libfiemap_writer: Improve device unwrapping to support simple linear devices.

Normally we reject complex dm targets that would invalidate or shift the
block mappings returned via FIEMAP/FIBMAP. Currently the only targets
allowed are crypt, default-key, and bow. This patch adds support for
"linear" as long as there is only one linear target and it targets
sector 0 of the underlying block device.

This is useful for testing gsid, so we can simulate how a
metadata-encrypted device works without having to create a dm-crypt or
dm-default-key node.

Bug: 134536978
Test: manual test
Change-Id: I7c12bc20d95ff4c90402e66bafb4cf2fce7818e2

fs_mgr/libdm/dm.cpp

@@ -368,5 +368,13 @@ void DeviceMapper::InitIo(struct dm_ioctl* io, const std::string& name) const {
     }
 }
 
+std::string DeviceMapper::GetTargetType(const struct dm_target_spec& spec) {
+    if (const void* p = memchr(spec.target_type, '\0', sizeof(spec.target_type))) {
+        ptrdiff_t length = reinterpret_cast<const char*>(p) - spec.target_type;
+        return std::string{spec.target_type, static_cast<size_t>(length)};
+    }
+    return std::string{spec.target_type, sizeof(spec.target_type)};
+}
+
 }  // namespace dm
 }  // namespace android

fs_mgr/libdm/dm_test.cpp

@@ -170,6 +170,10 @@ TEST(libdm, DmLinear) {
     EXPECT_EQ(targets[1].spec.sector_start, 1);
     EXPECT_EQ(targets[1].spec.length, 1);
 
+    // Test GetTargetType().
+    EXPECT_EQ(DeviceMapper::GetTargetType(targets[0].spec), std::string{"linear"});
+    EXPECT_EQ(DeviceMapper::GetTargetType(targets[1].spec), std::string{"linear"});
+
     // Normally the TestDevice destructor would delete this, but at least one
     // test should ensure that device deletion works.
     ASSERT_TRUE(dev.Destroy());

fs_mgr/libdm/include/libdm/dm.h

@@ -136,6 +136,8 @@ class DeviceMapper final {
     // mapper device from the kernel.
     bool GetTableInfo(const std::string& name, std::vector<TargetInfo>* table);
 
+    static std::string GetTargetType(const struct dm_target_spec& spec);
+
   private:
     // Maximum possible device mapper targets registered in the kernel.
     // This is only used to read the list of targets from kernel so we allocate

fs_mgr/libfiemap_writer/fiemap_writer.cpp

@@ -89,6 +89,31 @@ static bool BlockDeviceToName(uint32_t major, uint32_t minor, std::string* bdev_
     return true;
 }
 
+static bool ValidateDmTarget(const DeviceMapper::TargetInfo& target) {
+    const auto& entry = target.spec;
+    if (entry.sector_start != 0) {
+        LOG(INFO) << "Stopping at target with non-zero starting sector";
+        return false;
+    }
+
+    auto target_type = DeviceMapper::GetTargetType(entry);
+    if (target_type == "bow" || target_type == "default-key" || target_type == "crypt") {
+        return true;
+    }
+    if (target_type == "linear") {
+        auto pieces = android::base::Split(target.data, " ");
+        if (pieces[1] != "0") {
+            LOG(INFO) << "Stopping at complex linear target with non-zero starting sector: "
+                      << pieces[1];
+            return false;
+        }
+        return true;
+    }
+
+    LOG(INFO) << "Stopping at complex target type " << target_type;
+    return false;
+}
+
 static bool DeviceMapperStackPop(const std::string& bdev, std::string* bdev_raw) {
     *bdev_raw = bdev;
 
@@ -128,15 +153,7 @@ static bool DeviceMapperStackPop(const std::string& bdev, std::string* bdev_raw)
         LOG(INFO) << "Stopping at complex table for " << dm_name << " at " << bdev;
         return true;
     }
-    const auto& entry = table[0].spec;
-    std::string target_type(std::string(entry.target_type, sizeof(entry.target_type)).c_str());
-    if (target_type != "bow" && target_type != "default-key" && target_type != "crypt") {
-        LOG(INFO) << "Stopping at complex target-type " << target_type << " for " << dm_name
-                  << " at " << bdev;
-        return true;
-    }
-    if (entry.sector_start != 0) {
-        LOG(INFO) << "Stopping at target-type with non-zero starting sector";
+    if (!ValidateDmTarget(table[0])) {
         return true;
     }