libfs_avb: Allow overriding the slot suffix.

There is no easy way to override fs_mgr_get_slot_suffix for testing or
for simulating slot switches. It relies on global properties, and
since fstab is usually a static library, linked multiple times, we can't
just stick a global variable in fs_mgr_slotselect.

libfs_avb already takes slot suffix arguments in various places. This CL
extends that by adding arguments to AvbHandle and FsManagerAvbOps.

Bug: N/A
Test: snapshotctl
Change-Id: Ide7f052b38249d6bc79a6a2c124baf8ca08bcc3b

fs_mgr/libfs_avb/avb_ops.cpp

@@ -108,8 +108,8 @@ static AvbIOResult get_size_of_partition(AvbOps* ops ATTRIBUTE_UNUSED,
 // 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();
+static std::string DeriveMountPoint(const std::string& partition_name,
+                                    const std::string& ab_suffix) {
     std::string mount_point(partition_name);
     auto found = partition_name.rfind(ab_suffix);
     if (found != std::string::npos) {
@@ -119,7 +119,7 @@ static std::string DeriveMountPoint(const std::string& partition_name) {
     return "/" + mount_point;
 }
 
-FsManagerAvbOps::FsManagerAvbOps() {
+FsManagerAvbOps::FsManagerAvbOps(const std::string& slot_suffix) {
     // 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().
     // Other I/O operations are only required in bootloader but not in
@@ -135,6 +135,11 @@ FsManagerAvbOps::FsManagerAvbOps() {
 
     // Sets user_data for GetInstanceFromAvbOps() to convert it back to FsManagerAvbOps.
     avb_ops_.user_data = this;
+
+    slot_suffix_ = slot_suffix;
+    if (slot_suffix_.empty()) {
+        slot_suffix_ = fs_mgr_get_slot_suffix();
+    }
 }
 
 // Given a partition name (with ab_suffix), e.g., system_a, returns the corresponding
@@ -149,7 +154,7 @@ std::string FsManagerAvbOps::GetLogicalPath(const std::string& partition_name) {
         return "";
     }
 
-    const auto mount_point = DeriveMountPoint(partition_name);
+    const auto mount_point = DeriveMountPoint(partition_name, slot_suffix_);
     if (mount_point.empty()) return "";
 
     auto fstab_entry = GetEntryForMountPoint(&fstab_, mount_point);

fs_mgr/libfs_avb/avb_ops.h

@@ -48,7 +48,7 @@ namespace fs_mgr {
 //
 class FsManagerAvbOps {
   public:
-    FsManagerAvbOps();
+    explicit FsManagerAvbOps(const std::string& slot_suffix = {});
 
     static FsManagerAvbOps* GetInstanceFromAvbOps(AvbOps* ops) {
         return reinterpret_cast<FsManagerAvbOps*>(ops->user_data);
@@ -66,6 +66,7 @@ class FsManagerAvbOps {
     std::string GetPartitionPath(const char* partition_name);
     AvbOps avb_ops_;
     Fstab fstab_;
+    std::string slot_suffix_;
 };
 
 }  // namespace fs_mgr

fs_mgr/libfs_avb/fs_avb.cpp

@@ -182,6 +182,11 @@ bool AvbVerifier::VerifyVbmetaImages(const std::vector<VBMetaData>& vbmeta_image
 
 // class AvbHandle
 // ---------------
+AvbHandle::AvbHandle() : status_(AvbHandleStatus::kUninitialized) {
+    slot_suffix_ = fs_mgr_get_slot_suffix();
+    other_slot_suffix_ = fs_mgr_get_other_slot_suffix();
+}
+
 AvbUniquePtr AvbHandle::LoadAndVerifyVbmeta(
         const std::string& partition_name, const std::string& ab_suffix,
         const std::string& ab_other_suffix, const std::string& expected_public_key_path,
@@ -194,6 +199,9 @@ AvbUniquePtr AvbHandle::LoadAndVerifyVbmeta(
         return nullptr;
     }
 
+    avb_handle->slot_suffix_ = ab_suffix;
+    avb_handle->other_slot_suffix_ = ab_other_suffix;
+
     std::string expected_key_blob;
     if (!expected_public_key_path.empty()) {
         if (access(expected_public_key_path.c_str(), F_OK) != 0) {
@@ -373,9 +381,14 @@ AvbUniquePtr AvbHandle::LoadAndVerifyVbmeta(const FstabEntry& fstab_entry,
     return avb_handle;
 }
 
-AvbUniquePtr AvbHandle::LoadAndVerifyVbmeta() {
+AvbUniquePtr AvbHandle::LoadAndVerifyVbmeta(const std::string& slot_suffix) {
     // Loads inline vbmeta images, starting from /vbmeta.
-    return LoadAndVerifyVbmeta("vbmeta", fs_mgr_get_slot_suffix(), fs_mgr_get_other_slot_suffix(),
+    auto suffix = slot_suffix;
+    if (suffix.empty()) {
+        suffix = fs_mgr_get_slot_suffix();
+    }
+    auto other_suffix = android::fs_mgr::OtherSlotSuffix(suffix);
+    return LoadAndVerifyVbmeta("vbmeta", suffix, other_suffix,
                                {} /* expected_public_key, already checked by bootloader */,
                                HashAlgorithm::kSHA256,
                                IsAvbPermissive(), /* allow_verification_error */
@@ -399,7 +412,7 @@ AvbUniquePtr AvbHandle::Open() {
                                        ? AVB_SLOT_VERIFY_FLAGS_ALLOW_VERIFICATION_ERROR
                                        : AVB_SLOT_VERIFY_FLAGS_NONE;
     AvbSlotVerifyResult verify_result =
-            avb_ops.AvbSlotVerify(fs_mgr_get_slot_suffix(), flags, &avb_handle->vbmeta_images_);
+            avb_ops.AvbSlotVerify(avb_handle->slot_suffix_, flags, &avb_handle->vbmeta_images_);
 
     // Only allow the following verify results:
     //   - AVB_SLOT_VERIFY_RESULT_OK.
@@ -492,7 +505,7 @@ AvbHashtreeResult AvbHandle::SetUpAvbHashtree(FstabEntry* fstab_entry, bool wait
     }
 
     if (!LoadAvbHashtreeToEnableVerity(fstab_entry, wait_for_verity_dev, vbmeta_images_,
-                                       fs_mgr_get_slot_suffix(), fs_mgr_get_other_slot_suffix())) {
+                                       slot_suffix_, other_slot_suffix_)) {
         return AvbHashtreeResult::kFail;
     }
 
@@ -526,8 +539,8 @@ std::string AvbHandle::GetSecurityPatchLevel(const FstabEntry& fstab_entry) cons
     if (vbmeta_images_.size() < 1) {
         return "";
     }
-    std::string avb_partition_name = DeriveAvbPartitionName(fstab_entry, fs_mgr_get_slot_suffix(),
-                                                            fs_mgr_get_other_slot_suffix());
+    std::string avb_partition_name =
+            DeriveAvbPartitionName(fstab_entry, slot_suffix_, other_slot_suffix_);
     auto avb_prop_name = "com.android.build." + avb_partition_name + ".security_patch";
     return GetAvbPropertyDescriptor(avb_prop_name, vbmeta_images_);
 }

fs_mgr/libfs_avb/include/fs_avb/fs_avb.h

@@ -83,8 +83,8 @@ class AvbHandle {
     //     is verified and can be trusted.
     //
     // TODO(bowgotsai): remove Open() and switch to LoadAndVerifyVbmeta().
-    static AvbUniquePtr Open();                 // loads inline vbmeta, via libavb.
-    static AvbUniquePtr LoadAndVerifyVbmeta();  // loads inline vbmeta.
+    static AvbUniquePtr Open();  // loads inline vbmeta, via libavb.
+    static AvbUniquePtr LoadAndVerifyVbmeta(const std::string& slot_suffix = {});
 
     // The caller can specify optional preload_avb_key_blobs for public key matching.
     // This is mostly for init to preload AVB keys before chroot into /system.
@@ -137,12 +137,14 @@ class AvbHandle {
     AvbHandle& operator=(AvbHandle&&) noexcept = delete;  // no move assignment
 
   private:
-    AvbHandle() : status_(AvbHandleStatus::kUninitialized) {}
+    AvbHandle();
 
     std::vector<VBMetaData> vbmeta_images_;
     VBMetaInfo vbmeta_info_;  // A summary info for vbmeta_images_.
     AvbHandleStatus status_;
     std::string avb_version_;
+    std::string slot_suffix_;
+    std::string other_slot_suffix_;
 };
 
 }  // namespace fs_mgr

fs_mgr/libfstab/include/fstab/fstab.h

@@ -145,5 +145,8 @@ void ImportKernelCmdline(const std::function<void(std::string, std::string)>& fn
 // Otherwise returns false and |*out| is not modified.
 bool GetKernelCmdline(const std::string& key, std::string* out);
 
+// Return the "other" slot for the given slot suffix.
+std::string OtherSlotSuffix(const std::string& suffix);
+
 }  // namespace fs_mgr
 }  // namespace android

fs_mgr/libfstab/slotselect.cpp

@@ -74,3 +74,13 @@ bool fs_mgr_update_for_slotselect(Fstab* fstab) {
     }
     return true;
 }
+
+namespace android {
+namespace fs_mgr {
+
+std::string OtherSlotSuffix(const std::string& suffix) {
+    return other_suffix(suffix);
+}
+
+}  // namespace fs_mgr
+}  // namespace android