One chunked syscall can consume multiple open-file descriptions after fd reuse

[CvvT]

One bug found by specula

sys_readv validates the raw fd once, then read_from_iovec calls self.sys_read(fd, ...) again for each iovec chunk. If another task sharing FilesState closes and reopens the same raw fd between chunks, one readv can return bytes from two different open-file descriptions.

See the full report for more detail.