On Mo, 02.05.22 18:12, Jason A. Donenfeld (Jason@xxxxxxxxx) wrote: > > > In order to inform userspace of virtual machine forks, this commit adds > > > a "fork_event" sysctl, which does not return any data, but allows > > > userspace processes to poll() on it for notification of VM forks. > > > > > > It avoids exposing the actual vmgenid from the hypervisor to userspace, > > > in case there is any randomness value in keeping it secret. Rather, > > > userspace is expected to simply use getrandom() if it wants a fresh > > > value. > > > > Wouldn't it make sense to expose a monotonic 64bit counter of detected > > VM forks since boot through read()? It might be interesting to know > > for userspace how many forks it missed the fork events for. Moreover it > > might be interesting to userspace to know if any fork happened so far > > *at* *all*, by checking if the counter is non-zero. > > "Might be interesting" is different from "definitely useful". I'm not > going to add this without a clear use case. This feature is pretty > narrowly scoped in its objectives right now, and I intend to keep it > that way if possible. Sure, whatever. I mean, if you think it's preferable to have 3 API abstractions for the same concept each for it's special usecase, then that's certainly one way to do things. I personally would try to figure out a modicum of generalization for things like this. But maybe that' just me… I can just tell you, that in systemd we'd have a usecase for consuming such a generation counter: we try to provide stable MAC addresses for synthetic network interfaces managed by networkd, so we hash them from /etc/machine-id, but otoh people also want them to change when they clone their VMs. We could very nicely solve this if we had a generation counter easily accessible from userspace, that starts at 0 initially. Because then we can hash as we always did when the counter is zero, but otherwise use something else, possibly hashed from the generation counter. But anyway, I understand you are not interested in generalization/other usecases, so I'll shut up. Lennart -- Lennart Poettering, Berlin
