commit 7a8d8fd176dace4a3a8f99d7a51d251e9e1a58d2
author Yi-Yo Chiang <yochiang@google.com> Wed Jan 19 23:22:45 2022 +0800
committer Yi-yo Chiang <yochiang@google.com> Thu Jan 20 11:11:32 2022 +0000
tree c250346a6bae06705f5853f42972e28fdd441760
parent 87afd3983154c857098aa697f12628bb738887ef

Better heuristics for discovering boot sig for retrofitted image

For retrofitted images, DEFINE the last 16K bytes to be the boot
signature.
For boot v2, the new heuristic doesn't assume the internal structure of
the boot image, thus we don't need to calculate the boot signature
offset, making the boot signature excavation more deterministic and
robust.

Bug: 211741246
Test: Mock out boot image with a retrofitted image
Test: atest vts_gki_compliance_test
Change-Id: I1236d499a5987c6f1e0b814547afd8279ffa3280

avb/vts_gki_compliance_test.cpp

diff --git a/avb/vts_gki_compliance_test.cpp b/avb/vts_gki_compliance_test.cpp
index 2577dd5..e3f59cd 100644
--- a/avb/vts_gki_compliance_test.cpp
+++ b/avb/vts_gki_compliance_test.cpp
@@ -159,6 +159,8 @@
 
 class GkiBootImageV2 : public GkiBootImage {
  public:
+  static constexpr uint32_t kBootSignatureSize = 16_KiB;
+
   GkiBootImageV2(const uint8_t *data, size_t size) : GkiBootImage(data, size) {}
 
   const boot_img_hdr_v2 *boot_header() const {
@@ -172,21 +174,16 @@
   uint32_t ramdisk_size() const override { return boot_header()->ramdisk_size; }
 
   uint32_t signature_size() const override {
-    // Boot v2 header doesn't tell us the size of boot signature, so we just
-    // let GkiBootImage::GetBootSignatures() method heuristically carve out any
-    // boot signature blobs if it can find any. The size we return here is only
-    // a heuristic so we don't look too far.
-    return 16_KiB;
+    // Boot v2 header doesn't tell us the size of the boot signature, so we just
+    // define the last 16K bytes to be the boot signature for retrofitted GKI.
+    return kBootSignatureSize;
   }
 
   uint32_t signature_offset() const override {
-    const uint32_t second_pages = GetNumberOfPages(boot_header()->second_size);
-    const uint32_t recovery_dtbo_pages =
-        GetNumberOfPages(boot_header()->recovery_dtbo_size);
-    const uint32_t dtb_pages = GetNumberOfPages(boot_header()->dtb_size);
-    return ramdisk_offset() +
-           (ramdisk_pages() + second_pages + recovery_dtbo_pages + dtb_pages) *
-               page_size();
+    if (size() < signature_size()) {
+      return 0;
+    }
+    return size() - signature_size();
   }
 };
 
@@ -220,6 +217,8 @@
 // the |signature_size| field, so we would have to improvise.
 class GkiBootImageV3 : public GkiBootImageV4 {
  public:
+  static constexpr uint32_t kBootSignatureSize = 16_KiB;
+
   GkiBootImageV3(const uint8_t *data, size_t size)
       : GkiBootImageV4(data, size) {}
 
@@ -228,7 +227,7 @@
     // If |signature_size| is non-zero then this is actually a boot v4 image
     // wearing a boot v3 camouflage, else use the same heuristic as boot v2.
     const uint32_t value = GkiBootImageV4::boot_header()->signature_size;
-    return value ? value : 16_KiB;
+    return value ? value : kBootSignatureSize;
   }
 };
 
@@ -361,7 +360,8 @@
   GTEST_LOG_(INFO) << TAG << ": " + name + ".fingerprint: "
                    << GetAvbProperty(name + ".fingerprint",
                                      *boot_signature_images);
-  GTEST_LOG_(INFO) << TAG << ": kernel size: " << boot_image->kernel_size()
+  GTEST_LOG_(INFO) << TAG << ": header version: " << boot_header_version
+                   << ", kernel size: " << boot_image->kernel_size()
                    << ", ramdisk size: " << boot_image->ramdisk_size()
                    << ", signature size: " << boot_image->signature_size();