linux-kernel - Re: [PATCH] kfifo: overflow of unsigned integer

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Date:	Thu, 8 Feb 2007 01:38:45 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Cong WANG" <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org, Stelian Pop <stelian@...ies.net>
Subject: Re: [PATCH] kfifo: overflow of unsigned integer

On Thu, 8 Feb 2007 17:07:28 +0800 "Cong WANG" <xiyou.wangcong@...il.com> wrote:

> Kfifo is a ring-buffer in kernel which can be used as a lock-free way
> for concurrent read/write when there are only one producer and one
> consumer. Details of its design can be found in kernel/kfifo.c and
> include/linux/kfifo.h.
> 
> You will find that the 'in' and 'out' fields of 'struct kfifo' are
> both represented as 'unsigned int' and in most cases 'in' is larger
> than 'out' and their difference will NOT be  over 'size'.
> 
> Now the problem is that 'in' will be *smaller* than 'out' when 'in'
> overflows and 'out' doesn't (Yes, this may occur quietly.). This is
> NOT what we expect, though it may not cause any serious problems if we
> carefully use kfifo*() functions. And this is really a bug.

You seem to be saying that it's not a bug, but it's a bug.

Exactly what goes wrong?

> This bug
> affects the kernel since version 2.6.10. I have tested this patch on
> x86 machines.
> 
> Signed-off-by: WANG Cong  <xiyou.wangcong@...il.com>
> 
> ---
> 
> --- kernel/kfifo.c.orig	2007-02-07 19:42:51.000000000 +0800
> +++ kernel/kfifo.c	2007-02-07 19:43:31.000000000 +0800
> @@ -24,6 +24,7 @@
>  #include <linux/slab.h>
>  #include <linux/err.h>
>  #include <linux/kfifo.h>
> +#include <linux/compiler.h>
> 
>  /**
>   * kfifo_init - allocates a new FIFO using a preallocated buffer
> @@ -120,6 +121,12 @@ unsigned int __kfifo_put(struct kfifo *f
>  {
>  	unsigned int l;
> 
> +	/*If only fifo->in overflows, let both overflow!*/
> +	if (unlikely(fifo->in < fifo->out)) {
> +		fifo->out += fifo->size;
> +		fifo->in  += fifo->size;
> +	}
> +

hm.   That would indicate that there's a problem elsewhere in the logic.

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