ILLink produces invalid IL metadata with -b switch and Save assembly mode

[filipnavara]

As part of testing dotnet/macios#18268 we hit an odd case where invalid IL is produced as the output of ILLink, resulting in later crash when post-processing the data with ILStrip. There's actually several issues at play here:

• Roslyn produces an unused TypeRef for global using aliases (global using produces unnecessary TypeRef in output file roslyn#68250). Xamarin uses this to create an alias for nfloat.
• ILLink with -b switch strips the unused AssemblyRef but keeps the unused TypeRef in the Save assembly mode. Xamarin uses ILLink, among other things, to produce the Managed Static Registrar. In this configuration the original Copy mode of the input assembly is changed to Save in the process.
• ILStrip crashes on the invalid input (Xamarin uses pretty old version, bump is handled in [msbuild] Bump ILStrip and add a maestro dependency to keep it updated. macios#18291).

Repro:

• Download extra-typeref-linker.zip
• Update the illinktask.dll path in linker.rsp to an absolute path to the included task .dll; the custom task only changes the assembly action to Save since it cannot be done on command line
• Run dotnet <path to illink.dll> @linker.rsp
• Check out\extra-typeref.dll with the mdv tool (https://github.com/dotnet/metadata-tools) and observe that there's NFloat type reference that points to non-existent assembly reference

Observations:

• Tested with .NET 7.0.302 and .NET 8.0 Preview 5 linkers. The issue exists in both. On .NET 8 the original test case produces different linker input and doesn't hit the problem, but the underlying bug still exists.
• Removing the -b switch from linker.rsp makes the problem go away

cc @rolfbjarne @ivanpovazan

Tagging subscribers to this area: @agocke, @sbomer, @vitek-karas
See info in area-owners.md if you want to be subscribed.

Issue Details

---

As part of testing dotnet/macios#18268 we hit an odd case where invalid IL is produced as the output of ILLink, resulting in later crash when post-processing the data with ILStrip. There's actually several issues at play here:

• Roslyn produces an unused TypeRef for global using aliases (global using produces unnecessary TypeRef in output file roslyn#68250). Xamarin uses this to create an alias for nfloat.
• ILLink with -b switch strips the unused AssemblyRef but keeps the unused TypeRef in the Save assembly mode. Xamarin uses ILLink, among other things, to produce the Managed Static Registrar. In this configuration the original Copy mode of the input assembly is changed to Save in the process.
• ILStrip crashes on the invalid input (Xamarin uses pretty old version, bump is handled in [msbuild] Bump ILStrip and add a maestro dependency to keep it updated. macios#18291).

Repro:

• Download extra-typeref-linker.zip
• Update the illinktask.dll path in linker.rsp to an absolute path to the included task .dll; the custom task only changes the assembly action to Save since it cannot be done on command line
• Run dotnet <path to illink.dll> @linker.rsp
• Check out\extra-typeref.dll with the mdv tool (https://github.com/dotnet/metadata-tools) and observe that there's NFloat type reference that points to non-existent assembly reference

Observations:

• Tested with .NET 7.0.302 and .NET 8.0 Preview 5 linkers. The issue exists in both. On .NET 8 the original test case produces different linker input and doesn't hit the problem, but the underlying bug still exists.
• Removing the -b switch from linker.rsp makes the problem go away

cc @rolfbjarne @ivanpovazan

Author:	filipnavara
Assignees:	-
Labels:	area-Tools-ILLink
Milestone:	-

[filipnavara]

My suspicion is that the .pdb actually keeps the reference to the type. Roslyn generates it only for the debug info, and ILLink sweeps the assembly references without taking the extra .pdb reference into account. It then gets brought back by Cecil when the rewritten PDB gets saved.

Let's see how far is my guess from the truth. I have no knowledge of the ILLink code, so take it with a huge grain of salt.

[filipnavara]

I realized that I forgot to include the .pdb in the repro package. That's essential for the error to occur. Here's a fixed one:
extra-typeref-linker.zip

[filipnavara]

This fixes the output for me: filipnavara@7cca2fa

Unit tests will be easier to write once #86474 lands.

[filipnavara]

This is similar to this old issue btw: dotnet/linker#399

[filipnavara]

@vitek-karas This should be rather trivial to fix. I suppose that I can take a stab at it if you are busy...

[vitek-karas]

I'm definitely busy with other commitments... maybe @sbomer can take a look.
Or if you want to tackle it - we would be happy to help of course.

[sbomer]

Would love it if you were able to contribute a fix. :) Otherwise I can take a look next week.