[compiler-rt] [ASan][Windows] Fix rip-relative instruction replacement (PR #68432)

[Charlie Barto via llvm-commits]

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@@ -726,16 +726,22 @@ static bool CopyInstructions(uptr to, uptr from, size_t size) {
     size_t instruction_size = GetInstructionSize(from + cursor, &rel_offset);
     if (!instruction_size)
       return false;
-    _memcpy((void*)(to + cursor), (void*)(from + cursor),
+    _memcpy((void *)(to + cursor), (void *)(from + cursor),
             (size_t)instruction_size);
     if (rel_offset) {
-      uptr delta = to - from;
-      uptr relocated_offset = *(u32*)(to + cursor + rel_offset) - delta;
-#if SANITIZER_WINDOWS64
-      if (relocated_offset + 0x80000000U >= 0xFFFFFFFFU)
+#  if SANITIZER_WINDOWS64
+      // we want to make sure that the new relative offset still fits in 32-bits
+      // this will be untrue if relocated_offset \notin [-2**31, 2**31)
+      s64 delta = to - from;
+      s64 relocated_offset = *(s32 *)(to + cursor + rel_offset) - delta;
+      if (-0x8000'0000ll > relocated_offset || relocated_offset > 0x7FFF'FFFFll)
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barcharcraz wrote:

This is ... a lot less/more than -2**31/2**31

https://github.com/llvm/llvm-project/pull/68432