linux-kernel - Re: [PATCH] dmi: Prevent linked list corruption (resent)

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Date:	Mon, 11 Feb 2008 18:53:52 +0100
From:	Jean Delvare <khali@...ux-fr.org>
To:	"Parag Warudkar" <parag.warudkar@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, "Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: [PATCH] dmi: Prevent linked list corruption (resent)

Hi Parag,

On Mon, 11 Feb 2008 12:35:39 -0500, Parag Warudkar wrote:
> On Feb 11, 2008 12:24 PM, Jean Delvare <khali@...ux-fr.org> wrote:
> > [Once more without forgetting the last "quilt refresh", sorry.]
> >
> > Adding the same item to a given linked list more than once is guaranteed
> > to break and corrupt the list. This is however what we do in dmi_scan
> > since commit 79da4721117fcf188b4b007b775738a530f574da.
> >
> > Given that there is absolutely no interest in saving empty OEM
> > strings anyway, I propose the simple and efficient fix below: we
> > discard the empty OEM strings altogether.
> >
> > Signed-off-by: Jean Delvare <khali@...ux-fr.org>
> > Cc: Parag Warudkar <parag.warudkar@...il.com>
> > Cc: Ingo Molnar <mingo@...e.hu>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> 
> I suppose the list would be corrupted only if there are deletions from
> the list? (Which there aren't.)

As I understand the way doubly linked lists are implemented in Linux, I
think that the corruption exists even if you are only adding items to
the list. Each struct dmi_device contains a list_head which points to
the previous and next items in the list. If you add a struct dmi_device
that was already in the list, you are overwriting this list_head with
new pointers and you lose the pointers that were originally there. This
means that you have created a "shortcut" from one list item to another
item that is further in the list, and the items in-between them are no
longer reachable.

> Anyway not adding the empty strings is way better and I don't see now
> how they could've been useful.
> (I added them out of the doubt of breaking something.)
> 
> Acked-By: Parag Warudkar <parag.warudkar@...il.com>

Thanks,
-- 
Jean Delvare
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