`fillNeighborsMPI()` truncates byte offsets from `size_t` to `int`

[WeiqunZhang]

• Type: Correctness / Large-message robustness
• Severity: Medium
• Component: Neighbor particle CPU MPI unpack path
• Location:
  • Src/Particle/AMReX_NeighborParticlesCPUImpl.H:524
  • Src/Particle/AMReX_NeighborParticlesCPUImpl.H:573

Problem

Receive offsets are accumulated in Vector<std::size_t> rOffset, but unpack converts each offset to int:

const auto offset = int(rOffset[i]);
char* buffer = &recvdata[offset];

For large aggregate receive buffers, this can overflow/truncate and produce invalid buffer pointers.

Impact

• Potential out-of-bounds reads during unpack on large-scale runs with large ghost exchange payloads.
• Corrupted neighbor data or hard crashes.

Suggested patch

Keep offsets in std::size_t through pointer arithmetic.

--- a/Src/Particle/AMReX_NeighborParticlesCPUImpl.H
+++ b/Src/Particle/AMReX_NeighborParticlesCPUImpl.H
@@
-            const auto offset = int(rOffset[i]);
+            const std::size_t offset = rOffset[i];
             char* buffer = &recvdata[offset];

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