On Wed, Oct 05, 2022 at 11:48:39PM +0200, Jason A. Donenfeld wrote: > Hi folks, > > This is a five part treewide cleanup of random integer handling. The > rules for random integers are: > > - If you want a secure or an insecure random u64, use get_random_u64(). > - If you want a secure or an insecure random u32, use get_random_u32(). > * The old function prandom_u32() has been deprecated for a while now > and is just a wrapper around get_random_u32(). > - If you want a secure or an insecure random u16, use get_random_u16(). > - If you want a secure or an insecure random u8, use get_random_u8(). > - If you want secure or insecure random bytes, use get_random_bytes(). > * The old function prandom_bytes() has been deprecated for a while now > and has long been a wrapper around get_random_bytes(). > - If you want a non-uniform random u32, u16, or u8 bounded by a certain > open interval maximum, use prandom_u32_max(). > * I say "non-uniform", because it doesn't do any rejection sampling or > divisions. Hence, it stays within the prandom_* namespace. > > These rules ought to be applied uniformly, so that we can clean up the > deprecated functions, and earn the benefits of using the modern > functions. In particular, in addition to the boring substitutions, this > patchset accomplishes a few nice effects: > > - By using prandom_u32_max() with an upper-bound that the compiler can > prove at compile-time is ≤65536 or ≤256, internally get_random_u16() > or get_random_u8() is used, which wastes fewer batched random bytes, > and hence has higher throughput. > > - By using prandom_u32_max() instead of %, when the upper-bound is not a > constant, division is still avoided, because prandom_u32_max() uses > a faster multiplication-based trick instead. > > - By using get_random_u16() or get_random_u8() in cases where the return > value is intended to indeed be a u16 or a u8, we waste fewer batched > random bytes, and hence have higher throughput. > > So, based on those rules and benefits from following them, this patchset > breaks down into the following five steps: > > 1) Replace `prandom_u32() % max` and variants thereof with > prandom_u32_max(max). > > 2) Replace `(type)get_random_u32()` and variants thereof with > get_random_u16() or get_random_u8(). I took the pains to actually > look and see what every lvalue type was across the entire tree. > > 3) Replace remaining deprecated uses of prandom_u32() with > get_random_u32(). > > 4) Replace remaining deprecated uses of prandom_bytes() with > get_random_bytes(). > > 5) Remove the deprecated and now-unused prandom_u32() and > prandom_bytes() inline wrapper functions. > > I was thinking of taking this through my random.git tree (on which this > series is currently based) and submitting it near the end of the merge > window, or waiting for the very end of the 6.1 cycle when there will be > the fewest new patches brewing. If somebody with some treewide-cleanup > experience might share some wisdom about what the best timing usually > winds up being, I'm all ears. It'd be nice to capture some (all?) of the above somewhere. Perhaps just a massive comment in the header? > I've CC'd get_maintainers.pl, which is a pretty big list. Probably some > portion of those are going to bounce, too, and everytime you reply to > this thread, you'll have to deal with a bunch of bounces coming > immediately after. And a recipient list this big will probably dock my > email domain's spam reputation, at least temporarily. Sigh. I think > that's just how it goes with treewide cleanups though. Again, let me > know if I'm doing it wrong. I usually stick to just mailing lists and subsystem maintainers. If any of the subsystems ask you to break this up (I hope not), I've got this[1], which does a reasonable job of splitting a commit up into separate commits for each matching subsystem. Showing that a treewide change can be reproduced mechanically helps with keeping it together as one bit treewide patch, too, I've found. :) Thank you for the cleanup! The "u8 rnd = get_random_u32()" in the tree has bothered me for a loooong time. -Kees -- Kees Cook
