4 files changed, 390 insertions, 0 deletions
diff --git a/pkg/sentry/vfs/BUILD b/pkg/sentry/vfs/BUILD
index 4de6c41cf..0f247bf77 100644
--- a/pkg/sentry/vfs/BUILD
+++ b/pkg/sentry/vfs/BUILD
@@ -18,6 +18,7 @@ go_library(
         "permissions.go",
         "resolving_path.go",
         "syscalls.go",
+        "testutil.go",
         "vfs.go",
     ],
     importpath = "gvisor.dev/gvisor/pkg/sentry/vfs",
@@ -40,7 +41,16 @@ go_test(
     name = "vfs_test",
     size = "small",
     srcs = [
+        "file_description_impl_util_test.go",
         "mount_test.go",
     ],
     embed = [":vfs"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/sentry/context",
+        "//pkg/sentry/context/contexttest",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/usermem",
+        "//pkg/syserror",
+    ],
 )
diff --git a/pkg/sentry/vfs/file_description_impl_util.go b/pkg/sentry/vfs/file_description_impl_util.go
index 486893e70..4d1d447ed 100644
--- a/pkg/sentry/vfs/file_description_impl_util.go
+++ b/pkg/sentry/vfs/file_description_impl_util.go
@@ -15,6 +15,10 @@
 package vfs
 
 import (
+	"bytes"
+	"io"
+	"sync"
+
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/sentry/arch"
 	"gvisor.dev/gvisor/pkg/sentry/context"
@@ -140,3 +144,106 @@ func (DirectoryFileDescriptionDefaultImpl) PWrite(ctx context.Context, src userm
 func (DirectoryFileDescriptionDefaultImpl) Write(ctx context.Context, src usermem.IOSequence, opts WriteOptions) (int64, error) {
 	return 0, syserror.EISDIR
 }
+
+// DynamicBytesFileDescriptionImpl may be embedded by implementations of
+// FileDescriptionImpl that represent read-only regular files whose contents
+// are backed by a bytes.Buffer that is regenerated when necessary, consistent
+// with Linux's fs/seq_file.c:single_open().
+//
+// DynamicBytesFileDescriptionImpl.SetDataSource() must be called before first
+// use.
+type DynamicBytesFileDescriptionImpl struct {
+	FileDescriptionDefaultImpl
+
+	data     DynamicBytesSource // immutable
+	mu       sync.Mutex         // protects the following fields
+	buf      bytes.Buffer
+	off      int64
+	lastRead int64 // offset at which the last Read, PRead, or Seek ended
+}
+
+// DynamicBytesSource represents a data source for a
+// DynamicBytesFileDescriptionImpl.
+type DynamicBytesSource interface {
+	// Generate writes the file's contents to buf.
+	Generate(ctx context.Context, buf *bytes.Buffer) error
+}
+
+// SetDataSource must be called exactly once on fd before first use.
+func (fd *DynamicBytesFileDescriptionImpl) SetDataSource(data DynamicBytesSource) {
+	fd.data = data
+}
+
+// Preconditions: fd.mu must be locked.
+func (fd *DynamicBytesFileDescriptionImpl) preadLocked(ctx context.Context, dst usermem.IOSequence, offset int64, opts *ReadOptions) (int64, error) {
+	// Regenerate the buffer if it's empty, or before pread() at a new offset.
+	// Compare fs/seq_file.c:seq_read() => traverse().
+	switch {
+	case offset != fd.lastRead:
+		fd.buf.Reset()
+		fallthrough
+	case fd.buf.Len() == 0:
+		if err := fd.data.Generate(ctx, &fd.buf); err != nil {
+			fd.buf.Reset()
+			// fd.off is not updated in this case.
+			fd.lastRead = 0
+			return 0, err
+		}
+	}
+	bs := fd.buf.Bytes()
+	if offset >= int64(len(bs)) {
+		return 0, io.EOF
+	}
+	n, err := dst.CopyOut(ctx, bs[offset:])
+	fd.lastRead = offset + int64(n)
+	return int64(n), err
+}
+
+// PRead implements FileDescriptionImpl.PRead.
+func (fd *DynamicBytesFileDescriptionImpl) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts ReadOptions) (int64, error) {
+	fd.mu.Lock()
+	n, err := fd.preadLocked(ctx, dst, offset, &opts)
+	fd.mu.Unlock()
+	return n, err
+}
+
+// Read implements FileDescriptionImpl.Read.
+func (fd *DynamicBytesFileDescriptionImpl) Read(ctx context.Context, dst usermem.IOSequence, opts ReadOptions) (int64, error) {
+	fd.mu.Lock()
+	n, err := fd.preadLocked(ctx, dst, fd.off, &opts)
+	fd.off += n
+	fd.mu.Unlock()
+	return n, err
+}
+
+// Seek implements FileDescriptionImpl.Seek.
+func (fd *DynamicBytesFileDescriptionImpl) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	fd.mu.Lock()
+	defer fd.mu.Unlock()
+	switch whence {
+	case linux.SEEK_SET:
+		// Use offset as given.
+	case linux.SEEK_CUR:
+		offset += fd.off
+	default:
+		// fs/seq_file:seq_lseek() rejects SEEK_END etc.
+		return 0, syserror.EINVAL
+	}
+	if offset < 0 {
+		return 0, syserror.EINVAL
+	}
+	if offset != fd.lastRead {
+		// Regenerate the file's contents immediately. Compare
+		// fs/seq_file.c:seq_lseek() => traverse().
+		fd.buf.Reset()
+		if err := fd.data.Generate(ctx, &fd.buf); err != nil {
+			fd.buf.Reset()
+			fd.off = 0
+			fd.lastRead = 0
+			return 0, err
+		}
+		fd.lastRead = offset
+	}
+	fd.off = offset
+	return offset, nil
+}
diff --git a/pkg/sentry/vfs/file_description_impl_util_test.go b/pkg/sentry/vfs/file_description_impl_util_test.go
new file mode 100644
index 000000000..cef7e3ce5
--- /dev/null
+++ b/pkg/sentry/vfs/file_description_impl_util_test.go
@@ -0,0 +1,134 @@
+// 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 (
+	"bytes"
+	"fmt"
+	"io"
+	"sync/atomic"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"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/syserror"
+)
+
+// genCountFD is a read-only FileDescriptionImpl representing a regular file
+// that contains the number of times its DynamicBytesSource.Generate()
+// implementation has been called.
+type genCountFD struct {
+	vfsfd FileDescription
+	DynamicBytesFileDescriptionImpl
+
+	count uint64 // accessed using atomic memory ops
+}
+
+func newGenCountFD(mnt *Mount, vfsd *Dentry) *FileDescription {
+	var fd genCountFD
+	fd.vfsfd.Init(&fd, mnt, vfsd)
+	fd.DynamicBytesFileDescriptionImpl.SetDataSource(&fd)
+	return &fd.vfsfd
+}
+
+// Release implements FileDescriptionImpl.Release.
+func (fd *genCountFD) Release() {
+}
+
+// StatusFlags implements FileDescriptionImpl.StatusFlags.
+func (fd *genCountFD) StatusFlags(ctx context.Context) (uint32, error) {
+	return 0, nil
+}
+
+// SetStatusFlags implements FileDescriptionImpl.SetStatusFlags.
+func (fd *genCountFD) SetStatusFlags(ctx context.Context, flags uint32) error {
+	return syserror.EPERM
+}
+
+// Stat implements FileDescriptionImpl.Stat.
+func (fd *genCountFD) Stat(ctx context.Context, opts StatOptions) (linux.Statx, error) {
+	// Note that Statx.Mask == 0 in the return value.
+	return linux.Statx{}, nil
+}
+
+// SetStat implements FileDescriptionImpl.SetStat.
+func (fd *genCountFD) SetStat(ctx context.Context, opts SetStatOptions) error {
+	return syserror.EPERM
+}
+
+// Generate implements DynamicBytesSource.Generate.
+func (fd *genCountFD) Generate(ctx context.Context, buf *bytes.Buffer) error {
+	fmt.Fprintf(buf, "%d", atomic.AddUint64(&fd.count, 1))
+	return nil
+}
+
+func TestGenCountFD(t *testing.T) {
+	ctx := contexttest.Context(t)
+	creds := auth.CredentialsFromContext(ctx)
+
+	vfsObj := New() // vfs.New()
+	vfsObj.MustRegisterFilesystemType("testfs", FDTestFilesystemType{})
+	mntns, err := vfsObj.NewMountNamespace(ctx, creds, "", "testfs", &NewFilesystemOptions{})
+	if err != nil {
+		t.Fatalf("failed to create testfs root mount: %v", err)
+	}
+	vd := mntns.Root()
+	defer vd.DecRef()
+
+	fd := newGenCountFD(vd.Mount(), vd.Dentry())
+	defer fd.DecRef()
+
+	// The first read causes Generate to be called to fill the FD's buffer.
+	buf := make([]byte, 2)
+	ioseq := usermem.BytesIOSequence(buf)
+	n, err := fd.Impl().Read(ctx, ioseq, ReadOptions{})
+	if n != 1 || (err != nil && err != io.EOF) {
+		t.Fatalf("first Read: got (%d, %v), wanted (1, nil or EOF)", n, err)
+	}
+	if want := byte('1'); buf[0] != want {
+		t.Errorf("first Read: got byte %c, wanted %c", buf[0], want)
+	}
+
+	// A second read without seeking is still at EOF.
+	n, err = fd.Impl().Read(ctx, ioseq, ReadOptions{})
+	if n != 0 || err != io.EOF {
+		t.Fatalf("second Read: got (%d, %v), wanted (0, EOF)", n, err)
+	}
+
+	// Seeking to the beginning of the file causes it to be regenerated.
+	n, err = fd.Impl().Seek(ctx, 0, linux.SEEK_SET)
+	if n != 0 || err != nil {
+		t.Fatalf("Seek: got (%d, %v), wanted (0, nil)", n, err)
+	}
+	n, err = fd.Impl().Read(ctx, ioseq, ReadOptions{})
+	if n != 1 || (err != nil && err != io.EOF) {
+		t.Fatalf("Read after Seek: got (%d, %v), wanted (1, nil or EOF)", n, err)
+	}
+	if want := byte('2'); buf[0] != want {
+		t.Errorf("Read after Seek: got byte %c, wanted %c", buf[0], want)
+	}
+
+	// PRead at the beginning of the file also causes it to be regenerated.
+	n, err = fd.Impl().PRead(ctx, ioseq, 0, ReadOptions{})
+	if n != 1 || (err != nil && err != io.EOF) {
+		t.Fatalf("PRead: got (%d, %v), wanted (1, nil or EOF)", n, err)
+	}
+	if want := byte('3'); buf[0] != want {
+		t.Errorf("PRead: got byte %c, wanted %c", buf[0], want)
+	}
+}
diff --git a/pkg/sentry/vfs/testutil.go b/pkg/sentry/vfs/testutil.go
new file mode 100644
index 000000000..70b192ece
--- /dev/null
+++ b/pkg/sentry/vfs/testutil.go
@@ -0,0 +1,139 @@
+// 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 (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// FDTestFilesystemType is a test-only FilesystemType that produces Filesystems
+// for which all FilesystemImpl methods taking a path return EPERM. It is used
+// to produce Mounts and Dentries for testing of FileDescriptionImpls that do
+// not depend on their originating Filesystem.
+type FDTestFilesystemType struct{}
+
+// FDTestFilesystem is a test-only FilesystemImpl produced by
+// FDTestFilesystemType.
+type FDTestFilesystem struct {
+	vfsfs Filesystem
+}
+
+// NewFilesystem implements FilesystemType.NewFilesystem.
+func (fstype FDTestFilesystemType) NewFilesystem(ctx context.Context, creds *auth.Credentials, source string, opts NewFilesystemOptions) (*Filesystem, *Dentry, error) {
+	var fs FDTestFilesystem
+	fs.vfsfs.Init(&fs)
+	return &fs.vfsfs, fs.NewDentry(), nil
+}
+
+// Release implements FilesystemImpl.Release.
+func (fs *FDTestFilesystem) Release() {
+}
+
+// Sync implements FilesystemImpl.Sync.
+func (fs *FDTestFilesystem) Sync(ctx context.Context) error {
+	return nil
+}
+
+// GetDentryAt implements FilesystemImpl.GetDentryAt.
+func (fs *FDTestFilesystem) GetDentryAt(ctx context.Context, rp *ResolvingPath, opts GetDentryOptions) (*Dentry, error) {
+	return nil, syserror.EPERM
+}
+
+// LinkAt implements FilesystemImpl.LinkAt.
+func (fs *FDTestFilesystem) LinkAt(ctx context.Context, rp *ResolvingPath, vd VirtualDentry) error {
+	return syserror.EPERM
+}
+
+// MkdirAt implements FilesystemImpl.MkdirAt.
+func (fs *FDTestFilesystem) MkdirAt(ctx context.Context, rp *ResolvingPath, opts MkdirOptions) error {
+	return syserror.EPERM
+}
+
+// MknodAt implements FilesystemImpl.MknodAt.
+func (fs *FDTestFilesystem) MknodAt(ctx context.Context, rp *ResolvingPath, opts MknodOptions) error {
+	return syserror.EPERM
+}
+
+// OpenAt implements FilesystemImpl.OpenAt.
+func (fs *FDTestFilesystem) OpenAt(ctx context.Context, rp *ResolvingPath, opts OpenOptions) (*FileDescription, error) {
+	return nil, syserror.EPERM
+}
+
+// ReadlinkAt implements FilesystemImpl.ReadlinkAt.
+func (fs *FDTestFilesystem) ReadlinkAt(ctx context.Context, rp *ResolvingPath) (string, error) {
+	return "", syserror.EPERM
+}
+
+// RenameAt implements FilesystemImpl.RenameAt.
+func (fs *FDTestFilesystem) RenameAt(ctx context.Context, rp *ResolvingPath, vd VirtualDentry, opts RenameOptions) error {
+	return syserror.EPERM
+}
+
+// RmdirAt implements FilesystemImpl.RmdirAt.
+func (fs *FDTestFilesystem) RmdirAt(ctx context.Context, rp *ResolvingPath) error {
+	return syserror.EPERM
+}
+
+// SetStatAt implements FilesystemImpl.SetStatAt.
+func (fs *FDTestFilesystem) SetStatAt(ctx context.Context, rp *ResolvingPath, opts SetStatOptions) error {
+	return syserror.EPERM
+}
+
+// StatAt implements FilesystemImpl.StatAt.
+func (fs *FDTestFilesystem) StatAt(ctx context.Context, rp *ResolvingPath, opts StatOptions) (linux.Statx, error) {
+	return linux.Statx{}, syserror.EPERM
+}
+
+// StatFSAt implements FilesystemImpl.StatFSAt.
+func (fs *FDTestFilesystem) StatFSAt(ctx context.Context, rp *ResolvingPath) (linux.Statfs, error) {
+	return linux.Statfs{}, syserror.EPERM
+}
+
+// SymlinkAt implements FilesystemImpl.SymlinkAt.
+func (fs *FDTestFilesystem) SymlinkAt(ctx context.Context, rp *ResolvingPath, target string) error {
+	return syserror.EPERM
+}
+
+// UnlinkAt implements FilesystemImpl.UnlinkAt.
+func (fs *FDTestFilesystem) UnlinkAt(ctx context.Context, rp *ResolvingPath) error {
+	return syserror.EPERM
+}
+
+type fdTestDentry struct {
+	vfsd Dentry
+}
+
+// NewDentry returns a new Dentry.
+func (fs *FDTestFilesystem) NewDentry() *Dentry {
+	var d fdTestDentry
+	d.vfsd.Init(&d)
+	return &d.vfsd
+}
+
+// IncRef implements DentryImpl.IncRef.
+func (d *fdTestDentry) IncRef(vfsfs *Filesystem) {
+}
+
+// TryIncRef implements DentryImpl.TryIncRef.
+func (d *fdTestDentry) TryIncRef(vfsfs *Filesystem) bool {
+	return true
+}
+
+// DecRef implements DentryImpl.DecRef.
+func (d *fdTestDentry) DecRef(vfsfs *Filesystem) {
+}