On Wed, Aug 23, 2023 at 11:01:05AM +0200, Babis Chalios wrote: > Sometimes, PRNGs need to reseed. For example, on a regular timer > interval, to ensure nothing consumes a random value for longer than e.g. > 5 minutes, or when VMs get cloned, to ensure seeds don't leak in to > clones. > > The notification happens through a 32bit epoch value that changes every > time cached entropy is no longer valid, hence PRNGs need to reseed. User > space applications can get hold of a pointer to this value through > /dev/(u)random. We introduce a new ioctl() that returns an anonymous > file descriptor. From this file descriptor we can mmap() a single page > which includes the epoch at offset 0. > > random.c maintains the epoch value in a global shared page. It exposes > a registration API for kernel subsystems that are able to notify when > reseeding is needed. Notifiers register with random.c and receive a > unique 8bit ID and a pointer to the epoch. When they need to report a > reseeding event they write a new epoch value which includes the > notifier ID in the first 8 bits and an increasing counter value in the > remaining 24 bits: > > RNG epoch > *-------------*---------------------* > | notifier id | epoch counter value | > *-------------*---------------------* > 8 bits 24 bits Why not just use 32/32 for a full 64bit value, or better yet, 2 different variables? Why is 32bits and packing things together here somehow simpler? thanks, greg k-h
