Merge "Initial prototype of COW format and API." am: 34dda54411 (00138ea6) · Commits · e / os / android_system_core

fs_mgr/libsnapshot/Android.bp

+60 −0

Original line number	Diff line number	Diff line
		@@ -122,6 +122,39 @@ cc_library_static {
		],
		}

		cc_defaults {
		name: "libsnapshot_cow_defaults",
		defaults: [
		"fs_mgr_defaults",
		],
		cflags: [
		"-D_FILE_OFFSET_BITS=64",
		"-Wall",
		"-Werror",
		],
		export_include_dirs: ["include"],
		srcs: [
		"cow_reader.cpp",
		"cow_writer.cpp",
		],
		}

		cc_library_static {
		name: "libsnapshot_cow",
		defaults: [
		"libsnapshot_cow_defaults",
		],
		host_supported: true,
		shared_libs: [
		"libbase",
		"libcrypto",
		"liblog",
		],
		static_libs: [
		"libz",
		],
		}

		cc_library_static {
		name: "libsnapshot_test_helpers",
		defaults: ["libsnapshot_defaults"],
		@@ -343,3 +376,30 @@ cc_binary {
		static_executable: true,
		system_shared_libs: [],
		}

		cc_test {
		name: "cow_api_test",
		defaults: [
		"fs_mgr_defaults",
		],
		srcs: [
		"cow_api_test.cpp",
		],
		cflags: [
		"-Wall",
		"-Werror",
		],
		shared_libs: [
		"libbase",
		"libcrypto",
		"liblog",
		"libz",
		],
		static_libs: [
		"libgtest",
		"libsnapshot_cow",
		],
		test_min_api_level: 30,
		auto_gen_config: true,
		require_root: false,
		}

fs_mgr/libsnapshot/cow_api_test.cpp

0 → 100644

+244 −0

Original line number	Diff line number	Diff line
		// Copyright (C) 2018 The Android Open Source Project
		//
		// 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.

		#include <iostream>
		#include <memory>
		#include <string_view>

		#include <android-base/file.h>
		#include <gtest/gtest.h>
		#include <libsnapshot/cow_reader.h>
		#include <libsnapshot/cow_writer.h>

		namespace android {
		namespace snapshot {

		class CowTest : public ::testing::Test {
		protected:
		void SetUp() override {
		cow_ = std::make_unique<TemporaryFile>();
		ASSERT_GE(cow_->fd, 0) << strerror(errno);
		}

		void TearDown() override { cow_ = nullptr; }

		std::unique_ptr<TemporaryFile> cow_;
		};

		// Sink that always appends to the end of a string.
		class StringSink : public IByteSink {
		public:
		void* GetBuffer(size_t requested, size_t* actual) override {
		size_t old_size = stream_.size();
		stream_.resize(old_size + requested, '\0');
		*actual = requested;
		return stream_.data() + old_size;
		}
		bool ReturnData(void*, size_t) override { return true; }
		void Reset() { stream_.clear(); }

		std::string& stream() { return stream_; }

		private:
		std::string stream_;
		};

		TEST_F(CowTest, ReadWrite) {
		CowOptions options;
		CowWriter writer(options);

		ASSERT_TRUE(writer.Initialize(cow_->fd));

		std::string data = "This is some data, believe it";
		data.resize(options.block_size, '\0');

		ASSERT_TRUE(writer.AddCopy(10, 20));
		ASSERT_TRUE(writer.AddRawBlocks(50, data.data(), data.size()));
		ASSERT_TRUE(writer.AddZeroBlocks(51, 2));
		ASSERT_TRUE(writer.Finalize());

		ASSERT_EQ(lseek(cow_->fd, 0, SEEK_SET), 0);

		CowReader reader;
		CowHeader header;
		ASSERT_TRUE(reader.Parse(cow_->fd));
		ASSERT_TRUE(reader.GetHeader(&header));
		ASSERT_EQ(header.magic, kCowMagicNumber);
		ASSERT_EQ(header.major_version, kCowVersionMajor);
		ASSERT_EQ(header.minor_version, kCowVersionMinor);
		ASSERT_EQ(header.block_size, options.block_size);
		ASSERT_EQ(header.num_ops, 4);

		auto iter = reader.GetOpIter();
		ASSERT_NE(iter, nullptr);
		ASSERT_FALSE(iter->Done());
		auto op = &iter->Get();

		ASSERT_EQ(op->type, kCowCopyOp);
		ASSERT_EQ(op->compression, kCowCompressNone);
		ASSERT_EQ(op->data_length, 0);
		ASSERT_EQ(op->new_block, 10);
		ASSERT_EQ(op->source, 20);

		StringSink sink;

		iter->Next();
		ASSERT_FALSE(iter->Done());
		op = &iter->Get();

		ASSERT_EQ(op->type, kCowReplaceOp);
		ASSERT_EQ(op->compression, kCowCompressNone);
		ASSERT_EQ(op->data_length, 4096);
		ASSERT_EQ(op->new_block, 50);
		ASSERT_EQ(op->source, 104);
		ASSERT_TRUE(reader.ReadData(*op, &sink));
		ASSERT_EQ(sink.stream(), data);

		iter->Next();
		ASSERT_FALSE(iter->Done());
		op = &iter->Get();

		// Note: the zero operation gets split into two blocks.
		ASSERT_EQ(op->type, kCowZeroOp);
		ASSERT_EQ(op->compression, kCowCompressNone);
		ASSERT_EQ(op->data_length, 0);
		ASSERT_EQ(op->new_block, 51);
		ASSERT_EQ(op->source, 0);

		iter->Next();
		ASSERT_FALSE(iter->Done());
		op = &iter->Get();

		ASSERT_EQ(op->type, kCowZeroOp);
		ASSERT_EQ(op->compression, kCowCompressNone);
		ASSERT_EQ(op->data_length, 0);
		ASSERT_EQ(op->new_block, 52);
		ASSERT_EQ(op->source, 0);

		iter->Next();
		ASSERT_TRUE(iter->Done());
		}

		TEST_F(CowTest, CompressGz) {
		CowOptions options;
		options.compression = "gz";
		CowWriter writer(options);

		ASSERT_TRUE(writer.Initialize(cow_->fd));

		std::string data = "This is some data, believe it";
		data.resize(options.block_size, '\0');

		ASSERT_TRUE(writer.AddRawBlocks(50, data.data(), data.size()));
		ASSERT_TRUE(writer.Finalize());

		ASSERT_EQ(lseek(cow_->fd, 0, SEEK_SET), 0);

		CowReader reader;
		ASSERT_TRUE(reader.Parse(cow_->fd));

		auto iter = reader.GetOpIter();
		ASSERT_NE(iter, nullptr);
		ASSERT_FALSE(iter->Done());
		auto op = &iter->Get();

		StringSink sink;

		ASSERT_EQ(op->type, kCowReplaceOp);
		ASSERT_EQ(op->compression, kCowCompressGz);
		ASSERT_EQ(op->data_length, 56); // compressed!
		ASSERT_EQ(op->new_block, 50);
		ASSERT_EQ(op->source, 104);
		ASSERT_TRUE(reader.ReadData(*op, &sink));
		ASSERT_EQ(sink.stream(), data);

		iter->Next();
		ASSERT_TRUE(iter->Done());
		}

		TEST_F(CowTest, CompressTwoBlocks) {
		CowOptions options;
		options.compression = "gz";
		CowWriter writer(options);

		ASSERT_TRUE(writer.Initialize(cow_->fd));

		std::string data = "This is some data, believe it";
		data.resize(options.block_size * 2, '\0');

		ASSERT_TRUE(writer.AddRawBlocks(50, data.data(), data.size()));
		ASSERT_TRUE(writer.Finalize());

		ASSERT_EQ(lseek(cow_->fd, 0, SEEK_SET), 0);

		CowReader reader;
		ASSERT_TRUE(reader.Parse(cow_->fd));

		auto iter = reader.GetOpIter();
		ASSERT_NE(iter, nullptr);
		ASSERT_FALSE(iter->Done());
		iter->Next();
		ASSERT_FALSE(iter->Done());

		StringSink sink;

		auto op = &iter->Get();
		ASSERT_EQ(op->type, kCowReplaceOp);
		ASSERT_EQ(op->compression, kCowCompressGz);
		ASSERT_EQ(op->new_block, 51);
		ASSERT_TRUE(reader.ReadData(*op, &sink));
		}

		// Only return 1-byte buffers, to stress test the partial read logic in
		// CowReader.
		class HorribleStringSink : public StringSink {
		public:
		void* GetBuffer(size_t, size_t* actual) override { return StringSink::GetBuffer(1, actual); }
		};

		TEST_F(CowTest, HorribleSink) {
		CowOptions options;
		options.compression = "gz";
		CowWriter writer(options);

		ASSERT_TRUE(writer.Initialize(cow_->fd));

		std::string data = "This is some data, believe it";
		data.resize(options.block_size, '\0');

		ASSERT_TRUE(writer.AddRawBlocks(50, data.data(), data.size()));
		ASSERT_TRUE(writer.Finalize());

		ASSERT_EQ(lseek(cow_->fd, 0, SEEK_SET), 0);

		CowReader reader;
		ASSERT_TRUE(reader.Parse(cow_->fd));

		auto iter = reader.GetOpIter();
		ASSERT_NE(iter, nullptr);
		ASSERT_FALSE(iter->Done());

		HorribleStringSink sink;
		auto op = &iter->Get();
		ASSERT_TRUE(reader.ReadData(*op, &sink));
		ASSERT_EQ(sink.stream(), data);
		}

		} // namespace snapshot
		} // namespace android

		int main(int argc, char** argv) {
		::testing::InitGoogleTest(&argc, argv);
		return RUN_ALL_TESTS();
		}

fs_mgr/libsnapshot/cow_reader.cpp

0 → 100644

+264 −0

Original line number	Diff line number	Diff line
		//
		// Copyright (C) 2020 The Android Open Source Project
		//
		// 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.
		//

		#include <sys/types.h>
		#include <unistd.h>

		#include <android-base/file.h>
		#include <android-base/logging.h>
		#include <libsnapshot/cow_reader.h>
		#include <openssl/sha.h>
		#include <zlib.h>

		namespace android {
		namespace snapshot {

		CowReader::CowReader() : fd_(-1), header_(), fd_size_(0) {}

		static void SHA256(const void* data, size_t length, uint8_t out[32]) {
		SHA256_CTX c;
		SHA256_Init(&c);
		SHA256_Update(&c, data, length);
		SHA256_Final(out, &c);
		}

		bool CowReader::Parse(android::base::unique_fd&& fd) {
		owned_fd_ = std::move(fd);
		return Parse(android::base::borrowed_fd{owned_fd_});
		}

		bool CowReader::Parse(android::base::borrowed_fd fd) {
		fd_ = fd;

		auto pos = lseek(fd_.get(), 0, SEEK_END);
		if (pos < 0) {
		PLOG(ERROR) << "lseek end failed";
		return false;
		}
		fd_size_ = pos;

		if (lseek(fd_.get(), 0, SEEK_SET) < 0) {
		PLOG(ERROR) << "lseek header failed";
		return false;
		}
		if (!android::base::ReadFully(fd_, &header_, sizeof(header_))) {
		PLOG(ERROR) << "read header failed";
		return false;
		}

		// Validity check the ops range.
		if (header_.ops_offset >= fd_size_) {
		LOG(ERROR) << "ops offset " << header_.ops_offset << " larger than fd size " << fd_size_;
		return false;
		}
		if (fd_size_ - header_.ops_offset < header_.ops_size) {
		LOG(ERROR) << "ops size " << header_.ops_size << " is too large";
		return false;
		}

		uint8_t header_csum[32];
		{
		CowHeader tmp = header_;
		memset(&tmp.header_checksum, 0, sizeof(tmp.header_checksum));
		SHA256(&tmp, sizeof(tmp), header_csum);
		}
		if (memcmp(header_csum, header_.header_checksum, sizeof(header_csum)) != 0) {
		LOG(ERROR) << "header checksum is invalid";
		return false;
		}
		return true;
		}

		bool CowReader::GetHeader(CowHeader* header) {
		*header = header_;
		return true;
		}

		class CowOpIter final : public ICowOpIter {
		public:
		CowOpIter(std::unique_ptr<uint8_t[]>&& ops, size_t len);

		bool Done() override;
		const CowOperation& Get() override;
		void Next() override;

		private:
		bool HasNext();

		std::unique_ptr<uint8_t[]> ops_;
		const uint8_t* pos_;
		const uint8_t* end_;
		bool done_;
		};

		CowOpIter::CowOpIter(std::unique_ptr<uint8_t[]>&& ops, size_t len)
		: ops_(std::move(ops)), pos_(ops_.get()), end_(pos_ + len), done_(!HasNext()) {}

		bool CowOpIter::Done() {
		return done_;
		}

		bool CowOpIter::HasNext() {
		return pos_ < end_ && size_t(end_ - pos_) >= sizeof(CowOperation);
		}

		void CowOpIter::Next() {
		CHECK(!Done());

		pos_ += sizeof(CowOperation);
		if (!HasNext()) done_ = true;
		}

		const CowOperation& CowOpIter::Get() {
		CHECK(!Done());
		CHECK(HasNext());
		return reinterpret_cast<const CowOperation>(pos_);
		}

		std::unique_ptr<ICowOpIter> CowReader::GetOpIter() {
		if (lseek(fd_.get(), header_.ops_offset, SEEK_SET) < 0) {
		PLOG(ERROR) << "lseek ops failed";
		return nullptr;
		}
		auto ops_buffer = std::make_unique<uint8_t[]>(header_.ops_size);
		if (!android::base::ReadFully(fd_, ops_buffer.get(), header_.ops_size)) {
		PLOG(ERROR) << "read ops failed";
		return nullptr;
		}

		uint8_t csum[32];
		SHA256(ops_buffer.get(), header_.ops_size, csum);
		if (memcmp(csum, header_.ops_checksum, sizeof(csum)) != 0) {
		LOG(ERROR) << "ops checksum does not match";
		return nullptr;
		}

		return std::make_unique<CowOpIter>(std::move(ops_buffer), header_.ops_size);
		}

		bool CowReader::GetRawBytes(uint64_t offset, void* buffer, size_t len) {
		// Validate the offset, taking care to acknowledge possible overflow of offset+len.
		if (offset < sizeof(header_) \|\| offset >= header_.ops_offset \|\| len >= fd_size_ \|\|
		offset + len > header_.ops_offset) {
		LOG(ERROR) << "invalid data offset: " << offset << ", " << len << " bytes";
		return false;
		}
		if (lseek(fd_.get(), offset, SEEK_SET) < 0) {
		PLOG(ERROR) << "lseek to read raw bytes failed";
		return false;
		}
		if (!android::base::ReadFully(fd_, buffer, len)) {
		PLOG(ERROR) << "read raw bytes failed";
		return false;
		}
		return true;
		}

		bool CowReader::ReadData(const CowOperation& op, IByteSink* sink) {
		uint64_t offset = op.source;

		switch (op.compression) {
		case kCowCompressNone: {
		size_t remaining = op.data_length;
		while (remaining) {
		size_t amount = remaining;
		void* buffer = sink->GetBuffer(amount, &amount);
		if (!buffer) {
		LOG(ERROR) << "Could not acquire buffer from sink";
		return false;
		}
		if (!GetRawBytes(offset, buffer, amount)) {
		return false;
		}
		if (!sink->ReturnData(buffer, amount)) {
		LOG(ERROR) << "Could not return buffer to sink";
		return false;
		}
		remaining -= amount;
		offset += amount;
		}
		return true;
		}
		case kCowCompressGz: {
		auto input = std::make_unique<Bytef[]>(op.data_length);
		if (!GetRawBytes(offset, input.get(), op.data_length)) {
		return false;
		}

		z_stream z = {};
		z.next_in = input.get();
		z.avail_in = op.data_length;
		if (int rv = inflateInit(&z); rv != Z_OK) {
		LOG(ERROR) << "inflateInit returned error code " << rv;
		return false;
		}

		while (z.total_out < header_.block_size) {
		// If no more output buffer, grab a new buffer.
		if (z.avail_out == 0) {
		size_t amount = header_.block_size - z.total_out;
		z.next_out = reinterpret_cast<Bytef*>(sink->GetBuffer(amount, &amount));
		if (!z.next_out) {
		LOG(ERROR) << "Could not acquire buffer from sink";
		return false;
		}
		z.avail_out = amount;
		}

		// Remember the position of the output buffer so we can call ReturnData.
		auto buffer = z.next_out;
		auto avail_out = z.avail_out;

		// Decompress.
		int rv = inflate(&z, Z_NO_FLUSH);
		if (rv != Z_OK && rv != Z_STREAM_END) {
		LOG(ERROR) << "inflate returned error code " << rv;
		return false;
		}

		// Return the section of the buffer that was updated.
		if (z.avail_out < avail_out && !sink->ReturnData(buffer, avail_out - z.avail_out)) {
		LOG(ERROR) << "Could not return buffer to sink";
		return false;
		}

		if (rv == Z_STREAM_END) {
		// Error if the stream has ended, but we didn't fill the entire block.
		if (z.total_out != header_.block_size) {
		LOG(ERROR) << "Reached gz stream end but did not read a full block of data";
		return false;
		}
		break;
		}

		CHECK(rv == Z_OK);

		// Error if the stream is expecting more data, but we don't have any to read.
		if (z.avail_in == 0) {
		LOG(ERROR) << "Gz stream ended prematurely";
		return false;
		}
		}
		return true;
		}
		default:
		LOG(ERROR) << "Unknown compression type: " << op.compression;
		return false;
		}
		}

		} // namespace snapshot
		} // namespace android

fs_mgr/libsnapshot/cow_writer.cpp

0 → 100644

+230 −0

Original line number	Diff line number	Diff line
		//
		// Copyright (C) 2020 The Android Open Source Project
		//
		// 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.
		//

		#include <sys/types.h>
		#include <unistd.h>

		#include <limits>

		#include <android-base/file.h>
		#include <android-base/logging.h>
		#include <libsnapshot/cow_writer.h>
		#include <openssl/sha.h>
		#include <zlib.h>

		namespace android {
		namespace snapshot {

		static_assert(sizeof(off_t) == sizeof(uint64_t));

		CowWriter::CowWriter(const CowOptions& options) : ICowWriter(options), fd_(-1) {
		SetupHeaders();
		}

		void CowWriter::SetupHeaders() {
		header_ = {};
		header_.magic = kCowMagicNumber;
		header_.major_version = kCowVersionMajor;
		header_.minor_version = kCowVersionMinor;
		header_.block_size = options_.block_size;
		}

		bool CowWriter::Initialize(android::base::unique_fd&& fd) {
		owned_fd_ = std::move(fd);
		return Initialize(android::base::borrowed_fd{owned_fd_});
		}

		bool CowWriter::Initialize(android::base::borrowed_fd fd) {
		fd_ = fd;

		// This limitation is tied to the data field size in CowOperation.
		if (header_.block_size > std::numeric_limits<uint16_t>::max()) {
		LOG(ERROR) << "Block size is too large";
		return false;
		}

		if (lseek(fd_.get(), 0, SEEK_SET) < 0) {
		PLOG(ERROR) << "lseek failed";
		return false;
		}

		if (options_.compression == "gz") {
		compression_ = kCowCompressGz;
		} else if (!options_.compression.empty()) {
		LOG(ERROR) << "unrecognized compression: " << options_.compression;
		return false;
		}

		// Headers are not complete, but this ensures the file is at the right
		// position.
		if (!android::base::WriteFully(fd_, &header_, sizeof(header_))) {
		PLOG(ERROR) << "write failed";
		return false;
		}
		return true;
		}

		bool CowWriter::AddCopy(uint64_t new_block, uint64_t old_block) {
		header_.num_ops++;

		CowOperation op = {};
		op.type = kCowCopyOp;
		op.new_block = new_block;
		op.source = old_block;
		ops_ += std::basic_string<uint8_t>(reinterpret_cast<uint8_t*>(&op), sizeof(op));

		return true;
		}

		bool CowWriter::AddRawBlocks(uint64_t new_block_start, const void* data, size_t size) {
		if (size % header_.block_size != 0) {
		LOG(ERROR) << "AddRawBlocks: size " << size << " is not a multiple of "
		<< header_.block_size;
		return false;
		}

		uint64_t pos;
		if (!GetDataPos(&pos)) {
		return false;
		}

		const uint8_t* iter = reinterpret_cast<const uint8_t*>(data);
		for (size_t i = 0; i < size / header_.block_size; i++) {
		header_.num_ops++;

		CowOperation op = {};
		op.type = kCowReplaceOp;
		op.new_block = new_block_start + i;
		op.source = pos;

		if (compression_) {
		auto data = Compress(iter, header_.block_size);
		if (data.empty()) {
		PLOG(ERROR) << "AddRawBlocks: compression failed";
		return false;
		}
		if (data.size() > std::numeric_limits<uint16_t>::max()) {
		LOG(ERROR) << "Compressed block is too large: " << data.size() << " bytes";
		return false;
		}
		if (!android::base::WriteFully(fd_, data.data(), data.size())) {
		PLOG(ERROR) << "AddRawBlocks: write failed";
		return false;
		}
		op.compression = compression_;
		op.data_length = static_cast<uint16_t>(data.size());
		pos += data.size();
		} else {
		op.data_length = static_cast<uint16_t>(header_.block_size);
		pos += header_.block_size;
		}

		ops_ += std::basic_string<uint8_t>(reinterpret_cast<uint8_t*>(&op), sizeof(op));
		iter += header_.block_size;
		}

		if (!compression_ && !android::base::WriteFully(fd_, data, size)) {
		PLOG(ERROR) << "AddRawBlocks: write failed";
		return false;
		}
		return true;
		}

		bool CowWriter::AddZeroBlocks(uint64_t new_block_start, uint64_t num_blocks) {
		for (uint64_t i = 0; i < num_blocks; i++) {
		header_.num_ops++;

		CowOperation op = {};
		op.type = kCowZeroOp;
		op.new_block = new_block_start + i;
		op.source = 0;
		ops_ += std::basic_string<uint8_t>(reinterpret_cast<uint8_t*>(&op), sizeof(op));
		}
		return true;
		}

		std::basic_string<uint8_t> CowWriter::Compress(const void* data, size_t length) {
		switch (compression_) {
		case kCowCompressGz: {
		auto bound = compressBound(length);
		auto buffer = std::make_unique<uint8_t[]>(bound);

		uLongf dest_len = bound;
		auto rv = compress2(buffer.get(), &dest_len, reinterpret_cast<const Bytef*>(data),
		length, Z_BEST_COMPRESSION);
		if (rv != Z_OK) {
		LOG(ERROR) << "compress2 returned: " << rv;
		return {};
		}
		return std::basic_string<uint8_t>(buffer.get(), dest_len);
		}
		default:
		LOG(ERROR) << "unhandled compression type: " << compression_;
		break;
		}
		return {};
		}

		static void SHA256(const void* data, size_t length, uint8_t out[32]) {
		SHA256_CTX c;
		SHA256_Init(&c);
		SHA256_Update(&c, data, length);
		SHA256_Final(out, &c);
		}

		bool CowWriter::Finalize() {
		auto offs = lseek(fd_.get(), 0, SEEK_CUR);
		if (offs < 0) {
		PLOG(ERROR) << "lseek failed";
		return false;
		}
		header_.ops_offset = offs;
		header_.ops_size = ops_.size();

		SHA256(ops_.data(), ops_.size(), header_.ops_checksum);
		SHA256(&header_, sizeof(header_), header_.header_checksum);

		if (lseek(fd_.get(), 0, SEEK_SET) < 0) {
		PLOG(ERROR) << "lseek start failed";
		return false;
		}
		if (!android::base::WriteFully(fd_, &header_, sizeof(header_))) {
		PLOG(ERROR) << "write header failed";
		return false;
		}
		if (lseek(fd_.get(), header_.ops_offset, SEEK_SET) < 0) {
		PLOG(ERROR) << "lseek ops failed";
		return false;
		}
		if (!android::base::WriteFully(fd_, ops_.data(), ops_.size())) {
		PLOG(ERROR) << "write ops failed";
		return false;
		}
		return true;
		}

		bool CowWriter::GetDataPos(uint64_t* pos) {
		off_t offs = lseek(fd_.get(), 0, SEEK_CUR);
		if (offs < 0) {
		PLOG(ERROR) << "lseek failed";
		return false;
		}
		*pos = offs;
		return true;
		}

		} // namespace snapshot
		} // namespace android

fs_mgr/libsnapshot/include/libsnapshot/cow_format.h

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