linux-kernel - Re: scatterlist.c: bug in sg

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Date:	Sun, 28 Sep 2008 18:28:05 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Leon Woestenberg <leon.woestenberg@...il.com>
CC:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux-kernel@...r.kernel.org,
	James.Bottomley@...senpartnership.com, jens.axboe@...cle.com
Subject: Re: scatterlist.c: bug in sg_next()?

Leon Woestenberg wrote:
> Hello,
> 
> I was code-inspecting 2.6.27-r7 through git web, when I came across this:
> 
> In sg_next(), after following a chain_ptr, a few more checks should be
> performed.
> The rare case exists that the first entry in the chained list is a
> last marker, in case NULL must be returned.
> 
> Can someone confirm and cook a patch?
> 
> struct scatterlist *sg_next(struct scatterlist *sg)
> {
>   if (sg_is_last(sg))
>     return NULL;
>   sg++;
>   if (unlikely(sg_is_chain(sg))) {
>      sg = sg_chain_ptr(sg);
> +#ifdef CONFIG_DEBUG_SG
> + BUG_ON(sg->sg_magic != SG_MAGIC);
> +#endif
> +   if (sg_is_last(sg))
> +     return NULL;
>   }
>   return sg;
> }
> 
> Signed-off-by: Leon Woestenberg <leon@...ebranch.com>
> 
> Regards,

No! the last marker is set on a valid sg entry. Only it's next
is no longer valid. So the check at the top is for the Next-sg
not the passed-in-sg. What you thought of is a NULL terminating
sg-list. The end marker is so to save the extra NULL entry.

Boaz

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