On 16/08/06, Karsten Keil <[email protected]> wrote:
On Wed, Aug 16, 2006 at 10:22:46PM +0200, Jesper Juhl wrote:
> On 15/08/06, Karsten Keil <[email protected]> wrote:
> >On Tue, Aug 15, 2006 at 02:15:03AM -0700, David Miller wrote:
> >> From: "Jesper Juhl" <[email protected]>
> >> 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 ?
Well, the trigger for the initial patch was Coverity bug #1060 .
I looked at it and thought it looked valid, so I tried to cook up a
patch for was I believed looked like an actual problem.
Here's what Coverity had to say :
CID: 1060
Checker: USE_AFTER_FREE (help)
File: base/src/linux-2.6/drivers/isdn/i4l/isdn_net.c
...
1006 void isdn_net_writebuf_skb(isdn_net_local *lp, struct sk_buff *skb)
1007 {
1008 int ret;
1009 int len = skb->len; /* save len */
1010
1011 /* before obtaining the lock the caller should have checked that
1012 the lp isn't busy */
1013 if (isdn_net_lp_busy(lp)) {
1014 printk("isdn BUG at %s:%d!\n", __FILE__, __LINE__);
1015 goto error;
1016 }
1017
1018 if (!(lp->flags & ISDN_NET_CONNECTED)) {
1019 printk("isdn BUG at %s:%d!\n", __FILE__, __LINE__);
1020 goto error;
1021 }
Event freed_arg: Pointer "skb" freed by function
"isdn_writebuf_skb_stub" [model]
Also see events: [double_free]
1022 ret = isdn_writebuf_skb_stub(lp->isdn_device, lp->isdn_channel, 1, skb);
At conditional (1): "ret != len" taking true path
1023 if (ret != len) {
1024 /* we should never get here */
1025 printk(KERN_WARNING "%s: HL driver queue full\n", lp->name);
1026 goto error;
1027 }
1028
1029 lp->transcount += len;
1030 isdn_net_inc_frame_cnt(lp);
1031 return;
1032
1033 error:
Event double_free: Double free of pointer "skb" in call to "kfree_skb" [model]
Also see events: [freed_arg]
1034 dev_kfree_skb(skb);
1035 lp->stats.tx_errors++;
1036
1037 }
...
--
Jesper Juhl <[email protected]>
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