Re: [PATCH 5.10/5.15/6.1/6.6/6.12] agp/amd64: Fix broken error propagation in agp_amd64_probe()

[Mingyu Wang]

> What is special about this virtual environment? Nobody else
> seems to have seen that in 20+ years.
>
> Or maybe the Fixes tag is not quite correct and something else more
> recent has caused it.

Hi Andi,


You are right that normal users will not see this crash in the wild.

The environment is a QEMU-based driver fuzzing framework. Rather than 
functionally emulating specific hardware, the framework extracts device 
matching information from the driver and synthesizes a mock PCI device 
just to trigger the driver's binding and initialization paths.

In this case, the synthesized PCI device matched the AGP bridge's IDs, 
forcing `agp_amd64_probe()` to run. However, because this is a synthetic 
fuzzing environment, there was no physical or emulated AMD Northbridge 
present in the system.

In a real-world system, the AGP bridge and the Northbridge exist 
together. Because the fuzzing framework provided the mock PCI device but 
did not provide the Northbridge, `cache_nbs()` returned -ENODEV, 
exposing the flawed `== -1` error handling path.

The `Fixes` tag is correct. The logic flaw was introduced in that 
commit, but it remained dormant because standard hardware configurations 
do not produce this specific missing-hardware scenario.

Best regards,
Mingyu Wang