[llvm] [llvm-objcopy][COFF] Update WinCFGuard section contents after stripping (PR #153322)

[Evgenii Kudriashov via llvm-commits]

================
@@ -92,6 +93,63 @@ Error COFFWriter::finalizeSymbolContents() {
   return Error::success();
 }
 
+Error COFFWriter::finalizeCFGuardContents() {
+  DenseMap<size_t, size_t> SymIdMap;
+  bool NeedUpdate = false;
+  for (Symbol &Sym : Obj.getMutableSymbols()) {
+    NeedUpdate |= Sym.OriginalRawIndex == Sym.RawIndex;
+    SymIdMap[Sym.OriginalRawIndex] = Sym.RawIndex;
+  }
+
+  if (!NeedUpdate)
+    return Error::success();
+
+  for (auto &Sym : Obj.getMutableSymbols()) {
+    if (Sym.Name != ".gljmp$y" && Sym.Name != ".giats$y" &&
+        Sym.Name != ".gfids$y")
+      continue;
+
+    auto Sec = find_if(Obj.getMutableSections(),
+                       [&Sym](Section &S) { return S.Name == Sym.Name; });
+
+    if (Sec == Obj.getMutableSections().end() ||
+        Sec->UniqueId != Sym.TargetSectionId)
+      return createStringError(object_error::invalid_symbol_index,
+                               "symbol '%s' is missing its section",
+                               Sym.Name.str().c_str());
+
+    if (Sym.Sym.NumberOfAuxSymbols != 1 ||
+        Sym.Sym.StorageClass != IMAGE_SYM_CLASS_STATIC)
+      return createStringError(object_error::invalid_symbol_index,
+                               "symbol '%s' has unexpected section format",
+                               Sym.Name.str().c_str());
+
+    ArrayRef<uint8_t> RawIds = Sec->getContents();
+    // Nothing to do and also CheckSum will be -1 instead of 0 if we recalculate
+    // it on empty input.
+    if (RawIds.size() == 0)
+      return Error::success();
+
+    // Create updated content
+    ArrayRef<uint32_t> Ids(reinterpret_cast<const uint32_t *>(RawIds.data()),
+                           RawIds.size() / 4);
+    std::vector<uint32_t> NewIds;
+    for (auto Id : Ids)
+      NewIds.push_back(SymIdMap[Id]);
----------------
e-kud wrote:

Do we want to emit an error here or silently leave it as it is? The origin of the the initial problem is that MSVC linker complains on the corrupted object file. So we probably can complain as well.
`SymIdMap` is formed after stripping. So we actually can't say whether it is a garbage value or a removed one.

https://github.com/llvm/llvm-project/pull/153322