[WebGPU] Index resolution is potentially underspecified

[mwyrzykowski]

Under step 2.2.3 of the index resolution process it is stated:

Append drawCall.baseVertex + relativeVertexIndex to the vertexIndexList.

in the case drawCall.baseVertex is negative and relativeVertexIndex < -drawCall.baseVertex, i.e., drawCall.baseVertex + relativeVertexIndex < 0, is it expected for the vertex index to wrap around or is the call rejected / invalid memory reference applies?

I was unable to find corresponding reference in Metal, but the OpenGL specification for glDrawElementsBaseVertex says:

The operation is undefined if the sum would be negative.

Is it allowed for an implementation to reject the call in this case per the note under https://www.w3.org/TR/webgpu/#dom-gpurendercommandsmixin-drawindexed? In the case of 16-indices and negative baseIndex values, it is not clear that all APIs have the same semantics and polyfilling it would seem expensive.

[kainino0x]

It doesn't happen during index resolution; that just determines the index values that will be used, but doesn't actually use them. The checking happens during https://www.w3.org/TR/webgpu/#vertex-processing

However it looks like we are missing any check for negative values:

If attributeOffset + byteSize(attributeDesc.format) > vertexBufferOffset + vertexBufferBindingSize:

So I think that's the line that needs to be fixed.

If attributeOffset < 0 or ...

or something like that.

[kainino0x]

Actually:

If [attribute offset relative to start of vertex buffer binding] < 0 or ...

Because we don't want to allow accessing past the beginning of the binding.

[Kangz]

GPU Web WG 2025-02-19 Atlantic-time

• KN: Think just a typo basically; forgot to specify this bounds condition.
• JB: Agree, seems editorial.
• KN: Labeled as copyediting.