[libcu++] Fix Overflow in Dynamic Shared Memory Validation

[davebayer]

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Reviewing files that changed from the base of the PR and between a6109c1 and 3c29a94.

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• libcudacxx/include/cuda/__launch/configuration.h

🚧 Files skipped from review as they are similar to previous changes (1)

• libcudacxx/include/cuda/__launch/configuration.h

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved validation for dynamic shared memory requests that use unbounded arrays.
  • Added safer size computation to detect multiplication overflow and oversize byte totals, preventing incorrect calculations.
  • Now returns a clearer invalid_argument error when the requested dynamic shared memory exceeds the portable limit or cannot be represented safely.
  • Kept public interfaces unchanged while making the underlying check more reliable.

Walkthrough

dynamic_shared_memory(_Tp)(__n) now uses cuda::mul_overflow to compute requested bytes and rejects overflowed or over-limit sizes with std::invalid_argument. The header also includes cuda/__numeric/mul_overflow.h.

Changes

Dynamic Shared Memory Overflow Check

Layer / File(s)	Summary
Overflow-safe size computation libcudacxx/include/cuda/__launch/configuration.h	Adds <cuda/__numeric/mul_overflow.h> and changes the unbounded dynamic_shared_memory(__n) check to use mul_overflow(__n, sizeof(value_type)) before comparing against the portable dynamic shared memory limit.

Related PRs: None identified.

Suggested labels: bug, libcudacxx

Suggested reviewers: None identified.

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[Jacobfaib]

I would swap these conditions and check __overflow first.

Given the nature of overflow I am afraid that an overzealous compiler could interpret __dyn_smem_size > __max_portable_dyn_smem_size (which would be false on overflow) to mean that __overflow could not possibly be true, and therefore delete the check.

I.e. I am envisioning a scenario similar to deleted nullptr checks:

int *ptr = ...;
int x = *ptr;

if (!ptr) {
  // This entire branch is deleted because you deref ptr above
  // and because dereferencing a nullptr is UB, the compiler
  // is allowed to assume it cannot happen.
}

[davebayer]

mul_overflow always has well defined output, so that shouldn't ever happen. But yeah, why not

[github-actions]

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