[llvm][hwasan] Add Fuchsia shadow mapping configuration

[leonardchan]

Summary

Ensure that Fuchsia shadow memory starts at zero.

[phosek]

LGTM

llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp

1537

It may be worth leaving the same comment as we have for ASan: https://github.com/llvm/llvm-project/blob/4b5baa5b8244778b0e7253cdb98924c3dab611b7/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp#L488

[mcgrathr]

lgtm with comment added

llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp

1537

agreed

llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp

for (auto &P : PersonalityFns) {
     for (Function *F : P.second)
       F->setPersonalityFn(ThunkFn);
   }
 }
 
 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;
   } else if (ClEnableKhwasan || InstrumentWithCalls) {
     InGlobal = false;
     InTls = false;
     Offset = 0;
   } else if (ClWithIfunc) {

llvm/test/Instrumentation/HWAddressSanitizer/fuchsia.ll

+; 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
+}