linux-kernel - Re: [Cake] [PATCH] sch_cake: avoid possible divide by zero in cake

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Date:   Thu, 2 Jan 2020 23:58:02 +0200
From:   Jonathan Morton <chromatix99@...il.com>
To:     Wen Yang <wenyang@...ux.alibaba.com>
Cc:     Toke Høiland-Jørgensen <toke@...e.dk>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        cake@...ts.bufferbloat.net,
        Kevin Darbyshire-Bryant <ldir@...byshire-bryant.me.uk>,
        Cong Wang <xiyou.wangcong@...il.com>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [Cake] [PATCH] sch_cake: avoid possible divide by zero in
 cake_enqueue()

> On 2 Jan, 2020, at 11:21 am, Wen Yang <wenyang@...ux.alibaba.com> wrote:
> 
> The variables 'window_interval' is u64 and do_div()
> truncates it to 32 bits, which means it can test
> non-zero and be truncated to zero for division.
> The unit of window_interval is nanoseconds,
> so its lower 32-bit is relatively easy to exceed.
> Fix this issue by using div64_u64() instead.

That might actually explain a few things.  I approve.

Honestly the *correct* fix is for the compiler to implement division in a way that doesn't require substituting it with function calls.  As this shows, it's error-prone to do this manually.

 - Jonathan Morton