 0 {
+		return 0, syserror.EINVAL
+	}
+
+	// If the offset isn't changing (e.g. due to lseek(0, SEEK_CUR)), don't
+	// seek even if doing so might reposition the iterator due to concurrent
+	// mutation of the directory. Compare fs/libfs.c:dcache_dir_lseek().
+	if fd.off == offset {
+		return offset, nil
+	}
+
+	fd.off = offset
+	// Compensate for "." and "..".
+	remChildren := int64(0)
+	if offset >= 2 {
+		remChildren = offset - 2
+	}
+
+	dir := fd.inode().impl.(*directory)
+
+	// Ensure that fd.iter exists and is not linked into dir.childList.
+	if fd.iter == nil {
+		fd.iter = &dentry{}
+	} else {
+		dir.childList.Remove(fd.iter)
+	}
+	// Insert fd.iter before the remChildren'th child, or at the end of the
+	// list if remChildren >= number of children.
+	child := dir.childList.Front()
+	for child != nil {
+		// Skip other directoryFD iterators.
+		if child.inode != nil {
+			if remChildren == 0 {
+				dir.childList.InsertBefore(child, fd.iter)
+				return offset, nil
+			}
+			remChildren--
+		}
+		child = child.Next()
+	}
+	dir.childList.PushBack(fd.iter)
+	return offset, nil
+}

pkg/sentry/fsimpl/tmpfs/named_pipe.go

@@ -0,0 +1,60 @@
+// 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 tmpfs
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/pipe"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+)
+
+type namedPipe struct {
+	inode inode
+
+	pipe *pipe.VFSPipe
+}
+
+// Preconditions:
+//   * fs.mu must be locked.
+//   * rp.Mount().CheckBeginWrite() has been called successfully.
+func (fs *filesystem) newNamedPipe(creds *auth.Credentials, mode linux.FileMode) *inode {
+	file := &namedPipe{pipe: pipe.NewVFSPipe(pipe.DefaultPipeSize, usermem.PageSize)}
+	file.inode.init(file, fs, creds, mode)
+	file.inode.nlink = 1 // Only the parent has a link.
+	return &file.inode
+}
+
+// namedPipeFD implements vfs.FileDescriptionImpl. Methods are implemented
+// entirely via struct embedding.
+type namedPipeFD struct {
+	fileDescription
+
+	*pipe.VFSPipeFD
+}
+
+func newNamedPipeFD(ctx context.Context, np *namedPipe, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, flags uint32) (*vfs.FileDescription, error) {
+	var err error
+	var fd namedPipeFD
+	fd.VFSPipeFD, err = np.pipe.NewVFSPipeFD(ctx, rp, vfsd, &fd.vfsfd, flags)
+	if err != nil {
+		return nil, err
+	}
+	mnt := rp.Mount()
+	fd.vfsfd.Init(&fd, flags, mnt, vfsd, &vfs.FileDescriptionOptions{})
+	return &fd.vfsfd, nil
+}

pkg/sentry/fsimpl/tmpfs/pipe_test.go

@@ -0,0 +1,235 @@
+// 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 tmpfs
+
+import (
+	"bytes"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/context/contexttest"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+const fileName = "mypipe"
+
+func TestSeparateFDs(t *testing.T) {
+	ctx, creds, vfsObj, root := setup(t)
+	defer root.DecRef()
+
+	// Open the read side. This is done in a concurrently because opening
+	// One end the pipe blocks until the other end is opened.
+	pop := vfs.PathOperation{
+		Root:               root,
+		Start:              root,
+		Path:               fspath.Parse(fileName),
+		FollowFinalSymlink: true,
+	}
+	rfdchan := make(chan *vfs.FileDescription)
+	go func() {
+		openOpts := vfs.OpenOptions{Flags: linux.O_RDONLY}
+		rfd, _ := vfsObj.OpenAt(ctx, creds, &pop, &openOpts)
+		rfdchan <- rfd
+	}()
+
+	// Open the write side.
+	openOpts := vfs.OpenOptions{Flags: linux.O_WRONLY}
+	wfd, err := vfsObj.OpenAt(ctx, creds, &pop, &openOpts)
+	if err != nil {
+		t.Fatalf("failed to open pipe for writing %q: %v", fileName, err)
+	}
+	defer wfd.DecRef()
+
+	rfd, ok := <-rfdchan
+	if !ok {
+		t.Fatalf("failed to open pipe for reading %q", fileName)
+	}
+	defer rfd.DecRef()
+
+	const msg = "vamos azul"
+	checkEmpty(ctx, t, rfd)
+	checkWrite(ctx, t, wfd, msg)
+	checkRead(ctx, t, rfd, msg)
+}
+
+func TestNonblockingRead(t *testing.T) {
+	ctx, creds, vfsObj, root := setup(t)
+	defer root.DecRef()
+
+	// Open the read side as nonblocking.
+	pop := vfs.PathOperation{
+		Root:               root,
+		Start:              root,
+		Path:               fspath.Parse(fileName),
+		FollowFinalSymlink: true,
+	}
+	openOpts := vfs.OpenOptions{Flags: linux.O_RDONLY | linux.O_NONBLOCK}
+	rfd, err := vfsObj.OpenAt(ctx, creds, &pop, &openOpts)
+	if err != nil {
+		t.Fatalf("failed to open pipe for reading %q: %v", fileName, err)
+	}
+	defer rfd.DecRef()
+
+	// Open the write side.
+	openOpts = vfs.OpenOptions{Flags: linux.O_WRONLY}
+	wfd, err := vfsObj.OpenAt(ctx, creds, &pop, &openOpts)
+	if err != nil {
+		t.Fatalf("failed to open pipe for writing %q: %v", fileName, err)
+	}
+	defer wfd.DecRef()
+
+	const msg = "geh blau"
+	checkEmpty(ctx, t, rfd)
+	checkWrite(ctx, t, wfd, msg)
+	checkRead(ctx, t, rfd, msg)
+}
+
+func TestNonblockingWriteError(t *testing.T) {
+	ctx, creds, vfsObj, root := setup(t)
+	defer root.DecRef()
+
+	// Open the write side as nonblocking, which should return ENXIO.
+	pop := vfs.PathOperation{
+		Root:               root,
+		Start:              root,
+		Path:               fspath.Parse(fileName),
+		FollowFinalSymlink: true,
+	}
+	openOpts := vfs.OpenOptions{Flags: linux.O_WRONLY | linux.O_NONBLOCK}
+	_, err := vfsObj.OpenAt(ctx, creds, &pop, &openOpts)
+	if err != syserror.ENXIO {
+		t.Fatalf("expected ENXIO, but got error: %v", err)
+	}
+}
+
+func TestSingleFD(t *testing.T) {
+	ctx, creds, vfsObj, root := setup(t)
+	defer root.DecRef()
+
+	// Open the pipe as readable and writable.
+	pop := vfs.PathOperation{
+		Root:               root,
+		Start:              root,
+		Path:               fspath.Parse(fileName),
+		FollowFinalSymlink: true,
+	}
+	openOpts := vfs.OpenOptions{Flags: linux.O_RDWR}
+	fd, err := vfsObj.OpenAt(ctx, creds, &pop, &openOpts)
+	if err != nil {
+		t.Fatalf("failed to open pipe for writing %q: %v", fileName, err)
+	}
+	defer fd.DecRef()
+
+	const msg = "forza blu"
+	checkEmpty(ctx, t, fd)
+	checkWrite(ctx, t, fd, msg)
+	checkRead(ctx, t, fd, msg)
+}
+
+// setup creates a VFS with a pipe in the root directory at path fileName. The
+// returned VirtualDentry must be DecRef()'d be the caller. It calls t.Fatal
+// upon failure.
+func setup(t *testing.T) (context.Context, *auth.Credentials, *vfs.VirtualFilesystem, vfs.VirtualDentry) {
+	ctx := contexttest.Context(t)
+	creds := auth.CredentialsFromContext(ctx)
+
+	// Create VFS.
+	vfsObj := vfs.New()
+	vfsObj.MustRegisterFilesystemType("tmpfs", FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserMount: true,
+	})
+	mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "tmpfs", &vfs.GetFilesystemOptions{})
+	if err != nil {
+		t.Fatalf("failed to create tmpfs root mount: %v", err)
+	}
+
+	// Create the pipe.
+	root := mntns.Root()
+	pop := vfs.PathOperation{
+		Root:  root,
+		Start: root,
+		Path:  fspath.Parse(fileName),
+	}
+	mknodOpts := vfs.MknodOptions{Mode: linux.ModeNamedPipe | 0644}
+	if err := vfsObj.MknodAt(ctx, creds, &pop, &mknodOpts); err != nil {
+		t.Fatalf("failed to create file %q: %v", fileName, err)
+	}
+
+	// Sanity check: the file pipe exists and has the correct mode.
+	stat, err := vfsObj.StatAt(ctx, creds, &vfs.PathOperation{
+		Root:               root,
+		Start:              root,
+		Path:               fspath.Parse(fileName),
+		FollowFinalSymlink: true,
+	}, &vfs.StatOptions{})
+	if err != nil {
+		t.Fatalf("stat(%q) failed: %v", fileName, err)
+	}
+	if stat.Mode&^linux.S_IFMT != 0644 {
+		t.Errorf("got wrong permissions (%0o)", stat.Mode)
+	}
+	if stat.Mode&linux.S_IFMT != linux.ModeNamedPipe {
+		t.Errorf("got wrong file type (%0o)", stat.Mode)
+	}
+
+	return ctx, creds, vfsObj, root
+}
+
+// checkEmpty calls t.Fatal if the pipe in fd is not empty.
+func checkEmpty(ctx context.Context, t *testing.T, fd *vfs.FileDescription) {
+	readData := make([]byte, 1)
+	dst := usermem.BytesIOSequence(readData)
+	bytesRead, err := fd.Read(ctx, dst, vfs.ReadOptions{})
+	if err != syserror.ErrWouldBlock {
+		t.Fatalf("expected ErrWouldBlock reading from empty pipe %q, but got: %v", fileName, err)
+	}
+	if bytesRead != 0 {
+		t.Fatalf("expected to read 0 bytes, but got %d", bytesRead)
+	}
+}
+
+// checkWrite calls t.Fatal if it fails to write all of msg to fd.
+func checkWrite(ctx context.Context, t *testing.T, fd *vfs.FileDescription, msg string) {
+	writeData := []byte(msg)
+	src := usermem.BytesIOSequence(writeData)
+	bytesWritten, err := fd.Write(ctx, src, vfs.WriteOptions{})
+	if err != nil {
+		t.Fatalf("error writing to pipe %q: %v", fileName, err)
+	}
+	if bytesWritten != int64(len(writeData)) {
+		t.Fatalf("expected to write %d bytes, but wrote %d", len(writeData), bytesWritten)
+	}
+}
+
+// checkRead calls t.Fatal if it fails to read msg from fd.
+func checkRead(ctx context.Context, t *testing.T, fd *vfs.FileDescription, msg string) {
+	readData := make([]byte, len(msg))
+	dst := usermem.BytesIOSequence(readData)
+	bytesRead, err := fd.Read(ctx, dst, vfs.ReadOptions{})
+	if err != nil {
+		t.Fatalf("error reading from pipe %q: %v", fileName, err)
+	}
+	if bytesRead != int64(len(msg)) {
+		t.Fatalf("expected to read %d bytes, but got %d", len(msg), bytesRead)
+	}
+	if !bytes.Equal(readData, []byte(msg)) {
+		t.Fatalf("expected to read %q from pipe, but got %q", msg, string(readData))
+	}
+}

pkg/sentry/fsimpl/tmpfs/regular_file.go

@@ -0,0 +1,357 @@
+// 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 tmpfs
+
+import (
+	"io"
+	"math"
+	"sync"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/fs/fsutil"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
+	"gvisor.dev/gvisor/pkg/sentry/pgalloc"
+	"gvisor.dev/gvisor/pkg/sentry/safemem"
+	"gvisor.dev/gvisor/pkg/sentry/usage"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+type regularFile struct {
+	inode inode
+
+	// memFile is a platform.File used to allocate pages to this regularFile.
+	memFile *pgalloc.MemoryFile
+
+	// mu protects the fields below.
+	mu sync.RWMutex
+
+	// data maps offsets into the file to offsets into memFile that store
+	// the file's data.
+	data fsutil.FileRangeSet
+
+	// size is the size of data, but accessed using atomic memory
+	// operations to avoid locking in inode.stat().
+	size uint64
+
+	// seals represents file seals on this inode.
+	seals uint32
+}
+
+func (fs *filesystem) newRegularFile(creds *auth.Credentials, mode linux.FileMode) *inode {
+	file := &regularFile{
+		memFile: fs.memFile,
+	}
+	file.inode.init(file, fs, creds, mode)
+	file.inode.nlink = 1 // from parent directory
+	return &file.inode
+}
+
+type regularFileFD struct {
+	fileDescription
+
+	// These are immutable.
+	readable bool
+	writable bool
+
+	// off is the file offset. off is accessed using atomic memory operations.
+	// offMu serializes operations that may mutate off.
+	off   int64
+	offMu sync.Mutex
+}
+
+// Release implements vfs.FileDescriptionImpl.Release.
+func (fd *regularFileFD) Release() {
+	if fd.writable {
+		fd.vfsfd.VirtualDentry().Mount().EndWrite()
+	}
+}
+
+// PRead implements vfs.FileDescriptionImpl.PRead.
+func (fd *regularFileFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	if !fd.readable {
+		return 0, syserror.EINVAL
+	}
+	if offset < 0 {
+		return 0, syserror.EINVAL
+	}
+	if dst.NumBytes() == 0 {
+		return 0, nil
+	}
+	f := fd.inode().impl.(*regularFile)
+	rw := getRegularFileReadWriter(f, offset)
+	n, err := dst.CopyOutFrom(ctx, rw)
+	putRegularFileReadWriter(rw)
+	return int64(n), err
+}
+
+// Read implements vfs.FileDescriptionImpl.Read.
+func (fd *regularFileFD) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	fd.offMu.Lock()
+	n, err := fd.PRead(ctx, dst, fd.off, opts)
+	fd.off += n
+	fd.offMu.Unlock()
+	return n, err
+}
+
+// PWrite implements vfs.FileDescriptionImpl.PWrite.
+func (fd *regularFileFD) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
+	if !fd.writable {
+		return 0, syserror.EINVAL
+	}
+	if offset < 0 {
+		return 0, syserror.EINVAL
+	}
+	srclen := src.NumBytes()
+	if srclen == 0 {
+		return 0, nil
+	}
+	f := fd.inode().impl.(*regularFile)
+	end := offset + srclen
+	if end < offset {
+		// Overflow.
+		return 0, syserror.EFBIG
+	}
+	rw := getRegularFileReadWriter(f, offset)
+	n, err := src.CopyInTo(ctx, rw)
+	putRegularFileReadWriter(rw)
+	return n, err
+}
+
+// Write implements vfs.FileDescriptionImpl.Write.
+func (fd *regularFileFD) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
+	fd.offMu.Lock()
+	n, err := fd.PWrite(ctx, src, fd.off, opts)
+	fd.off += n
+	fd.offMu.Unlock()
+	return n, err
+}
+
+// Seek implements vfs.FileDescriptionImpl.Seek.
+func (fd *regularFileFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	fd.offMu.Lock()
+	defer fd.offMu.Unlock()
+	switch whence {
+	case linux.SEEK_SET:
+		// use offset as specified
+	case linux.SEEK_CUR:
+		offset += fd.off
+	case linux.SEEK_END:
+		offset += int64(atomic.LoadUint64(&fd.inode().impl.(*regularFile).size))
+	default:
+		return 0, syserror.EINVAL
+	}
+	if offset < 0 {
+		return 0, syserror.EINVAL
+	}
+	fd.off = offset
+	return offset, nil
+}
+
+// Sync implements vfs.FileDescriptionImpl.Sync.
+func (fd *regularFileFD) Sync(ctx context.Context) error {
+	return nil
+}
+
+// regularFileReadWriter implements safemem.Reader and Safemem.Writer.
+type regularFileReadWriter struct {
+	file *regularFile
+
+	// Offset into the file to read/write at. Note that this may be
+	// different from the FD offset if PRead/PWrite is used.
+	off uint64
+}
+
+var regularFileReadWriterPool = sync.Pool{
+	New: func() interface{} {
+		return &regularFileReadWriter{}
+	},
+}
+
+func getRegularFileReadWriter(file *regularFile, offset int64) *regularFileReadWriter {
+	rw := regularFileReadWriterPool.Get().(*regularFileReadWriter)
+	rw.file = file
+	rw.off = uint64(offset)
+	return rw
+}
+
+func putRegularFileReadWriter(rw *regularFileReadWriter) {
+	rw.file = nil
+	regularFileReadWriterPool.Put(rw)
+}
+
+// ReadToBlocks implements safemem.Reader.ReadToBlocks.
+func (rw *regularFileReadWriter) ReadToBlocks(dsts safemem.BlockSeq) (uint64, error) {
+	rw.file.mu.RLock()
+
+	// Compute the range to read (limited by file size and overflow-checked).
+	if rw.off >= rw.file.size {
+		rw.file.mu.RUnlock()
+		return 0, io.EOF
+	}
+	end := rw.file.size
+	if rend := rw.off + dsts.NumBytes(); rend > rw.off && rend < end {
+		end = rend
+	}
+
+	var done uint64
+	seg, gap := rw.file.data.Find(uint64(rw.off))
+	for rw.off < end {
+		mr := memmap.MappableRange{uint64(rw.off), uint64(end)}
+		switch {
+		case seg.Ok():
+			// Get internal mappings.
+			ims, err := rw.file.memFile.MapInternal(seg.FileRangeOf(seg.Range().Intersect(mr)), usermem.Read)
+			if err != nil {
+				rw.file.mu.RUnlock()
+				return done, err
+			}
+
+			// Copy from internal mappings.
+			n, err := safemem.CopySeq(dsts, ims)
+			done += n
+			rw.off += uint64(n)
+			dsts = dsts.DropFirst64(n)
+			if err != nil {
+				rw.file.mu.RUnlock()
+				return done, err
+			}
+
+			// Continue.
+			seg, gap = seg.NextNonEmpty()
+
+		case gap.Ok():
+			// Tmpfs holes are zero-filled.
+			gapmr := gap.Range().Intersect(mr)
+			dst := dsts.TakeFirst64(gapmr.Length())
+			n, err := safemem.ZeroSeq(dst)
+			done += n
+			rw.off += uint64(n)
+			dsts = dsts.DropFirst64(n)
+			if err != nil {
+				rw.file.mu.RUnlock()
+				return done, err
+			}
+
+			// Continue.
+			seg, gap = gap.NextSegment(), fsutil.FileRangeGapIterator{}
+		}
+	}
+	rw.file.mu.RUnlock()
+	return done, nil
+}
+
+// WriteFromBlocks implements safemem.Writer.WriteFromBlocks.
+func (rw *regularFileReadWriter) WriteFromBlocks(srcs safemem.BlockSeq) (uint64, error) {
+	rw.file.mu.Lock()
+
+	// Compute the range to write (overflow-checked).
+	end := rw.off + srcs.NumBytes()
+	if end <= rw.off {
+		end = math.MaxInt64
+	}
+
+	// Check if seals prevent either file growth or all writes.
+	switch {
+	case rw.file.seals&linux.F_SEAL_WRITE != 0: // Write sealed
+		rw.file.mu.Unlock()
+		return 0, syserror.EPERM
+	case end > rw.file.size && rw.file.seals&linux.F_SEAL_GROW != 0: // Grow sealed
+		// When growth is sealed, Linux effectively allows writes which would
+		// normally grow the file to partially succeed up to the current EOF,
+		// rounded down to the page boundary before the EOF.
+		//
+		// This happens because writes (and thus the growth check) for tmpfs
+		// files proceed page-by-page on Linux, and the final write to the page
+		// containing EOF fails, resulting in a partial write up to the start of
+		// that page.
+		//
+		// To emulate this behaviour, artifically truncate the write to the
+		// start of the page containing the current EOF.
+		//
+		// See Linux, mm/filemap.c:generic_perform_write() and
+		// mm/shmem.c:shmem_write_begin().
+		if pgstart := uint64(usermem.Addr(rw.file.size).RoundDown()); end > pgstart {
+			end = pgstart
+		}
+		if end <= rw.off {
+			// Truncation would result in no data being written.
+			rw.file.mu.Unlock()
+			return 0, syserror.EPERM
+		}
+	}
+
+	// Page-aligned mr for when we need to allocate memory. RoundUp can't
+	// overflow since end is an int64.
+	pgstartaddr := usermem.Addr(rw.off).RoundDown()
+	pgendaddr, _ := usermem.Addr(end).RoundUp()
+	pgMR := memmap.MappableRange{uint64(pgstartaddr), uint64(pgendaddr)}
+
+	var (
+		done   uint64
+		retErr error
+	)
+	seg, gap := rw.file.data.Find(uint64(rw.off))
+	for rw.off < end {
+		mr := memmap.MappableRange{uint64(rw.off), uint64(end)}
+		switch {
+		case seg.Ok():
+			// Get internal mappings.
+			ims, err := rw.file.memFile.MapInternal(seg.FileRangeOf(seg.Range().Intersect(mr)), usermem.Write)
+			if err != nil {
+				retErr = err
+				goto exitLoop
+			}
+
+			// Copy to internal mappings.
+			n, err := safemem.CopySeq(ims, srcs)
+			done += n
+			rw.off += uint64(n)
+			srcs = srcs.DropFirst64(n)
+			if err != nil {
+				retErr = err
+				goto exitLoop
+			}
+
+			// Continue.
+			seg, gap = seg.NextNonEmpty()
+
+		case gap.Ok():
+			// Allocate memory for the write.
+			gapMR := gap.Range().Intersect(pgMR)
+			fr, err := rw.file.memFile.Allocate(gapMR.Length(), usage.Tmpfs)
+			if err != nil {
+				retErr = err
+				goto exitLoop
+			}
+
+			// Write to that memory as usual.
+			seg, gap = rw.file.data.Insert(gap, gapMR, fr.Start), fsutil.FileRangeGapIterator{}
+		}
+	}
+exitLoop:
+	// If the write ends beyond the file's previous size, it causes the
+	// file to grow.
+	if rw.off > rw.file.size {
+		atomic.StoreUint64(&rw.file.size, rw.off)
+	}
+
+	rw.file.mu.Unlock()
+	return done, retErr
+}

pkg/sentry/fsimpl/tmpfs/regular_file_test.go

@@ -0,0 +1,224 @@
+// 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 tmpfs
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/contexttest"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+)
+
+// newFileFD creates a new file in a new tmpfs mount, and returns the FD. If
+// the returned err is not nil, then cleanup should be called when the FD is no
+// longer needed.
+func newFileFD(ctx context.Context, filename string) (*vfs.FileDescription, func(), error) {
+	creds := auth.CredentialsFromContext(ctx)
+
+	vfsObj := vfs.New()
+	vfsObj.MustRegisterFilesystemType("tmpfs", FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserMount: true,
+	})
+	mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "tmpfs", &vfs.GetFilesystemOptions{})
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to create tmpfs root mount: %v", err)
+	}
+	root := mntns.Root()
+
+	// Create the file that will be write/read.
+	fd, err := vfsObj.OpenAt(ctx, creds, &vfs.PathOperation{
+		Root:               root,
+		Start:              root,
+		Path:               fspath.Parse(filename),
+		FollowFinalSymlink: true,
+	}, &vfs.OpenOptions{
+		Flags: linux.O_RDWR | linux.O_CREAT | linux.O_EXCL,
+		Mode:  0644,
+	})
+	if err != nil {
+		root.DecRef()
+		mntns.DecRef(vfsObj)
+		return nil, nil, fmt.Errorf("failed to create file %q: %v", filename, err)
+	}
+
+	return fd, func() {
+		root.DecRef()
+		mntns.DecRef(vfsObj)
+	}, nil
+}
+
+// Test that we can write some data to a file and read it back.`
+func TestSimpleWriteRead(t *testing.T) {
+	ctx := contexttest.Context(t)
+	fd, cleanup, err := newFileFD(ctx, "simpleReadWrite")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cleanup()
+
+	// Write.
+	data := []byte("foobarbaz")
+	n, err := fd.Write(ctx, usermem.BytesIOSequence(data), vfs.WriteOptions{})
+	if err != nil {
+		t.Fatalf("fd.Write failed: %v", err)
+	}
+	if n != int64(len(data)) {
+		t.Errorf("fd.Write got short write length %d, want %d", n, len(data))
+	}
+	if got, want := fd.Impl().(*regularFileFD).off, int64(len(data)); got != want {
+		t.Errorf("fd.Write left offset at %d, want %d", got, want)
+	}
+
+	// Seek back to beginning.
+	if _, err := fd.Seek(ctx, 0, linux.SEEK_SET); err != nil {
+		t.Fatalf("fd.Seek failed: %v", err)
+	}
+	if got, want := fd.Impl().(*regularFileFD).off, int64(0); got != want {
+		t.Errorf("fd.Seek(0) left offset at %d, want %d", got, want)
+	}
+
+	// Read.
+	buf := make([]byte, len(data))
+	n, err = fd.Read(ctx, usermem.BytesIOSequence(buf), vfs.ReadOptions{})
+	if err != nil && err != io.EOF {
+		t.Fatalf("fd.Read failed: %v", err)
+	}
+	if n != int64(len(data)) {
+		t.Errorf("fd.Read got short read length %d, want %d", n, len(data))
+	}
+	if got, want := string(buf), string(data); got != want {
+		t.Errorf("Read got %q want %s", got, want)
+	}
+	if got, want := fd.Impl().(*regularFileFD).off, int64(len(data)); got != want {
+		t.Errorf("fd.Write left offset at %d, want %d", got, want)
+	}
+}
+
+func TestPWrite(t *testing.T) {
+	ctx := contexttest.Context(t)
+	fd, cleanup, err := newFileFD(ctx, "PRead")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cleanup()
+
+	// Fill file with 1k 'a's.
+	data := bytes.Repeat([]byte{'a'}, 1000)
+	n, err := fd.Write(ctx, usermem.BytesIOSequence(data), vfs.WriteOptions{})
+	if err != nil {
+		t.Fatalf("fd.Write failed: %v", err)
+	}
+	if n != int64(len(data)) {
+		t.Errorf("fd.Write got short write length %d, want %d", n, len(data))
+	}
+
+	// Write "gVisor is awesome" at various offsets.
+	buf := []byte("gVisor is awesome")
+	offsets := []int{0, 1, 2, 10, 20, 50, 100, len(data) - 100, len(data) - 1, len(data), len(data) + 1}
+	for _, offset := range offsets {
+		name := fmt.Sprintf("PWrite offset=%d", offset)
+		t.Run(name, func(t *testing.T) {
+			n, err := fd.PWrite(ctx, usermem.BytesIOSequence(buf), int64(offset), vfs.WriteOptions{})
+			if err != nil {
+				t.Errorf("fd.PWrite got err %v want nil", err)
+			}
+			if n != int64(len(buf)) {
+				t.Errorf("fd.PWrite got %d bytes want %d", n, len(buf))
+			}
+
+			// Update data to reflect expected file contents.
+			if len(data) < offset+len(buf) {
+				data = append(data, make([]byte, (offset+len(buf))-len(data))...)
+			}
+			copy(data[offset:], buf)
+
+			// Read the whole file and compare with data.
+			readBuf := make([]byte, len(data))
+			n, err = fd.PRead(ctx, usermem.BytesIOSequence(readBuf), 0, vfs.ReadOptions{})
+			if err != nil {
+				t.Fatalf("fd.PRead failed: %v", err)
+			}
+			if n != int64(len(data)) {
+				t.Errorf("fd.PRead got short read length %d, want %d", n, len(data))
+			}
+			if got, want := string(readBuf), string(data); got != want {
+				t.Errorf("PRead got %q want %s", got, want)
+			}
+
+		})
+	}
+}
+
+func TestPRead(t *testing.T) {
+	ctx := contexttest.Context(t)
+	fd, cleanup, err := newFileFD(ctx, "PRead")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cleanup()
+
+	// Write 100 sequences of 'gVisor is awesome'.
+	data := bytes.Repeat([]byte("gVisor is awsome"), 100)
+	n, err := fd.Write(ctx, usermem.BytesIOSequence(data), vfs.WriteOptions{})
+	if err != nil {
+		t.Fatalf("fd.Write failed: %v", err)
+	}
+	if n != int64(len(data)) {
+		t.Errorf("fd.Write got short write length %d, want %d", n, len(data))
+	}
+
+	// Read various sizes from various offsets.
+	sizes := []int{0, 1, 2, 10, 20, 50, 100, 1000}
+	offsets := []int{0, 1, 2, 10, 20, 50, 100, 1000, len(data) - 100, len(data) - 1, len(data), len(data) + 1}
+
+	for _, size := range sizes {
+		for _, offset := range offsets {
+			name := fmt.Sprintf("PRead offset=%d size=%d", offset, size)
+			t.Run(name, func(t *testing.T) {
+				var (
+					wantRead []byte
+					wantErr  error
+				)
+				if offset < len(data) {
+					wantRead = data[offset:]
+				} else if size > 0 {
+					wantErr = io.EOF
+				}
+				if offset+size < len(data) {
+					wantRead = wantRead[:size]
+				}
+				buf := make([]byte, size)
+				n, err := fd.PRead(ctx, usermem.BytesIOSequence(buf), int64(offset), vfs.ReadOptions{})
+				if err != wantErr {
+					t.Errorf("fd.PRead got err %v want %v", err, wantErr)
+				}
+				if n != int64(len(wantRead)) {
+					t.Errorf("fd.PRead got %d bytes want %d", n, len(wantRead))
+				}
+				if got := string(buf[:n]); got != string(wantRead) {
+					t.Errorf("fd.PRead got %q want %q", got, string(wantRead))
+				}
+			})
+		}
+	}
+}

pkg/sentry/fsimpl/tmpfs/symlink.go

@@ -0,0 +1,36 @@
+// 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 tmpfs
+
+import (
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+)
+
+type symlink struct {
+	inode  inode
+	target string // immutable
+}
+
+func (fs *filesystem) newSymlink(creds *auth.Credentials, target string) *inode {
+	link := &symlink{
+		target: target,
+	}
+	link.inode.init(link, fs, creds, 0777)
+	link.inode.nlink = 1 // from parent directory
+	return &link.inode
+}
+
+// O_PATH is unimplemented, so there's no way to get a FileDescription
+// representing a symlink yet.

pkg/sentry/fsimpl/tmpfs/tmpfs.go

@@ -0,0 +1,299 @@
+// 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 tmpfs provides a filesystem implementation that behaves like tmpfs:
+// the Dentry tree is the sole source of truth for the state of the filesystem.
+//
+// Lock order:
+//
+// filesystem.mu
+//   regularFileFD.offMu
+//     regularFile.mu
+//   inode.mu
+package tmpfs
+
+import (
+	"fmt"
+	"math"
+	"sync"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/pgalloc"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// FilesystemType implements vfs.FilesystemType.
+type FilesystemType struct{}
+
+// filesystem implements vfs.FilesystemImpl.
+type filesystem struct {
+	vfsfs vfs.Filesystem
+
+	// memFile is used to allocate pages to for regular files.
+	memFile *pgalloc.MemoryFile
+
+	// mu serializes changes to the Dentry tree.
+	mu sync.RWMutex
+
+	nextInoMinusOne uint64 // accessed using atomic memory operations
+}
+
+// GetFilesystem implements vfs.FilesystemType.GetFilesystem.
+func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
+	memFileProvider := pgalloc.MemoryFileProviderFromContext(ctx)
+	if memFileProvider == nil {
+		panic("MemoryFileProviderFromContext returned nil")
+	}
+	fs := filesystem{
+		memFile: memFileProvider.MemoryFile(),
+	}
+	fs.vfsfs.Init(vfsObj, &fs)
+	root := fs.newDentry(fs.newDirectory(creds, 01777))
+	return &fs.vfsfs, &root.vfsd, nil
+}
+
+// Release implements vfs.FilesystemImpl.Release.
+func (fs *filesystem) Release() {
+}
+
+// dentry implements vfs.DentryImpl.
+type dentry struct {
+	vfsd vfs.Dentry
+
+	// inode is the inode represented by this dentry. Multiple Dentries may
+	// share a single non-directory inode (with hard links). inode is
+	// immutable.
+	inode *inode
+
+	// tmpfs doesn't count references on dentries; because the dentry tree is
+	// the sole source of truth, it is by definition always consistent with the
+	// state of the filesystem. However, it does count references on inodes,
+	// because inode resources are released when all references are dropped.
+	// (tmpfs doesn't really have resources to release, but we implement
+	// reference counting because tmpfs regular files will.)
+
+	// dentryEntry (ugh) links dentries into their parent directory.childList.
+	dentryEntry
+}
+
+func (fs *filesystem) newDentry(inode *inode) *dentry {
+	d := &dentry{
+		inode: inode,
+	}
+	d.vfsd.Init(d)
+	return d
+}
+
+// IncRef implements vfs.DentryImpl.IncRef.
+func (d *dentry) IncRef() {
+	d.inode.incRef()
+}
+
+// TryIncRef implements vfs.DentryImpl.TryIncRef.
+func (d *dentry) TryIncRef() bool {
+	return d.inode.tryIncRef()
+}
+
+// DecRef implements vfs.DentryImpl.DecRef.
+func (d *dentry) DecRef() {
+	d.inode.decRef()
+}
+
+// inode represents a filesystem object.
+type inode struct {
+	// refs is a reference count. refs is accessed using atomic memory
+	// operations.
+	//
+	// A reference is held on all inodes that are reachable in the filesystem
+	// tree. For non-directories (which may have multiple hard links), this
+	// means that a reference is dropped when nlink reaches 0. For directories,
+	// nlink never reaches 0 due to the "." entry; instead,
+	// filesystem.RmdirAt() drops the reference.
+	refs int64
+
+	// Inode metadata; protected by mu and accessed using atomic memory
+	// operations unless otherwise specified.
+	mu    sync.RWMutex
+	mode  uint32 // excluding file type bits, which are based on impl
+	nlink uint32 // protected by filesystem.mu instead of inode.mu
+	uid   uint32 // auth.KUID, but stored as raw uint32 for sync/atomic
+	gid   uint32 // auth.KGID, but ...
+	ino   uint64 // immutable
+
+	impl interface{} // immutable
+}
+
+const maxLinks = math.MaxUint32
+
+func (i *inode) init(impl interface{}, fs *filesystem, creds *auth.Credentials, mode linux.FileMode) {
+	i.refs = 1
+	i.mode = uint32(mode)
+	i.uid = uint32(creds.EffectiveKUID)
+	i.gid = uint32(creds.EffectiveKGID)
+	i.ino = atomic.AddUint64(&fs.nextInoMinusOne, 1)
+	// i.nlink initialized by caller
+	i.impl = impl
+}
+
+// incLinksLocked increments i's link count.
+//
+// Preconditions: filesystem.mu must be locked for writing. i.nlink != 0.
+// i.nlink < maxLinks.
+func (i *inode) incLinksLocked() {
+	if i.nlink == 0 {
+		panic("tmpfs.inode.incLinksLocked() called with no existing links")
+	}
+	if i.nlink == maxLinks {
+		panic("memfs.inode.incLinksLocked() called with maximum link count")
+	}
+	atomic.AddUint32(&i.nlink, 1)
+}
+
+// decLinksLocked decrements i's link count.
+//
+// Preconditions: filesystem.mu must be locked for writing. i.nlink != 0.
+func (i *inode) decLinksLocked() {
+	if i.nlink == 0 {
+		panic("tmpfs.inode.decLinksLocked() called with no existing links")
+	}
+	atomic.AddUint32(&i.nlink, ^uint32(0))
+}
+
+func (i *inode) incRef() {
+	if atomic.AddInt64(&i.refs, 1) <= 1 {
+		panic("tmpfs.inode.incRef() called without holding a reference")
+	}
+}
+
+func (i *inode) tryIncRef() bool {
+	for {
+		refs := atomic.LoadInt64(&i.refs)
+		if refs == 0 {
+			return false
+		}
+		if atomic.CompareAndSwapInt64(&i.refs, refs, refs+1) {
+			return true
+		}
+	}
+}
+
+func (i *inode) decRef() {
+	if refs := atomic.AddInt64(&i.refs, -1); refs == 0 {
+		// This is unnecessary; it's mostly to simulate what tmpfs would do.
+		if regFile, ok := i.impl.(*regularFile); ok {
+			regFile.mu.Lock()
+			regFile.data.DropAll(regFile.memFile)
+			atomic.StoreUint64(&regFile.size, 0)
+			regFile.mu.Unlock()
+		}
+	} else if refs < 0 {
+		panic("tmpfs.inode.decRef() called without holding a reference")
+	}
+}
+
+func (i *inode) checkPermissions(creds *auth.Credentials, ats vfs.AccessTypes, isDir bool) error {
+	return vfs.GenericCheckPermissions(creds, ats, isDir, uint16(atomic.LoadUint32(&i.mode)), auth.KUID(atomic.LoadUint32(&i.uid)), auth.KGID(atomic.LoadUint32(&i.gid)))
+}
+
+// Go won't inline this function, and returning linux.Statx (which is quite
+// big) means spending a lot of time in runtime.duffcopy(), so instead it's an
+// output parameter.
+func (i *inode) statTo(stat *linux.Statx) {
+	stat.Mask = linux.STATX_TYPE | linux.STATX_MODE | linux.STATX_NLINK | linux.STATX_UID | linux.STATX_GID | linux.STATX_INO
+	stat.Blksize = 1 // usermem.PageSize in tmpfs
+	stat.Nlink = atomic.LoadUint32(&i.nlink)
+	stat.UID = atomic.LoadUint32(&i.uid)
+	stat.GID = atomic.LoadUint32(&i.gid)
+	stat.Mode = uint16(atomic.LoadUint32(&i.mode))
+	stat.Ino = i.ino
+	// TODO: device number
+	switch impl := i.impl.(type) {
+	case *regularFile:
+		stat.Mode |= linux.S_IFREG
+		stat.Mask |= linux.STATX_SIZE | linux.STATX_BLOCKS
+		stat.Size = uint64(atomic.LoadUint64(&impl.size))
+		// In tmpfs, this will be FileRangeSet.Span() / 512 (but also cached in
+		// a uint64 accessed using atomic memory operations to avoid taking
+		// locks).
+		stat.Blocks = allocatedBlocksForSize(stat.Size)
+	case *directory:
+		stat.Mode |= linux.S_IFDIR
+	case *symlink:
+		stat.Mode |= linux.S_IFLNK
+		stat.Mask |= linux.STATX_SIZE | linux.STATX_BLOCKS
+		stat.Size = uint64(len(impl.target))
+		stat.Blocks = allocatedBlocksForSize(stat.Size)
+	case *namedPipe:
+		stat.Mode |= linux.S_IFIFO
+	default:
+		panic(fmt.Sprintf("unknown inode type: %T", i.impl))
+	}
+}
+
+// allocatedBlocksForSize returns the number of 512B blocks needed to
+// accommodate the given size in bytes, as appropriate for struct
+// stat::st_blocks and struct statx::stx_blocks. (Note that this 512B block
+// size is independent of the "preferred block size for I/O", struct
+// stat::st_blksize and struct statx::stx_blksize.)
+func allocatedBlocksForSize(size uint64) uint64 {
+	return (size + 511) / 512
+}
+
+func (i *inode) direntType() uint8 {
+	switch i.impl.(type) {
+	case *regularFile:
+		return linux.DT_REG
+	case *directory:
+		return linux.DT_DIR
+	case *symlink:
+		return linux.DT_LNK
+	default:
+		panic(fmt.Sprintf("unknown inode type: %T", i.impl))
+	}
+}
+
+// fileDescription is embedded by tmpfs implementations of
+// vfs.FileDescriptionImpl.
+type fileDescription struct {
+	vfsfd vfs.FileDescription
+	vfs.FileDescriptionDefaultImpl
+}
+
+func (fd *fileDescription) filesystem() *filesystem {
+	return fd.vfsfd.Mount().Filesystem().Impl().(*filesystem)
+}
+
+func (fd *fileDescription) inode() *inode {
+	return fd.vfsfd.Dentry().Impl().(*dentry).inode
+}
+
+// Stat implements vfs.FileDescriptionImpl.Stat.
+func (fd *fileDescription) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
+	var stat linux.Statx
+	fd.inode().statTo(&stat)
+	return stat, nil
+}
+
+// SetStat implements vfs.FileDescriptionImpl.SetStat.
+func (fd *fileDescription) SetStat(ctx context.Context, opts vfs.SetStatOptions) error {
+	if opts.Stat.Mask == 0 {
+		return nil
+	}
+	// TODO: implement inode.setStat
+	return syserror.EPERM
+}