Hi Greg, On 23/8/23 11:08, Greg KH wrote:
CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe. 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 The notification happens through a 32bit epoch value that changes every time cached entropy is no longer valid, hence PRNGs need to reseed.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 bitsWhy 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?
We made it 32 bits so that we can read/write it atomically in all 32bit architectures.
Do you think that's not a problem? Cheers, Babis
