Allow arbitrary `BorrowedFd` for the ioctls needing an `EventFd`

[DemiMarie]

Ensuring that this is actually an eventfd can be left to the kernel. If that isn’t sufficient, there is also an implementation via /proc/thread-self/fdinfo, though that would be slower.

Needed for cloud-hypervisor/cloud-hypervisor#7739 where the file descriptors will come from another process over SCM_RIGHTS. Emulation is possible but will have poor performance.

MSHV version is rust-vmm/mshv#309.

[roypat]

Doesn't SCM_RIGHTs effectively dup() the file descriptor, meaning you get an OwnedFd instead of a borrowed one? In that case, could we instead just convert that OwnedFd into a EventFd using the constructors available in vmm-sys-util?

[DemiMarie]

That’s definitely an option, but it requires either checking that the file descriptor is an eventfd or potentially violating the invariant that an EventFd always holds an eventfd. One can work around this by validating the eventfd manually, and I wrote code that does this via /proc/thread-self/fdinfo/{}. However, this is clumsy. I believe the kernel will validate eventfd itself, which makes userspace validation redundant.

An &EventFd that doesn’t necessarily contain an eventfd is logically equivalent to BorrowedFd. However, the latter is more flexible. For instance, one can go from &OwnedFd to BorrowedFd without unsafe code, but not the reverse.