linux-kernel - Re: [PATCH v1 0/5] treewide cleanup of random integer usage

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Date:   Thu, 06 Oct 2022 08:17:18 +0000
From:   "Yann Droneaud" <ydroneaud@...eya.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        linux-crypto@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-janitors@...r.kernel.org,
        "Julia Lawall" <Julia.Lawall@...ia.fr>,
        "Nicolas Palix" <nicolas.palix@...g.fr>, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 0/5] treewide cleanup of random integer usage

Hi,

6 octobre 2022 à 04:51 "Jason A. Donenfeld" a écrit:

> 
> 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.
> 

Did you use some coccinelle patches ? Or other semantic patch tool ?

Maybe we could introduce some coccinelle patch to ensure future get_random_u{16,32,64} usages be checked and patched to use the best fit.

Regards.

-- 
Yann Droneaud
OPTEYA