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Fix 32-bit zstdmt round buffer size overflow (memory OOB) - #4936
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On 32-bit builds, ZSTDMT_initCStream_internal() computed the round buffer size in size_t. The total can reach or exceed 4 GiB and wrap: - windowLog 30 with btultra2 and LDM disabled makes the overlap 1 GiB, which raises the job size to 1 GiB after the job size cap is applied. The 4 GiB total wraps to 0, the allocation is skipped, and the first streaming copy writes through a NULL buffer. - Many workers with large jobs wrap to a smaller non-zero size, so the buffer is allocated but is too small for the configuration. Compute the slack/section totals in 64 bits and reject the configuration with memory_allocation when the total exceeds SIZE_MAX, before allocating. Add a 32-bit-only fuzzer regression test that expects both cases to fail cleanly with memory_allocation.
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On 32-bit builds,
ZSTDMT_initCStream_internal()computed the round buffer size insize_t. The total can reach or exceed 4 GiB and wrap.Instead, compute the slack/section totals in 64 bits and reject the configuration with
memory_allocationwhen the total exceedsSIZE_MAX, before allocating.Add a 32-bit-only fuzzer regression test that expects both cases to fail cleanly with
memory_allocation, which prior to the fix crashes.Test plan: the new
fuzzertest (oversized round buffer must fail cleanly on 32-bit) is a no-op on 64-bit builds;fuzzerpasses there.