External builder fails silently when compiling generic implants.

[catmandx]

Describe the bug
I am customizing the sliver implant for an engagement, per this url, I should modify the source code under implant and register the resulting sliver-server binary as an external builder. The targets of this engagement is running ARMv7 Linux (which is not a supported compiler target, I know).

To Reproduce
Steps to reproduce the behavior:

1. Register the default sliver-server binary as an external builder

$ ./sliver-server builder -c custom-builder-multiplayer-profile.cfg --log-level 5
....
INFO[2024-11-19T21:53:22+07:00] [sliver/server/builder/builder.go:56] Attempting to register builder: sliver-dev 

2. Generate an implant using an unsupported combo of OS / Arch:

sliver > generate beacon --arch arm --os linux --http http://test.c2.com -s /tmp/custom-implant --external-builder

⚠️  Unsupported compiler target linux/arm, but we can try to compile a generic implant.
⚠️  Generic implants do not support all commands/features.
? Attempt to build generic implant? Yes

sliver>....

On the external builder side, I don't see it outputting anything

Expected behavior
The external builder should be able to build generic implants. If not, it should tell the operator.

Screenshots

Desktop (please complete the following information):

• OS: Linux AMD64
• Version 1.5.42

Additional context
Add any other context about the problem here.

[catmandx]

This can be worked around by manually inserting the desired combo to this function: /root/sliver/server/generate/binaries.go GetCompilerTargets()

	targets = append(targets, &clientpb.CompilerTarget{
		GOOS:   "linux",
		GOARCH: "arm",
		Format: clientpb.OutputFormat_EXECUTABLE,
	})

[rkervella]

I think this has been fixed in master but I'll need to double check.

[catmandx]

I'll recompile using the master branch and try again