CNA-BUG-076: Color::FromNonPremultiplied(int r, int g, int b, int a) multiplies in 32-bit int, so large arguments overflow (undefined behaviour)

CNA snapshot 009d40f5  ·  Known Issues › Current bugs  ·  source links pinned to 009d40f5

CNA computes r * a / 255 in int; XNA 4.0 widens to 64-bit long before multiplying and clamps the result, so arguments whose product exceeds INT_MAX are defined in XNA and undefined in CNA.

Identifier

CNA-BUG-076

Category

Bug

Subsystem

Math & geometry

Status

Open

Verified against

CNA 009d40f5 (009d40f5dd085c4e674d3479675fac84b12b3e0a)

Severity

Low (a triage suggestion, not a project priority)

Evidence basis

Source-verified: read at TARGET, not executed

Tests touching this area

Yes: see Current tests

Affected contract

Microsoft::Xna::Framework::Color::FromNonPremultiplied(intcs r, intcs g, intcs b, intcs a)

Expected behaviour

XNA 4.0 (IL): each channel is ClampToByte64((long)r * a / 255) - conv.i8 on both operands, a 64-bit multiply and divide, then a clamp.

Actual behaviour at TARGET

Color.cpp computes static_cast<intcs>(r * a / ByteMax) with intcs operands. For in-range byte values this matches; for out-of-range inputs whose product exceeds INT_MAX (for example r = a = 50000) the multiplication overflows.

Source locations

• modules/math/src/Color.cpp — Color::FromNonPremultiplied(intcs, intcs, intcs, intcs)

Evidence

Checked by reading the named sources at 009d40f5; nothing was built or executed for this entry. XNA arithmetic from the genuine XNA 4.0 IL.

Focused reproduction

No focused reproduction is known. Nothing has been invented here; the evidence above is what exists.

Current tests

ColorTests.cpp uses byte-range inputs.

Regression test

A test with FromNonPremultiplied(50000, 0, 0, 50000) expecting XNA's clamped (255, 0, 0, 255).

Blast radius

Only callers that pass values far outside 0-255; ordinary colour code is unaffected.

Workaround

Clamp arguments to 0-255 before the call.