[avb] Check partition before allocating 64KB memory for VBMeta
This cl is an optimization for load_and_verify_vbmeta(). When
we want to load the VBMeta from the partition `vbmeta` and we
are the main VBMeta, prior to this cl, we first allocated 64KB
memory and then tried to read the partition `vbmeta`.
This cl added a check before this action. After this cl, we
first try to read 1 byte from the partition, if this fails
because no such partition exists, we fallback directly on
the `boot` partition. With this approach, we avoid allocating
the big chunk of memory on heap for VBMeta when the partition
doesn't exist.
This unblocks the cl aosp/2388176 where we don't have the
`vbmeta` partition and also cannot afford allocating the 64KB
memory in pVM firmware.
Bug: 256148034
Test: atest libavb_host_unittest libpvmfw_avb.integration_test
Change-Id: Ia242ddedc8bb5e8a4277a9a766d563d8ae8cdd60
diff --git a/lib/libavb/avb_slot_verify.c b/lib/libavb/avb_slot_verify.c
index 7467daa..3e76deb 100644
--- a/lib/libavb/avb_slot_verify.c
+++ b/lib/libavb/avb_slot_verify.c
@@ -26,6 +26,9 @@
/* Maximum size of a vbmeta image - 64 KiB. */
#define VBMETA_MAX_SIZE (64 * 1024)
+/* Test buffer used to check the existence of a partition. */
+#define TEST_BUFFER_SIZE 1
+
static AvbSlotVerifyResult initialize_persistent_digest(
AvbOps* ops,
const char* part_name,
@@ -538,6 +541,22 @@
return ret;
}
+static AvbIOResult read_one_byte_from_partition(AvbOps* ops,
+ const char* partition_name) {
+ /* test_buf is a one-byte buffer used to check the existence of the
+ * current partition before allocating the big chunk of memory on heap
+ * for vbmeta later.
+ */
+ uint8_t test_buf[TEST_BUFFER_SIZE];
+ size_t test_num_read;
+ return ops->read_from_partition(ops,
+ partition_name,
+ 0 /* offset */,
+ TEST_BUFFER_SIZE,
+ test_buf,
+ &test_num_read);
+}
+
static AvbSlotVerifyResult load_and_verify_vbmeta(
AvbOps* ops,
const char* const* requested_partitions,
@@ -657,30 +676,38 @@
}
}
- vbmeta_buf = avb_malloc(vbmeta_size);
- if (vbmeta_buf == NULL) {
- ret = AVB_SLOT_VERIFY_RESULT_ERROR_OOM;
- goto out;
- }
+ /* Read one byte from the partition to check the existence of the
+ * partition before allocating the big chunk of memory on heap
+ * for vbmeta later. `io_ret` will be used later to decide whether
+ * to fallback on the `boot` partition.
+ */
+ io_ret = read_one_byte_from_partition(ops, full_partition_name);
+ if (io_ret != AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION) {
+ vbmeta_buf = avb_malloc(vbmeta_size);
+ if (vbmeta_buf == NULL) {
+ ret = AVB_SLOT_VERIFY_RESULT_ERROR_OOM;
+ goto out;
+ }
- if (vbmeta_offset != 0) {
- avb_debugv("Loading vbmeta struct in footer from partition '",
- full_partition_name,
- "'.\n",
- NULL);
- } else {
- avb_debugv("Loading vbmeta struct from partition '",
- full_partition_name,
- "'.\n",
- NULL);
- }
+ if (vbmeta_offset != 0) {
+ avb_debugv("Loading vbmeta struct in footer from partition '",
+ full_partition_name,
+ "'.\n",
+ NULL);
+ } else {
+ avb_debugv("Loading vbmeta struct from partition '",
+ full_partition_name,
+ "'.\n",
+ NULL);
+ }
- io_ret = ops->read_from_partition(ops,
- full_partition_name,
- vbmeta_offset,
- vbmeta_size,
- vbmeta_buf,
- &vbmeta_num_read);
+ io_ret = ops->read_from_partition(ops,
+ full_partition_name,
+ vbmeta_offset,
+ vbmeta_size,
+ vbmeta_buf,
+ &vbmeta_num_read);
+ }
if (io_ret == AVB_IO_RESULT_ERROR_OOM) {
ret = AVB_SLOT_VERIFY_RESULT_ERROR_OOM;
goto out;
