[libcxx] Fix PR21580 - Undefined behavior in readEncodedPointer()

[EricWF]

Summary

This patch fixes a bug in readEncodedPointer() where it would read from memory that was not suitably aligned. This patch fixes it by using memcpy.

[compnerd]

I imagine that you couldn't give a bit more context on what platform/environment you managed to get the unaligned read for future reference. We should get the change in irrespective of that.

src/cxa_personality.cpp

145

Any reason to have this in the __cxxabiv1::(anonymous namespace) rather than (anonymous namespace)? Maybe mark the function static and ignore the namespacing as the trend in the rest of LLVM has been?

148

FromType is poorly chosen I think. AsType might be better. The data is being read from a uint8_t *, but being read as the type specified.

151

Please use the appropriate new (C++) style cast here.

[EricWF]

Address @compnerd's comments.

src/cxa_personality.cpp

 
 #include <assert.h>
 #include <stdlib.h>
+#include <string.h>
 #include <typeinfo>
 
 #include "config.h"

 namespace __cxxabiv1
 {
 
+namespace
+{
+
+template <class AsType>
+uintptr_t readPointerHelper(const uint8_t*& p) {
+    AsType value;
+    memcpy(&value, const_cast<uint8_t*>(p), sizeof(AsType));
+    p += sizeof(AsType);
+    return static_cast<uintptr_t>(value);
+}
+
+} // end namespace
+
 extern "C"
 {
 

         result = static_cast<uintptr_t>(readSLEB128(&p));
         break;
     case DW_EH_PE_udata2:
-        result = *((uint16_t*)p);
-        p += sizeof(uint16_t);
+        result = readPointerHelper<uint16_t>(p);
         break;
     case DW_EH_PE_udata4:
-        result = *((uint32_t*)p);
-        p += sizeof(uint32_t);
+        result = readPointerHelper<uint32_t>(p);
         break;
     case DW_EH_PE_udata8:
-        result = static_cast<uintptr_t>(*((uint64_t*)p));
-        p += sizeof(uint64_t);
+        result = readPointerHelper<uint64_t>(p);
         break;
     case DW_EH_PE_sdata2:
-        result = static_cast<uintptr_t>(*((int16_t*)p));
-        p += sizeof(int16_t);
+        result = readPointerHelper<int16_t>(p);
         break;
     case DW_EH_PE_sdata4:
-        result = static_cast<uintptr_t>(*((int32_t*)p));
-        p += sizeof(int32_t);
+        result = readPointerHelper<int32_t>(p);
         break;
     case DW_EH_PE_sdata8:
-        result = static_cast<uintptr_t>(*((int64_t*)p));
-        p += sizeof(int64_t);
+        result = readPointerHelper<int64_t>(p);
         break;
     default:
         // not supported