linux-kernel - Re: [RFC PATCH v1 18/50] net/ipv6/addrconf.c: Use prandom_u32

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Message-ID: <CANP3RGean6M7PuTMXKJrXSdU+2RgzqsoEvnQK5C0RFoXGfFwBA@mail.gmail.com>
Date:   Sat, 28 Mar 2020 09:56:58 -0700
From:   Maciej Żenczykowski <maze@...gle.com>
To:     George Spelvin <lkml@....org>
Cc:     Kernel hackers <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Linux NetDev <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH v1 18/50] net/ipv6/addrconf.c: Use prandom_u32_max for
 rfc3315 backoff time computation

>         /* multiply 'initial retransmission time' by 0.9 .. 1.1 */
> -       u64 tmp = (900000 + prandom_u32() % 200001) * (u64)irt;
> -       do_div(tmp, 1000000);
> -       return (s32)tmp;
> +       s32 range = irt / 5;
> +       return irt - (s32)(range/2) + (s32)prandom_u32_max(range);

The cast on range/2 looks entirely spurious

>         /* multiply 'retransmission timeout' by 1.9 .. 2.1 */
> -       u64 tmp = (1900000 + prandom_u32() % 200001) * (u64)rt;
> -       do_div(tmp, 1000000);
> -       if ((s32)tmp > mrt) {
> +       s32 range = rt / 5;
> +       s32 tmp = 2*rt - (s32)(range/2) + (s32)prandom_u32_max(range);

Here as well.  Honestly the cast on prandom might also not be
necessary, but that at least has a reason.

> +       if (tmp > mrt) {
>                 /* multiply 'maximum retransmission time' by 0.9 .. 1.1 */
> -               tmp = (900000 + prandom_u32() % 200001) * (u64)mrt;
> -               do_div(tmp, 1000000);
> +               range = mrt / 5;
> +               tmp = mrt - (s32)(range/2) + (s32)prandom_u32_max(range);

Ditto.

>         }
>         return (s32)tmp;
>  }

- Maciej