[compiler-rt] [hwasan] Add fixed_shadow_base flag (PR #73980)

[Thurston Dang via llvm-commits]

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@@ -106,8 +106,14 @@ static uptr GetHighMemEnd() {
 }
 
 static void InitializeShadowBaseAddress(uptr shadow_size_bytes) {
-  __hwasan_shadow_memory_dynamic_address =
-      FindDynamicShadowStart(shadow_size_bytes);
+  // NULL is generally address zero, so it is not a valid location for the
+  // shadow.
+  if (flags()->fixed_shadow_base != 0) {
+    __hwasan_shadow_memory_dynamic_address = flags()->fixed_shadow_base;
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thurstond wrote:

Added compiler-rt/test/hwasan/TestCases/Linux/fixed-shadow.c

https://github.com/llvm/llvm-project/pull/73980