[compiler-rt] [llvm] [ASan][Fuchsia] Have Fuchsia use a dynamic shadow start (PR #180880)

[via llvm-commits]

https://github.com/PiJoules created https://github.com/llvm/llvm-project/pull/180880

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

>From b008eb388410b11048b077abf87cb7e5941b051b Mon Sep 17 00:00:00 2001
From: Leonard Chan <leonardchan at google.com>
Date: Tue, 10 Feb 2026 20:26:19 -0800
Subject: [PATCH] [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.
---
 compiler-rt/lib/asan/CMakeLists.txt                    |  4 +++-
 compiler-rt/lib/asan/asan_fuchsia.cpp                  | 10 ++++------
 compiler-rt/lib/asan/asan_mapping.h                    |  2 +-
 .../Transforms/Instrumentation/AddressSanitizer.cpp    |  7 ++++---
 4 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/compiler-rt/lib/asan/CMakeLists.txt b/compiler-rt/lib/asan/CMakeLists.txt
index 232bb2574a778..4db12ea93270d 100644
--- a/compiler-rt/lib/asan/CMakeLists.txt
+++ b/compiler-rt/lib/asan/CMakeLists.txt
@@ -61,7 +61,9 @@ set(ASAN_STATIC_SOURCES
   asan_rtl_static.cpp
   )
 
-if ("x86_64" IN_LIST ASAN_SUPPORTED_ARCH AND NOT WIN32 AND NOT APPLE)
+# TODO: These particular 64-bit targets use a dynamic shadow, so we should
+# probably have a cmake variable for this.
+if ("x86_64" IN_LIST ASAN_SUPPORTED_ARCH AND NOT WIN32 AND NOT APPLE AND NOT FUCHSIA)
   list(APPEND ASAN_STATIC_SOURCES
     asan_rtl_x86_64.S
   )
diff --git a/compiler-rt/lib/asan/asan_fuchsia.cpp b/compiler-rt/lib/asan/asan_fuchsia.cpp
index a9e5dad91b973..52fcd789e168f 100644
--- a/compiler-rt/lib/asan/asan_fuchsia.cpp
+++ b/compiler-rt/lib/asan/asan_fuchsia.cpp
@@ -40,18 +40,16 @@ 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;
+  // Shadow on Fuchsia starts as zero for now.
+  __asan_shadow_memory_dynamic_address = 0;
 
   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) {
diff --git a/compiler-rt/lib/asan/asan_mapping.h b/compiler-rt/lib/asan/asan_mapping.h
index 338324686a31d..e1be633a99bd6 100644
--- a/compiler-rt/lib/asan/asan_mapping.h
+++ b/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
diff --git a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
index 2d4c4c9b057c6..4f495b4814d92 100644
--- a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -549,9 +549,10 @@ static ShadowMapping getShadowMapping(const Triple &TargetTriple, int LongSize,
   } else {  // LongSize == 64
     // Fuchsia is always PIE, which means that the beginning of the address
     // space is always available.
-    if (IsFuchsia)
-      Mapping.Offset = 0;
-    else if (IsPPC64)
+    if (IsFuchsia) {
+      // kDynamicShadowSentinel tells instrumentation to use the dynamic shadow.
+      Mapping.Offset = kDynamicShadowSentinel;
+    } else if (IsPPC64)
       Mapping.Offset = kPPC64_ShadowOffset64;
     else if (IsSystemZ)
       Mapping.Offset = kSystemZ_ShadowOffset64;