ext4: disklayout: Directory Entry implementation.

PiperOrigin-RevId: 257314911

pkg/sentry/fs/ext4/disklayout/BUILD

@@ -8,6 +8,9 @@ go_library(
         "block_group.go",
         "block_group_32.go",
         "block_group_64.go",
+        "dirent.go",
+        "dirent_new.go",
+        "dirent_old.go",
         "disklayout.go",
         "inode.go",
         "inode_new.go",
@@ -22,6 +25,7 @@ go_library(
     deps = [
         "//pkg/abi/linux",
         "//pkg/binary",
+        "//pkg/sentry/fs",
         "//pkg/sentry/kernel/auth",
         "//pkg/sentry/kernel/time",
     ],
@@ -32,6 +36,7 @@ go_test(
     size = "small",
     srcs = [
         "block_group_test.go",
+        "dirent_test.go",
         "inode_test.go",
         "superblock_test.go",
     ],

pkg/sentry/fs/ext4/disklayout/dirent.go

@@ -0,0 +1,69 @@
+// 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 disklayout
+
+import (
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+)
+
+const (
+	// MaxFileName is the maximum length of an ext fs file's name.
+	MaxFileName = 255
+)
+
+var (
+	// inodeTypeByFileType maps ext4 file types to vfs inode types.
+	//
+	// See https://www.kernel.org/doc/html/latest/filesystems/ext4/dynamic.html#ftype.
+	inodeTypeByFileType = map[uint8]fs.InodeType{
+		0: fs.Anonymous,
+		1: fs.RegularFile,
+		2: fs.Directory,
+		3: fs.CharacterDevice,
+		4: fs.BlockDevice,
+		5: fs.Pipe,
+		6: fs.Socket,
+		7: fs.Symlink,
+	}
+)
+
+// The Dirent interface should be implemented by structs representing ext
+// directory entries. These are for the linear classical directories which
+// just store a list of dirent structs. A directory is a series of data blocks
+// where is each data block contains a linear array of dirents. The last entry
+// of the block has a record size that takes it to the end of the block. The
+// end of the directory is when you read dirInode.Size() bytes from the blocks.
+//
+// See https://www.kernel.org/doc/html/latest/filesystems/ext4/dynamic.html#linear-classic-directories.
+type Dirent interface {
+	// Inode returns the absolute inode number of the underlying inode.
+	// Inode number 0 signifies an unused dirent.
+	Inode() uint32
+
+	// RecordSize returns the record length of this dirent on disk. The next
+	// dirent in the dirent list should be read after these many bytes from
+	// the current dirent. Must be a multiple of 4.
+	RecordSize() uint16
+
+	// FileName returns the name of the file. Can be at most 255 is length.
+	FileName() string
+
+	// FileType returns the inode type of the underlying inode. This is a
+	// performance hack so that we do not have to read the underlying inode struct
+	// to know the type of inode. This will only work when the SbDirentFileType
+	// feature is set. If not, the second returned value will be false indicating
+	// that user code has to use the inode mode to extract the file type.
+	FileType() (fs.InodeType, bool)
+}

pkg/sentry/fs/ext4/disklayout/dirent_new.go

@@ -0,0 +1,61 @@
+// 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 disklayout
+
+import (
+	"fmt"
+
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+)
+
+// DirentNew represents the ext4 directory entry struct. This emulates Linux's
+// ext4_dir_entry_2 struct. The FileName can not be more than 255 bytes so we
+// only need 8 bits to store the NameLength. As a result, NameLength has been
+// shortened and the other 8 bits are used to encode the file type. Use the
+// FileTypeRaw field only if the SbDirentFileType feature is set.
+//
+// Note: This struct can be of variable size on disk. The one described below
+// is of maximum size and the FileName beyond NameLength bytes might contain
+// garbage.
+type DirentNew struct {
+	InodeNumber  uint32
+	RecordLength uint16
+	NameLength   uint8
+	FileTypeRaw  uint8
+	FileNameRaw  [MaxFileName]byte
+}
+
+// Compiles only if DirentNew implements Dirent.
+var _ Dirent = (*DirentNew)(nil)
+
+// Inode implements Dirent.Inode.
+func (d *DirentNew) Inode() uint32 { return d.InodeNumber }
+
+// RecordSize implements Dirent.RecordSize.
+func (d *DirentNew) RecordSize() uint16 { return d.RecordLength }
+
+// FileName implements Dirent.FileName.
+func (d *DirentNew) FileName() string {
+	return string(d.FileNameRaw[:d.NameLength])
+}
+
+// FileType implements Dirent.FileType.
+func (d *DirentNew) FileType() (fs.InodeType, bool) {
+	if inodeType, ok := inodeTypeByFileType[d.FileTypeRaw]; ok {
+		return inodeType, true
+	}
+
+	panic(fmt.Sprintf("unknown file type %v", d.FileTypeRaw))
+}

pkg/sentry/fs/ext4/disklayout/dirent_old.go

@@ -0,0 +1,50 @@
+// 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 disklayout
+
+import "gvisor.dev/gvisor/pkg/sentry/fs"
+
+// DirentOld represents the old directory entry struct which does not contain
+// the file type. This emulates Linux's ext4_dir_entry struct. This is used in
+// ext2, ext3 and sometimes in ext4.
+//
+// Note: This struct can be of variable size on disk. The one described below
+// is of maximum size and the FileName beyond NameLength bytes might contain
+// garbage.
+type DirentOld struct {
+	InodeNumber  uint32
+	RecordLength uint16
+	NameLength   uint16
+	FileNameRaw  [MaxFileName]byte
+}
+
+// Compiles only if DirentOld implements Dirent.
+var _ Dirent = (*DirentOld)(nil)
+
+// Inode implements Dirent.Inode.
+func (d *DirentOld) Inode() uint32 { return d.InodeNumber }
+
+// RecordSize implements Dirent.RecordSize.
+func (d *DirentOld) RecordSize() uint16 { return d.RecordLength }
+
+// FileName implements Dirent.FileName.
+func (d *DirentOld) FileName() string {
+	return string(d.FileNameRaw[:d.NameLength])
+}
+
+// FileType implements Dirent.FileType.
+func (d *DirentOld) FileType() (fs.InodeType, bool) {
+	return fs.Anonymous, false
+}

pkg/sentry/fs/ext4/disklayout/dirent_test.go

@@ -0,0 +1,28 @@
+// 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 disklayout
+
+import (
+	"testing"
+)
+
+// TestDirentSize tests that the dirent structs are of the correct
+// size.
+func TestDirentSize(t *testing.T) {
+	want := uintptr(263)
+
+	assertSize(t, DirentOld{}, want)
+	assertSize(t, DirentNew{}, want)
+}