fix(asan): Poison memory between stack and globals

This fixes compiler-rt/test/asan/TestCases/global-underflow.cpp , which
would sometimes fail to detect the underfloa if the tested global was the very
first one

compiler-rt/lib/asan/asan_shadow_setup.cpp

@@ -22,6 +22,8 @@
 #  if SANITIZER_CHEERPWASM
 #    include "asan_poisoning.h"
 extern char* volatile _stackTop;
+extern char* volatile _stackBottom;
+extern char* volatile _globalsStart;
 #  endif
 
 namespace __asan {
@@ -107,6 +109,8 @@ void InitializeShadowMemory() {
     // CHEERP: Poison everything from 0x0 up to stack top to detect null
     // derefences
     FastPoisonShadow(0, reinterpret_cast<uptr>(_stackTop), 0xfe);
+    // CHEERP: Poison the space between the stack and the globals
+    PoisonShadow(reinterpret_cast<uptr>(_stackBottom), reinterpret_cast<uptr>(_globalsStart)-reinterpret_cast<uptr>(_stackBottom), 0xfe);
 #  endif
     CHECK_EQ(kShadowGapEnd, kHighShadowBeg - 1);
   } else if (kMidMemBeg &&

compiler-rt/lib/sanitizer_common/sanitizer_cheerpwasm.cpp

@@ -19,6 +19,7 @@
 
 __attribute__((cheerp_asmjs)) char *volatile _stackBottom = (char *)0xdeadbeef;
 __attribute__((cheerp_asmjs)) char *volatile _stackTop = (char *)0xdeadbeef;
+__attribute__((cheerp_asmjs)) char *volatile _globalsStart = (char *)0xdeadbeef;
 
 static __asan::atomic_sint32_t _collect_traces;
 

llvm/include/llvm/Cheerp/LinearMemoryHelper.h

@@ -434,6 +434,8 @@ class LinearMemoryHelper
 	uint32_t stackSize;
 	// Stack start (it grows downwards)
 	uint32_t stackStart;
+	// Globals start
+	uint32_t globalsStart;
 	// Offset from 0x0 to the stack top. Primarily used with Asan to reserve
 	// the lower addresses for null pointer checks
 	uint32_t stackOffset;

llvm/lib/CheerpUtils/LinearMemoryHelper.cpp

@@ -426,6 +426,7 @@ void LinearMemoryHelper::addGlobals()
 	// Also, for thread locals, calculate offsets to the image start, and the total size of the image.
 	threadLocalImageSize = 0;
 	threadLocalStart = 0;
+	globalsStart = 0;
 	for (const auto G: asmjsGlobals) {
 		//Globalized globals do not need an address
 		if (globalizedGlobalsUsage.count(G))
@@ -437,6 +438,8 @@ void LinearMemoryHelper::addGlobals()
 		uint32_t alignment = std::max<uint32_t>(TypeSupport::getAlignmentAsmJS(targetData, ty), G->getAlignment());
 		// The following is correct if alignment is a power of 2 (which it should be)
 		heapStart = (heapStart + alignment - 1) & ~(alignment - 1);
+		if (globalsStart == 0)
+			globalsStart = heapStart;
 		globalAddresses.emplace(G, heapStart);
 		inverseGlobalAddresses.emplace(heapStart, G);
 		if (G->isThreadLocal())
@@ -688,6 +691,7 @@ void LinearMemoryHelper::addMemoryInfo()
 {
 	setGlobalPtrIfPresent("_stackBottom", stackStart);
 	setGlobalPtrIfPresent("_stackTop", stackStart + 8 - stackSize);
+	setGlobalPtrIfPresent("_globalsStart", globalsStart);
 
 	//Align to 8 bytes
 	heapStart = (heapStart + 7) & ~7;