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

[Evgenii Kudriashov via llvm-commits]

================
@@ -92,6 +94,79 @@ Error COFFWriter::finalizeSymbolContents() {
   return Error::success();
 }
 
+Error COFFWriter::finalizeSymIdxContents() {
+  // CFGuards shouldn't be present in PE
+  if (Obj.IsPE)
+    return Error::success();
+
+  // Currently handle only sections consisting only of .symidx.
+  // TODO: other sections such as .impcall and .hybmp$x require more complex
+  // handling as they have more complex layout.
+  auto IsSymIdxSection = [](StringRef Name) {
+    return Name == ".gljmp$y" || Name == ".giats$y" || Name == ".gfids$y" ||
+           Name == ".gehcont$y";
+  };
+
+  DenseMap<size_t, size_t> SymIdMap;
+  SmallDenseMap<ssize_t, coff_aux_section_definition *, 4> SecIdMap;
+  bool NeedUpdate = false;
+  for (auto &Sym : Obj.getMutableSymbols()) {
+    NeedUpdate |= Sym.OriginalRawIndex == Sym.RawIndex;
+    SymIdMap[Sym.OriginalRawIndex] = Sym.RawIndex;
+
+    // We collect only definition symbols of the sections to update checksum
+    if (Sym.Sym.NumberOfAuxSymbols == 1 &&
+        Sym.Sym.StorageClass == IMAGE_SYM_CLASS_STATIC && Sym.Sym.Value == 0 &&
+        IsSymIdxSection(Sym.Name))
+      SecIdMap[Sym.TargetSectionId] =
+          reinterpret_cast<coff_aux_section_definition *>(
+              Sym.AuxData[0].Opaque);
+  }
+
+  if (!NeedUpdate)
+    return Error::success();
+
+  for (auto &Sec : Obj.getMutableSections()) {
+    if (!IsSymIdxSection(Sec.Name))
+      continue;
+
+    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)
+      continue;
+
+    if (!SecIdMap.contains(Sec.UniqueId))
+      return createStringError(object_error::invalid_symbol_index,
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e-kud wrote:

Covered with 3 test cases for most of the options why we may miss an entry.

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