avb_ops: support reading from a logical partition

On some devices (e.g., emulator), init needs to read AVB footer from
a logical partition because:

   1) Dynamic/logical partition is enabled
   2) The partition is AVB chained, i.e., need to locate footer from the end
   3) Logical partition is not understandable by bootloader,
      but there is no bootloader in this case

Bug: 125540538
Bug: 128434470
Test: boot and force the fallback path, to check it can get logical path
Change-Id: Ie304bce234cbf0f938f386f7ce59235c851e0e2d

fs_mgr/libfs_avb/avb_ops.cpp

@@ -36,6 +36,7 @@
 #include <android-base/strings.h>
 #include <android-base/unique_fd.h>
 #include <libavb/libavb.h>
+#include <libdm/dm.h>
 #include <utils/Compat.h>
 
 #include "util.h"
@@ -104,6 +105,20 @@ static AvbIOResult dummy_get_size_of_partition(AvbOps* ops ATTRIBUTE_UNUSED,
     return AVB_IO_RESULT_OK;
 }
 
+// Converts a partition name (with ab_suffix) to the corresponding mount point.
+// e.g., "system_a" => "/system",
+// e.g., "vendor_a" => "/vendor",
+static std::string DeriveMountPoint(const std::string& partition_name) {
+    const std::string ab_suffix = fs_mgr_get_slot_suffix();
+    std::string mount_point(partition_name);
+    auto found = partition_name.rfind(ab_suffix);
+    if (found != std::string::npos) {
+        mount_point.erase(found);  // converts system_a => system
+    }
+
+    return "/" + mount_point;
+}
+
 FsManagerAvbOps::FsManagerAvbOps() {
     // We only need to provide the implementation of read_from_partition()
     // operation since that's all what is being used by the avb_slot_verify().
@@ -122,14 +137,53 @@ FsManagerAvbOps::FsManagerAvbOps() {
     avb_ops_.user_data = this;
 }
 
+// Given a partition name (with ab_suffix), e.g., system_a, returns the corresponding
+// dm-linear path for it. e.g., /dev/block/dm-0. If not found, returns an empty string.
+// This assumes that the prefix of the partition name and the mount point are the same.
+// e.g., partition vendor_a is mounted under /vendor, product_a is mounted under /product, etc.
+// This might not be true for some special fstab files, e.g., fstab.postinstall.
+// But it's good enough for the default fstab. Also note that the logical path is a
+// fallback solution when the physical path (/dev/block/by-name/<partition>) cannot be found.
+std::string FsManagerAvbOps::GetLogicalPath(const std::string& partition_name) {
+    if (fstab_.empty() && !ReadDefaultFstab(&fstab_)) {
+        return "";
+    }
+
+    const auto mount_point = DeriveMountPoint(partition_name);
+    if (mount_point.empty()) return "";
+
+    auto fstab_entry = GetEntryForMountPoint(&fstab_, mount_point);
+    if (!fstab_entry) return "";
+
+    std::string device_path;
+    if (fstab_entry->fs_mgr_flags.logical) {
+        dm::DeviceMapper& dm = dm::DeviceMapper::Instance();
+        if (!dm.GetDmDevicePathByName(fstab_entry->blk_device, &device_path)) {
+            LERROR << "Failed to resolve logical device path for: " << fstab_entry->blk_device;
+            return "";
+        }
+        return device_path;
+    }
+
+    return "";
+}
+
 AvbIOResult FsManagerAvbOps::ReadFromPartition(const char* partition, int64_t offset,
                                                size_t num_bytes, void* buffer,
                                                size_t* out_num_read) {
-    const std::string path = "/dev/block/by-name/"s + partition;
+    std::string path = "/dev/block/by-name/"s + partition;
 
     // Ensures the device path (a symlink created by init) is ready to access.
     if (!WaitForFile(path, 1s)) {
-        return AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION;
+        LERROR << "Device path not found: " << path;
+        // Falls back to logical path if the physical path is not found.
+        // This mostly only works for emulator (no bootloader). Because in normal
+        // device, bootloader is unable to read logical partitions. So if libavb in
+        // the bootloader failed to read a physical partition, it will failed to boot
+        // the HLOS and we won't reach the code here.
+        path = GetLogicalPath(partition);
+        if (path.empty() || !WaitForFile(path, 1s)) return AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION;
+        LINFO << "Fallback to use logical device path: " << path;
     }
 
     android::base::unique_fd fd(TEMP_FAILURE_RETRY(open(path.c_str(), O_RDONLY | O_CLOEXEC)));

fs_mgr/libfs_avb/avb_ops.h

@@ -28,6 +28,7 @@
 #include <vector>
 
 #include <fs_avb/types.h>
+#include <fstab/fstab.h>
 #include <libavb/libavb.h>
 
 namespace android {
@@ -60,7 +61,9 @@ class FsManagerAvbOps {
                                       std::vector<VBMetaData>* out_vbmeta_images);
 
   private:
+    std::string GetLogicalPath(const std::string& partition_name);
     AvbOps avb_ops_;
+    Fstab fstab_;
 };
 
 }  // namespace fs_mgr