3 files changed, 115 insertions, 0 deletions

diff --git a/fs_mgr/liblp/Android.bp b/fs_mgr/liblp/Android.bp
index 176671a59..1434b212e 100644
--- a/fs_mgr/liblp/Android.bp
+++ b/fs_mgr/liblp/Android.bp
@@ -34,6 +34,7 @@ cc_library_static {
         "liblog",
         "libcrypto",
         "libcrypto_utils",
+        "libsparse",
     ],
     whole_static_libs: [
         "libext2_uuid",
diff --git a/fs_mgr/liblp/images.cpp b/fs_mgr/liblp/images.cpp
index c8d9f13d8..93c56180f 100644
--- a/fs_mgr/liblp/images.cpp
+++ b/fs_mgr/liblp/images.cpp
@@ -16,8 +16,11 @@
 
 #include "images.h"
 
+#include <limits.h>
+
 #include <android-base/file.h>
 #include <android-base/unique_fd.h>
+#include <sparse/sparse.h>
 
 #include "reader.h"
 #include "utility.h"
@@ -75,5 +78,115 @@ bool WriteToImageFile(const char* file, const LpMetadata& input) {
     return WriteToImageFile(fd, input);
 }
 
+// We use an object to build the sparse file since it requires that data
+// pointers be held alive until the sparse file is destroyed. It's easier
+// to do this when the data pointers are all in one place.
+class SparseBuilder {
+  public:
+    explicit SparseBuilder(const LpMetadata& metadata);
+
+    bool Build();
+    bool Export(const char* file);
+    bool IsValid() const { return file_ != nullptr; }
+
+  private:
+    bool AddData(const std::string& blob, uint32_t block);
+
+    const LpMetadata& metadata_;
+    const LpMetadataGeometry& geometry_;
+    std::unique_ptr<sparse_file, decltype(&sparse_file_destroy)> file_;
+    std::string geometry_blob_;
+    std::string metadata_blob_;
+};
+
+SparseBuilder::SparseBuilder(const LpMetadata& metadata)
+    : metadata_(metadata),
+      geometry_(metadata.geometry),
+      file_(sparse_file_new(LP_SECTOR_SIZE, geometry_.block_device_size), sparse_file_destroy) {}
+
+bool SparseBuilder::Export(const char* file) {
+    android::base::unique_fd fd(open(file, O_CREAT | O_RDWR | O_TRUNC, 0644));
+    if (fd < 0) {
+        PERROR << "open failed: " << file;
+        return false;
+    }
+    // No gzip compression; sparseify; no checksum.
+    int ret = sparse_file_write(file_.get(), fd, false, true, false);
+    if (ret != 0) {
+        LERROR << "sparse_file_write failed (error code " << ret << ")";
+        return false;
+    }
+    return true;
+}
+
+bool SparseBuilder::AddData(const std::string& blob, uint32_t block) {
+    void* data = const_cast<char*>(blob.data());
+    int ret = sparse_file_add_data(file_.get(), data, blob.size(), block);
+    if (ret != 0) {
+        LERROR << "sparse_file_add_data failed (error code " << ret << ")";
+        return false;
+    }
+    return true;
+}
+
+bool SparseBuilder::Build() {
+    geometry_blob_ = SerializeGeometry(geometry_);
+    geometry_blob_.resize(LP_METADATA_GEOMETRY_SIZE);
+    if (!AddData(geometry_blob_, 0)) {
+        return false;
+    }
+
+    // Metadata immediately follows geometry, and we write the same metadata
+    // to all slots.
+    uint32_t metadata_block = LP_METADATA_GEOMETRY_SIZE / LP_SECTOR_SIZE;
+    metadata_blob_ = SerializeMetadata(metadata_);
+    for (size_t i = 0; i < geometry_.metadata_slot_count; i++) {
+        if (!AddData(metadata_blob_, metadata_block)) {
+            return false;
+        }
+        metadata_block += geometry_.metadata_max_size / LP_SECTOR_SIZE;
+    }
+
+    // The backup area contains all metadata slots, and then geometry. Similar
+    // to before we write the metadata to every slot.
+    int64_t backup_offset = GetBackupMetadataOffset(geometry_, 0);
+    uint64_t backups_start = geometry_.block_device_size + backup_offset;
+    uint64_t backup_sector = backups_start / LP_SECTOR_SIZE;
+    for (size_t i = 0; i < geometry_.metadata_slot_count; i++) {
+        if (!AddData(metadata_blob_, backup_sector)) {
+            return false;
+        }
+        backup_sector += geometry_.metadata_max_size / LP_SECTOR_SIZE;
+    }
+    if (!AddData(geometry_blob_, backup_sector)) {
+        return false;
+    }
+    return true;
+}
+
+bool WriteToSparseFile(const char* file, const LpMetadata& metadata) {
+    uint64_t num_blocks =
+            AlignTo(metadata.geometry.block_device_size, LP_SECTOR_SIZE) / LP_SECTOR_SIZE;
+    if (num_blocks >= UINT_MAX) {
+        // libsparse counts blocks in unsigned 32-bit integers, but our block
+        // size is rather low (512 bytes), since we operate in sectors.
+        // Therefore the maximum block device size we can represent with a
+        // sparse file is 2TB for now.
+        LERROR << "Block device is too large to encode with libsparse.";
+        return false;
+    }
+
+    SparseBuilder builder(metadata);
+    if (!builder.IsValid()) {
+        LERROR << "Could not allocate sparse file of size " << metadata.geometry.block_device_size;
+        return false;
+    }
+    if (!builder.Build()) {
+        return false;
+    }
+
+    return builder.Export(file);
+}
+
 }  // namespace fs_mgr
 }  // namespace android
diff --git a/fs_mgr/liblp/include/liblp/liblp.h b/fs_mgr/liblp/include/liblp/liblp.h
index 469ef9e59..c8d34d969 100644
--- a/fs_mgr/liblp/include/liblp/liblp.h
+++ b/fs_mgr/liblp/include/liblp/liblp.h
@@ -59,6 +59,7 @@ std::unique_ptr<LpMetadata> ReadMetadata(const char* block_device, uint32_t slot
 
 // Read/Write logical partition metadata to an image file, for diagnostics or
 // flashing.
+bool WriteToSparseFile(const char* file, const LpMetadata& metadata);
 bool WriteToImageFile(const char* file, const LpMetadata& metadata);
 std::unique_ptr<LpMetadata> ReadFromImageFile(const char* file);