[ASan][Fuchsia] Have Fuchsia use a dynamic shadow start

The dynamic shadow global is still set to zero, but this will change in
the future.

Author: PiJoules

compiler-rt/lib/asan/CMakeLists.txt

@@ -59,14 +59,11 @@ set(ASAN_CXX_SOURCES
 
 set(ASAN_STATIC_SOURCES
   asan_rtl_static.cpp
+  # Note that only x86_64 targets using a constant shadow offset use functions
+  # in this, but that's gated via macro checks in the file itself.
+  asan_rtl_x86_64.S
   )
 
-if ("x86_64" IN_LIST ASAN_SUPPORTED_ARCH AND NOT WIN32 AND NOT APPLE)
-  list(APPEND ASAN_STATIC_SOURCES
-    asan_rtl_x86_64.S
-  )
-endif()
-
 set(ASAN_PREINIT_SOURCES
   asan_preinit.cpp
   )

compiler-rt/lib/asan/asan_fuchsia.cpp

@@ -40,18 +40,18 @@ void InitializeShadowMemory() {
   if (Verbosity())
     PrintAddressSpaceLayout();
 
-  // Make sure SHADOW_OFFSET doesn't use __asan_shadow_memory_dynamic_address.
-  __asan_shadow_memory_dynamic_address = kDefaultShadowSentinel;
-  DCHECK(kLowShadowBeg != kDefaultShadowSentinel);
-  __asan_shadow_memory_dynamic_address = kLowShadowBeg;
+  // TODO(https://fxbug.dev/42085278): Shadow on Fuchsia starts as zero for now.
+  // __asan_shadow_memory_dynamic_address is an uninitialized global that's
+  // zero. This is temporary and will be changed to a non-zero value in the
+  // future.
 
   CHECK_EQ(kShadowGapEnd, kHighShadowBeg - 1);
   CHECK_EQ(kHighMemEnd, __sanitizer::ShadowBounds.memory_limit - 1);
   CHECK_EQ(kHighMemBeg, __sanitizer::ShadowBounds.shadow_limit);
   CHECK_EQ(kHighShadowBeg, __sanitizer::ShadowBounds.shadow_base);
   CHECK_EQ(kShadowGapEnd, __sanitizer::ShadowBounds.shadow_base - 1);
-  CHECK_EQ(kLowShadowEnd, 0);
-  CHECK_EQ(kLowShadowBeg, 0);
+  CHECK_EQ(kLowShadowEnd, MEM_TO_SHADOW(kLowMemEnd));
+  CHECK_EQ(kLowShadowBeg, __asan_shadow_memory_dynamic_address);
 }
 
 void AsanApplyToGlobals(globals_op_fptr op, const void *needle) {

compiler-rt/lib/asan/asan_mapping.h

@@ -175,7 +175,7 @@
 #define ASAN_SHADOW_SCALE 3
 
 #if SANITIZER_FUCHSIA
-#  define ASAN_SHADOW_OFFSET_CONST (0)
+#  define ASAN_SHADOW_OFFSET_DYNAMIC
 #elif SANITIZER_WORDSIZE == 32
 #  if SANITIZER_ANDROID
 #    define ASAN_SHADOW_OFFSET_DYNAMIC

compiler-rt/lib/asan/asan_rtl_x86_64.S

@@ -1,7 +1,7 @@
 #include "asan_mapping.h"
 #include "sanitizer_common/sanitizer_asm.h"
 
-#if defined(__x86_64__)
+#if defined(__x86_64__) && defined(ASAN_SHADOW_OFFSET_CONST)
 #include "sanitizer_common/sanitizer_platform.h"
 
 .file "asan_rtl_x86_64.S"