Demand read load bias for a map.

Add a static GetLoadBias method to the Elf object that only reads just
enough to get the load bias.

Add a method to MapInfo that gets the load bias. First attempt to get
it if the elf object already exists. If no elf object was created, use
the new static method to get the load bias.

In BacktraceMap, add a custom iterator so that when code dereferences
a map element, that's when the load bias will be retrieved if it hasn't
already been set.

Bug: 69871050

Test: New unit tests, verify tombstones have non-zero load bias values for
Test: libraries with a non-zero load bias.
Change-Id: I125f4abc827589957fce2f0df24b0f25d037d732

1 files changed, 13 insertions, 2 deletions

diff --git a/libbacktrace/UnwindStackMap.cpp b/libbacktrace/UnwindStackMap.cpp
index 9ac0a0b54..836a774b9 100644
--- a/libbacktrace/UnwindStackMap.cpp
+++ b/libbacktrace/UnwindStackMap.cpp
@@ -71,8 +71,19 @@ void UnwindStackMap::FillIn(uintptr_t addr, backtrace_map_t* map) {
   if (map_info == nullptr) {
     return;
   }
-  unwindstack::Elf* elf = map_info->GetElf(process_memory_, true);
-  map->load_bias = elf->GetLoadBias();
+  map->load_bias = map_info->GetLoadBias(process_memory_);
+}
+
+uint64_t UnwindStackMap::GetLoadBias(size_t index) {
+  if (index >= stack_maps_->Total()) {
+    return 0;
+  }
+
+  unwindstack::MapInfo* map_info = stack_maps_->Get(index);
+  if (map_info == nullptr) {
+    return 0;
+  }
+  return map_info->GetLoadBias(process_memory_);
 }
 
 std::string UnwindStackMap::GetFunctionName(uintptr_t pc, uintptr_t* offset) {