avbtool: open hash image read-only when possible

The `add_hash_footer` command normally adds the hash descriptor into the
image directly, except when the `--do_not_append_vbmeta_image` flag is
given. In this case we don't touch the input image so can open it
read-only.

An example use case is:
  1. Android source tree is checked out read-only (e.g. infra bots)
  2. Some test image is checked into the tree as a binary
  3. We want to call `add_hash_footer` on test image as a build rule
In this case, `avbtool` will fail during build if it attempts to open
the read-only test image as writable, so must open it read-only.

Bug: b/324451620
Test: atest libavb_host_unittest
Change-Id: I438030fed51244769100f305fcec7116fc679f4f

2 files changed, 8 insertions, 4 deletions

diff --git a/avbtool.py b/avbtool.py
index 5be377f..e46d042 100755
--- a/avbtool.py
+++ b/avbtool.py
@@ -3441,7 +3441,10 @@ class Avb(object):
       print('{}'.format(partition_size - max_metadata_size))
       return
 
-    image = ImageHandler(image_filename)
+    # If we aren't appending the vbmeta footer to the input image we can
+    # open it in read-only mode.
+    image = ImageHandler(image_filename,
+                         read_only=do_not_append_vbmeta_image)
 
     # If there's already a footer, truncate the image to its original
     # size. This way 'avbtool add_hash_footer' is idempotent (modulo
@@ -4382,8 +4385,7 @@ class AvbTool(object):
     sub_parser = subparsers.add_parser('add_hash_footer',
                                        help='Add hashes and footer to image.')
     sub_parser.add_argument('--image',
-                            help='Image to add hashes to',
-                            type=argparse.FileType('rb+'))
+                            help='Image to add hashes to')
     sub_parser.add_argument('--partition_size',
                             help='Partition size',
                             type=parse_number)
@@ -4818,7 +4820,7 @@ class AvbTool(object):
   def add_hash_footer(self, args):
     """Implements the 'add_hash_footer' sub-command."""
     args = self._fixup_common_args(args)
-    self.avb.add_hash_footer(args.image.name if args.image else None,
+    self.avb.add_hash_footer(args.image,
                              args.partition_size, args.dynamic_partition_size,
                              args.partition_name, args.hash_algorithm,
                              args.salt, args.chain_partition,
diff --git a/test/avbtool_unittest.cc b/test/avbtool_unittest.cc
index 93cae41..753aaca 100644
--- a/test/avbtool_unittest.cc
+++ b/test/avbtool_unittest.cc
@@ -656,6 +656,8 @@ void AvbToolTest::AddHashFooterTest(bool sparse_image) {
   EXPECT_EQ(static_cast<size_t>(erased_footer_file_size), rootfs_size);
 
   // Check that --do_not_append_vbmeta_image works as intended.
+  // In this case we don't modify the input image so it should work read-only.
+  EXPECT_COMMAND(0, "chmod a-w %s", rootfs_path.value().c_str());
   EXPECT_COMMAND(0,
                  "./avbtool.py add_hash_footer --salt d00df00d "
                  "--hash_algorithm sha256 --image %s "