Validate 'num_scan_inputs' attribute in Scan kernel construction

[Ti-Tai Wang (titaiwangms)]

Description

The Scan operator's num_scan_inputs attribute determines how many of the node's variadic inputs are scan inputs (the remainder are loop state variables). In scan::detail::Info::Info (shared by the CPU Scan-8/Scan-9 kernels, and reused as-is by the CUDA Scan kernel), this attribute was used in two unguarded subtractions:

num_loop_state_variables = num_variadic_inputs - num_scan_inputs;
...
num_scan_outputs = num_outputs - num_loop_state_variables;

If num_scan_inputs is outside [0, num_variadic_inputs], num_loop_state_variables becomes negative. That value is later used, unguarded, as a loop-start index into the node's inputs during Compute(), resulting in out-of-range indexing.

This PR adds validation of num_scan_inputs (and the derived num_loop_state_variables) in the constructor, rejecting invalid values up front with a clear error message before any arithmetic derived from them is used for indexing.

Testing

Added regression tests for both opset 8 and opset 9+ with an out-of-range num_scan_inputs value:

• Opset 8 hits the new kernel-construction-time check directly.
• Opset 9+ is caught earlier during graph resolution's standard ONNX-level shape inference (before the kernel is even constructed), so its test asserts on the shape-inference error message instead.

Both tests are guarded by #if !defined(ORT_NO_EXCEPTIONS) since they rely on exception-based failure reporting.

Ran locally (CPU-only Debug build):

• onnxruntime_provider_test --gtest_filter='Scan*' — all 34 tests pass, including the 2 new ones.

[Copilot]

Pull request overview

This PR hardens the CPU (and shared CUDA) Scan kernel setup by validating the num_scan_inputs attribute early, preventing negative/invalid derived counts that can later lead to out-of-range indexing. It also adds regression coverage for invalid num_scan_inputs for opset 8 and opset 9+.

Changes:

• Add ORT_ENFORCE validation in scan::detail::Info::Info to reject num_scan_inputs outside [0, num_variadic_inputs] and ensure the implied loop-state-variable count is compatible with the node’s outputs.
• Add new CPU provider tests that exercise invalid num_scan_inputs for opset 8 (kernel-level validation) and opset 9+ (graph-resolution/shape-inference failure path).
• Guard the new negative tests with #if !defined(ORT_NO_EXCEPTIONS).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
onnxruntime/core/providers/cpu/controlflow/scan_utils.cc	Adds constructor-time validation for num_scan_inputs and derived counts to prevent invalid indexing.
onnxruntime/test/providers/cpu/controlflow/scan_test.cc	Adds regression tests for out-of-range num_scan_inputs across opset 8 and opset 9+.

[Ti-Tai Wang (titaiwangms)]

cc Akshay Sonawane (@apsonawane) the ci failures are unrelated.

[Ti-Tai Wang (titaiwangms)]

cc Akshay Sonawane (@apsonawane)