3 files changed, 149 insertions, 0 deletions
diff --git a/pkg/merkletree/BUILD b/pkg/merkletree/BUILD
new file mode 100644
index 000000000..5b0e4143a
--- /dev/null
+++ b/pkg/merkletree/BUILD
@@ -0,0 +1,16 @@
+load("//tools:defs.bzl", "go_library", "go_test")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "merkletree",
+    srcs = ["merkletree.go"],
+    deps = ["//pkg/usermem"],
+)
+
+go_test(
+    name = "merkletree_test",
+    srcs = ["merkletree_test.go"],
+    library = ":merkletree",
+    deps = ["//pkg/usermem"],
+)
diff --git a/pkg/merkletree/merkletree.go b/pkg/merkletree/merkletree.go
new file mode 100644
index 000000000..965f3670b
--- /dev/null
+++ b/pkg/merkletree/merkletree.go
@@ -0,0 +1,71 @@
+// Copyright 2020 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 merkletree implements Merkle tree generating and verification.
+package merkletree
+
+import (
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+const (
+	// sha256DigestSize specifies the digest size of a SHA256 hash.
+	sha256DigestSize = 32
+)
+
+// Size defines the scale of a Merkle tree.
+type Size struct {
+	// blockSize is the size of a data block to be hashed.
+	blockSize int64
+	// digestSize is the size of a generated hash.
+	digestSize int64
+	// hashesPerBlock is the number of hashes in a block. For example, if
+	// blockSize is 4096 bytes, and digestSize is 32 bytes, there will be 128
+	// hashesPerBlock. Therefore 128 hashes in a lower level will be put into a
+	// block and generate a single hash in an upper level.
+	hashesPerBlock int64
+	// levelStart is the start block index of each level. The number of levels in
+	// the tree is the length of the slice. The leafs (level 0) are hashes of
+	// blocks in the input data. The levels above are hashes of lower level
+	// hashes.  The highest level is the root hash.
+	levelStart []int64
+}
+
+// MakeSize initializes and returns a new Size object describing the structure
+// of a tree. dataSize specifies the number of the file system size in bytes.
+func MakeSize(dataSize int64) Size {
+	size := Size{
+		blockSize: usermem.PageSize,
+		// TODO(b/156980949): Allow config other hash methods (SHA384/SHA512).
+		digestSize:     sha256DigestSize,
+		hashesPerBlock: usermem.PageSize / sha256DigestSize,
+	}
+	numBlocks := (dataSize + size.blockSize - 1) / size.blockSize
+	level := int64(0)
+	offset := int64(0)
+
+	// Calcuate the number of levels in the Merkle tree and the beginning offset
+	// of each level. Level 0 is the level directly above the data blocks, while
+	// level NumLevels - 1 is the root.
+	for numBlocks > 1 {
+		size.levelStart = append(size.levelStart, offset)
+		// Round numBlocks up to fill up a block.
+		numBlocks += (size.hashesPerBlock - numBlocks%size.hashesPerBlock) % size.hashesPerBlock
+		offset += numBlocks / size.hashesPerBlock
+		numBlocks = numBlocks / size.hashesPerBlock
+		level++
+	}
+	size.levelStart = append(size.levelStart, offset)
+	return size
+}
diff --git a/pkg/merkletree/merkletree_test.go b/pkg/merkletree/merkletree_test.go
new file mode 100644
index 000000000..6221eec07
--- /dev/null
+++ b/pkg/merkletree/merkletree_test.go
@@ -0,0 +1,62 @@
+// Copyright 2020 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 merkletree
+
+import (
+	"fmt"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+func TestSize(t *testing.T) {
+	testCases := []struct {
+		dataSize           int64
+		expectedLevelStart []int64
+	}{
+		{
+			dataSize:           100,
+			expectedLevelStart: []int64{0},
+		},
+		{
+			dataSize:           1000000,
+			expectedLevelStart: []int64{0, 2, 3},
+		},
+		{
+			dataSize:           4096 * int64(usermem.PageSize),
+			expectedLevelStart: []int64{0, 32, 33},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%d", tc.dataSize), func(t *testing.T) {
+			s := MakeSize(tc.dataSize)
+			if s.blockSize != int64(usermem.PageSize) {
+				t.Errorf("got blockSize %d, want %d", s.blockSize, usermem.PageSize)
+			}
+			if s.digestSize != sha256DigestSize {
+				t.Errorf("got digestSize %d, want %d", s.digestSize, sha256DigestSize)
+			}
+			if len(s.levelStart) != len(tc.expectedLevelStart) {
+				t.Errorf("got levels %d, want %d", len(s.levelStart), len(tc.expectedLevelStart))
+			}
+			for i := 0; i < len(s.levelStart) && i < len(tc.expectedLevelStart); i++ {
+				if s.levelStart[i] != tc.expectedLevelStart[i] {
+					t.Errorf("got levelStart[%d] %d, want %d", i, s.levelStart[i], tc.expectedLevelStart[i])
+				}
+			}
+		})
+	}
+}