UDP receive buffers grow beyond the maximum IPv4 payload #1695

UDP.bind is IPv4-only, but UDP receive currently sizes its buffer from the caller's max: the JS backend reserves max bytes, and C reserves min(max, INT32_MAX) plus one spare byte. An IPv4 UDP payload cannot exceed 65,535 - 20 - 8 = 65,507 bytes.

On main e5527d5, I sent a real one-byte loopback datagram and received it with max=1,048,576. A typed-array constructor trace records a 1,048,576-byte JS buffer; a native build linked with --wrap=malloc records a 1,048,577-byte allocation. Both consumers receive x successfully. This measures requested buffer sizes, not RSS; I did not try allocating 4 GiB.

Bounding the payload capacity to min(max, 65507) reduces those requests to 65,507 and 65,508 bytes with the same received data. max=0 must still fail without consuming a datagram; smaller positive limits must keep their existing truncation behavior. This applies to the text/byte and blocking/try receive variants sharing these helpers.

Observed with Bun 1.3.13 and clang 22 on Linux x86-64.