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MACH_RCV_INVALID_TYPE
The error is in the caller’s scatter receive specification
MACH_RCV_INVALID_TYPE means an invalid message-type specification was supplied in the scatter list. It is not a finding that the sender used an invalid descriptor in its original message. The failure occurs in the optional receive-side overwrite/scatter machinery that describes where out-of-line data may be placed.
This distinction matters when debugging RPC code with generated bindings. A normal receive buffer can be perfectly valid while the supplemental scatter description is not. The nearby status MACH_RCV_SCATTER_SMALL instead indicates a valid overwrite region that lacks capacity; this code points at the descriptor type or specification itself.
What to inspect
- Confirm that the receive path actually uses the intended overwrite or scatter API and that its auxiliary structures follow the matching header version.
- Validate each scatter descriptor’s declared type before comparing byte counts or allocating larger storage.
- Keep out-of-line-data handling separate from the ordinary inline message buffer in logs and error handling.
- Prefer the framework’s documented receive helper when it owns descriptor construction; hand-written scatter lists are ABI-sensitive.
References
- Apple XNU source: mach/message.h
- Mach Kernel Interface Reference: mach_msg
- GNU Mach Reference Manual: interprocess communication
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