[llvm][hwasan] Add Fuchsia shadow mapping configuration

Ensure that Fuchsia shadow memory starts at zero.

Differential Revision: https://reviews.llvm.org/D99380

Author: PiJoules

llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp

@@ -1531,7 +1531,13 @@ void HWAddressSanitizer::instrumentPersonalityFunctions() {
 void HWAddressSanitizer::ShadowMapping::init(Triple &TargetTriple,
                                              bool InstrumentWithCalls) {
   Scale = kDefaultShadowScale;
-  if (ClMappingOffset.getNumOccurrences() > 0) {
+  if (TargetTriple.isOSFuchsia()) {
+    // Fuchsia is always PIE, which means that the beginning of the address
+    // space is always available.
+    InGlobal = false;
+    InTls = false;
+    Offset = 0;
+  } else if (ClMappingOffset.getNumOccurrences() > 0) {
     InGlobal = false;
     InTls = false;
     Offset = ClMappingOffset;

llvm/test/Instrumentation/HWAddressSanitizer/fuchsia.ll

@@ -0,0 +1,9 @@
+; Check HWASan shadow mapping on Fuchsia.
+; RUN: opt -hwasan -S -mtriple=aarch64-unknown-fuchsia < %s | FileCheck %s
+
+define i32 @test_load(i32* %a) sanitize_hwaddress {
+; CHECK: %.hwasan.shadow = call i8* asm "", "=r,0"(i8* null)
+entry:
+  %x = load i32, i32* %a, align 4
+  ret i32 %x
+}