[llvm-ar] Reject bitcode which can't be parsed.

[efriedma]

Summary

If llvm-ar is older than the compiler that produced the bitcode, we should produce an error message, instead of silently generating a corrupt archive. This has, unfortunately, bit me multiple times now.

Doing this only for bitcode for now; I'm not confident that the "magic" detection in other cases is good enough to avoid compatibility issues in other cases, and other cases are much less likely to cause trouble anyway.

[rupprecht]

D88288 landed this morning. Does that work for you?

[efriedma]

Sure, that works. Thanks. :)

[MaskRay]

In D88693#2307037, @rupprecht wrote:

D88288 landed this morning. Does that work for you?

In D88693#2307066, @efriedma wrote:

Sure, that works. Thanks. :)

The test coverage from printf "\xDE\xC0\x17\x0B" > %t.o (bitcode file, rather than an unknown file) may still be useful.

[efriedma]

The test in D88288 does %python -c "with open('input.bc', 'a') as f: f.truncate(10)", which is essentially the same idea.

llvm/lib/Object/ArchiveWriter.cpp

getSymbols(MemoryBufferRef Buf, raw_ostream &SymNames, bool &HasObject) {
 
   // In the scenario when LLVMContext is populated SymbolicFile will contain a
   // reference to it, thus SymbolicFile should be destroyed first.
   LLVMContext Context;
   std::unique_ptr<object::SymbolicFile> Obj;
   if (identify_magic(Buf.getBuffer()) == file_magic::bitcode) {
     auto ObjOrErr = object::SymbolicFile::createSymbolicFile(
         Buf, file_magic::bitcode, &Context);
-    if (!ObjOrErr) {
-      // FIXME: check only for "not an object file" errors.
-      consumeError(ObjOrErr.takeError());
-      return Ret;
-    }
+    if (auto E = ObjOrErr.takeError())
+      return std::move(E);
     Obj = std::move(*ObjOrErr);
   } else {
     auto ObjOrErr = object::SymbolicFile::createSymbolicFile(Buf);
     if (!ObjOrErr) {
       // FIXME: check only for "not an object file" errors.
       consumeError(ObjOrErr.takeError());
       return Ret;
     }

llvm/test/tools/llvm-ar/invalid-bitcode.test

+; RUN: printf "\xDE\xC0\x17\x0B" > %t.o
+; RUN: not llvm-ar cr %t.a %t.o 2>&1 | grep "Invalid bitcode wrapper header"