Avoid UAF in WinCOFFObjectWriter with weak symbols.

[maleadt]

Summary

When using weak symbols, the WinCOFFObjectWriter keeps a list (WeakDefaults)
that's used to make names unique. This list should be reset when the object
writer is reset, because otherwise reuse of the object writer can result in
freed symbols being accessed. With some added output, this becomes clear when
using llc in --run-twice mode:

$ ./llc --compile-twice -mtriple=x86_64-pc-win32 trivial.ll -filetype=obj

DefineSymbol::WeakDefaults
 - .weak.foo.default
 - .weak.bar.default

DefineSymbol::WeakDefaults
 - .weak.foo.default
 - áÑJij⌂  p§┼Ø┐☺
 - .debug_macinfo.dw
 - .weak.bar.default

This does not seem to leak into the output object file though, so I couldn't
come up with a test. I added one that just does --run-twice (and verified
that it does access freed memory), which should result in detecting the
invalid memory accesses when running under ASAN.

Observed in a Julia PR where we started using weak symbols:
https://github.com/JuliaLang/julia/pull/45649

[mstorsjo]

LGTM, thanks! Do you have commit access, or do you want me to push it for you? In that case, can you provide your preferred git author line (Real Name <email@address>)?

[maleadt]

Looks like I botched the test here (missing a RUN directive), so let me update that first.

I don't have commit access; you can use Tim Besard <[e-mail]>.

[maleadt]

Fix test.

[maleadt]

Fix tests again.

llvm/lib/MC/WinCOFFObjectWriter.cpp

public:
   void reset() override {
     memset(&Header, 0, sizeof(Header));
     Header.Machine = TargetObjectWriter->getMachine();
     Sections.clear();
     Symbols.clear();
     Strings.clear();
     SectionMap.clear();
     SymbolMap.clear();
+    WeakDefaults.clear();
     MCObjectWriter::reset();
   }
 
   COFFSymbol *createSymbol(StringRef Name);
   COFFSymbol *GetOrCreateCOFFSymbol(const MCSymbol *Symbol);
   COFFSection *createSection(StringRef Name);
 
   void defineSection(MCSectionCOFF const &Sec, const MCAsmLayout &Layout);

llvm/test/MC/COFF/weak-uaf.ll

+; RUN: llc --compile-twice -mtriple=x86_64-pc-win32 -filetype=obj < %s
+
+; UAF when re-using the MCObjectWriter. does not leak into the output,
+; but should be detectable with --compile-twice under ASAN or so.
+
+define weak void @foo() nounwind {
+  ret void
+}
+
+define weak void @bar() nounwind {
+  ret void
+}