Issues with malformed cyclical TypeSpec

[nguerrera]

In the process of developing signature parsing facilities for System.Reflection.Metadata, I came across some discrepancies in the CLI specification vs. various signature parsing implementations.

My notes on that are here: https://github.com/dotnet/corefx/pull/5435

One such discrepancy is that the specification states that the encoded token following CMOD_OPT/CMOD_REQ is a TypeDef or TypeRef, but it can be TypeSpec in practice.

This opens up a loophole to cycles as TypeSpecs can contain modifiers, and modifiers can contain TypeSpecs. I put together some quick code to create such bad TypeSpecs here in https://github.com/nguerrera/signature-repros and experimented with ilasm, peverify, and running the resulting programs.

Repro steps

1. git clone https://github.com/nguerrera/signature-repros
2. cd signature-repros
3. build.bat
4. ildasm SignatureCycle1.exe
5. peverify SignatureCycle1.exe
6. SignatureCycle1.exe
7. repeat steps 4-6 for SignatureCycle2.exe and SignatureCycle3.exe

   Expected results

For all 3 programs:

• ildasm succeeds, but indicates a bad signature as it does in other cases.
• peverify reports the signature as invalid

If the particular program causes the runtime to have to inspect the bad modifiers contents rather than just skip the bad typespec token:

• An appropriate BadImageFormat/InvalidProgram/TypeLoadException is raised.

  Actual results

• ildasm crashes in all 3 cases
• peverify sometimes succeeds without error, sometimes crashes
• program sometimes crashes with stack overflow rather than a meaningful BadImageFormat/InvalidProgram/TypeLoadException.

[mangod9]

Moving to 6 to triage.