Re: [PATCH v2 3/3] treewide: use get_random_u32_between() when possible
From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: 2022-11-17 02:05:34
Also in:
linux-arm-kernel, linux-block, linux-crypto, linux-fsdevel, linux-media, linux-mips, linux-mmc, linux-patches, lkml, loongarch, netdev
On Thu, Nov 17, 2022 at 12:55:47AM +0100, Jason A. Donenfeld wrote:
1) How/whether to make f(0, UR2_MAX) safe, - without additional 64-bit arithmetic, - minimizing the number of branches. I have a few ideas I'll code golf for a bit. I think I can make progress with (1) alone by fiddling around with godbolt enough, like usual.
The code gen is definitely worse.
Original half-open interval:
return floor + get_random_u32_below(ceil - floor);
Suggested fully closed interval:
ceil = ceil - floor + 1;
return likely(ceil) ? floor + get_random_u32_below(ceil) : get_random_u32();
Is the worse code gen actually worth it? Options:
a) Decide worse codegen is worth it.
b) Declare f(0, U32_MAX) undefined and just not handle it.
c) Stick with original half-open interval that doesn't have this problem.
Jason