libfiemap_writer: Allow callers to query the block device of a file.

Bug: 126249541
Test: manual test
Change-Id: I478bcf60caf0cedc84189c39fee4b6c4f94b6ed6

2 files changed, 14 insertions, 2 deletions

diff --git a/fs_mgr/libfiemap_writer/fiemap_writer.cpp b/fs_mgr/libfiemap_writer/fiemap_writer.cpp
index b9b75f87e..fc24a173f 100644
--- a/fs_mgr/libfiemap_writer/fiemap_writer.cpp
+++ b/fs_mgr/libfiemap_writer/fiemap_writer.cpp
@@ -126,7 +126,8 @@ static bool DeviceMapperStackPop(const std::string& bdev, std::string* bdev_raw)
     return DeviceMapperStackPop(bdev_next, bdev_raw);
 }
 
-static bool FileToBlockDevicePath(const std::string& file_path, std::string* bdev_path) {
+bool FiemapWriter::GetBlockDeviceForFile(const std::string& file_path, std::string* bdev_path,
+                                         bool* uses_dm) {
     struct stat sb;
     if (stat(file_path.c_str(), &sb)) {
         PLOG(ERROR) << "Failed to get stat for: " << file_path;
@@ -146,6 +147,10 @@ static bool FileToBlockDevicePath(const std::string& file_path, std::string* bde
         return false;
     }
 
+    if (uses_dm) {
+        *uses_dm = (bdev_raw != bdev);
+    }
+
     LOG(DEBUG) << "Popped device (" << bdev_raw << ") from device mapper stack starting with ("
                << bdev << ")";
 
@@ -458,7 +463,7 @@ FiemapUniquePtr FiemapWriter::Open(const std::string& file_path, uint64_t file_s
     }
 
     std::string bdev_path;
-    if (!FileToBlockDevicePath(abs_path, &bdev_path)) {
+    if (!GetBlockDeviceForFile(abs_path, &bdev_path)) {
         LOG(ERROR) << "Failed to get block dev path for file: " << file_path;
         cleanup(abs_path, create);
         return nullptr;
diff --git a/fs_mgr/libfiemap_writer/include/libfiemap_writer/fiemap_writer.h b/fs_mgr/libfiemap_writer/include/libfiemap_writer/fiemap_writer.h
index edbae7721..b5472c9f7 100644
--- a/fs_mgr/libfiemap_writer/include/libfiemap_writer/fiemap_writer.h
+++ b/fs_mgr/libfiemap_writer/include/libfiemap_writer/fiemap_writer.h
@@ -57,6 +57,13 @@ class FiemapWriter final {
     // FiemapWriter::Open).
     static bool HasPinnedExtents(const std::string& file_path);
 
+    // Returns the underlying block device of a file. This will look past device-mapper layers.
+    // If an intermediate device-mapper layer would not maintain a 1:1 mapping (i.e. is a non-
+    // trivial dm-linear), then this will fail. If device-mapper nodes are encountered, then
+    // |uses_dm| will be set to true.
+    static bool GetBlockDeviceForFile(const std::string& file_path, std::string* bdev_path,
+                                      bool* uses_dm = nullptr);
+
     // The counter part of Write(). It is an error for the offset to be unaligned with
     // the block device's block size.
     // In case of error, the contents of buffer MUST be discarded.