Re: [PATCH] ISDN: fix double free bug in isdn_net

From: Karsten Keil
Date: Wed Aug 16 2006 - 16:37:13 EST

On Wed, Aug 16, 2006 at 10:22:46PM +0200, Jesper Juhl wrote:
> On 15/08/06, Karsten Keil <kkeil@xxxxxxx> wrote:
> >On Tue, Aug 15, 2006 at 02:15:03AM -0700, David Miller wrote:
> >> From: "Jesper Juhl" <jesper.juhl@xxxxxxxxx>
> >> Date: Tue, 15 Aug 2006 11:08:35 +0200
> >>
> >> > Hmm, perhaps I made a mistake and missed a path. Maybe it would be
> >> > better to fix if by making isdn_writebuf_skb_stub() always set the skb
> >> > to NULL when it does free it. That would add a few more assignments
> >> > but should ensure the right result always.
> >> > What do you say?
> >>
> >> Do we know if the ->writebuf_skb() method ever frees the skb? If it
> >> never does, then yes your suggestion would be one way to handle this.
> >
> >
> >It does if it consumes the skb (then it returns skb->len).
> >But the skb have not to be freed imediately in this case, it maybe
> >queued or used until all bytes are written to the physical device.
> >
> >If it returns any other value the skb is not freed.
> >
> >This logic came from using skb for transparent data too.
> >Here it was possible, that the hw driver only take some bytes from the
> >skb (so it returns < skb->len), then the isdn layer should requeue
> >the skb so no transparent data get lost.
> >
> >But this mechanism was never used in drivers, only 3 states:
> >
> >The driver accept the packet then it is responsible for the skb
> >and return skb->len or the driver do not accept it (e.g. buffer full,
> >conntection is going down), then it return 0 and does not free the
> >skb.
> >
> >If some internal error in the HW driver occur, it should return a
> >negative value and it also do not free the skb.
> >
> Ok, if I understand you correctly, then there's no actual problem here.
> right?

I not aware of any problems in this code (besides it should be cleaned up
and rewritten).

Was here any trigger for your patch ?

Karsten Keil
SuSE Labs
ISDN development
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