The issue described pertains to a problem with the `sendmsg` system call when used within QEMU, specifically when handling messages that contain ancillary data but no regular data (i.e., zero-length `iov`). This discrepancy in behavior between native Linux and QEMU indicates an error in how QEMU emulates or handles the `sendmsg` syscall. The fact that the test case works on native Linux but fails under QEMU suggests that there's a bug in QEMU's implementation of this system call.

**Answer: syscall**