fix(udp): cap the payload at the server's advertised buffer - #142
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`SMPUDPTransport.max_unencoded_size` overrode the base implementation with the MSS alone, so the MCUmgr parameters that `SMPClient` reads on connect never reached the UDP transport: `initialize(buf_size)` stored the value and nothing read it. Zephyr's UDP SMP transport receives each request as a single datagram into one MCUmgr buffer of `CONFIG_MCUMGR_TRANSPORT_NETBUF_SIZE` bytes, the value advertised as `buf_size`. A request larger than that cannot be received. The defaults (2048-byte buffer, 1500-byte MTU) hide this. A build that lowers the buffer, e.g. to its 384-byte non-UDP default, does not. The payload is now `min(MSS, buf_size)`. Before the params are known, `buf_size or mtu` makes that the MSS, so nothing changes for a server that does not advertise them. Verified: `camas check` green; the new parametrized test fails on the 384-byte case without the fix. https://github.com/zephyrproject-rtos/zephyr/blob/70be2ff0b565a3313128f5577f51cfeb3ebcf602/subsys/mgmt/mcumgr/grp/os_mgmt/src/os_mgmt.c#L551-L554 https://github.com/zephyrproject-rtos/zephyr/blob/70be2ff0b565a3313128f5577f51cfeb3ebcf602/subsys/mgmt/mcumgr/transport/Kconfig#L33-L58 Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
JPHutchins
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Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
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LLM Disclosure
This PR was authored by
claude-opus-5-5[1m]on behalf of @JPHutchins. A survey of Zephyr's SMP server transports for the #58 design found that smpclient's UDP transport ignores the server's advertised buffer, and @JPHutchins asked for a small fix tomain.SMPUDPTransportsized every message by the MSS alone, so the MCUmgr params thatSMPClientreads on connect never reached it. Zephyr receives each UDP request into a single buffer of the advertisedbuf_size, so a request larger than that can't be received.Non-breaking. Before params are known, or when a server doesn't advertise them, the result is still the MSS. Zephyr's defaults (2048-byte buffer, 1500-byte MTU) behave exactly as before; only builds with a smaller buffer change.
Sources and verification
buf_sizeisCONFIG_MCUMGR_TRANSPORT_NETBUF_SIZE(os_mgmt.c). It defaults to 2048 with UDP and 384 without (Kconfig).camas checkgreen (ruff, pydoclint, mypy, pyright, tests).test_max_unencoded_size_capped_by_server_buffercoversbuf_sizebelow, equal to, and above the MSS. Without the fix, it fails on the 384-byte case.screaming-goblin) replaces this with UDP's ownAuto()sizing arm, which uses the same arithmetic.🤖 Generated with Claude Code