[Devel] [PATCH rh7] netlink: Don't manipulate @sk_peek_off if data fetching failed

[Andrey Vagin]

On Thu, Dec 22, 2016 at 04:12:13PM -0800, Andrey Vagin wrote:
> On Thu, Dec 22, 2016 at 03:34:15PM -0800, Andrey Vagin wrote:
> > On Thu, Dec 22, 2016 at 06:41:42PM +0300, Cyrill Gorcunov wrote:
> > > When skb_copy_datagram_iovec called to fetch queued data
> > > it may fail with EFAULT and if MSG_PEEK set by a caller
> > > the position get advanced even if data hasn't been read.
> > > So we might loose data bits here on subsequent recvmsg
> > > calls. Instead lets exit early with error.
> > > 
> > > In sake of https://jira.sw.ru/browse/PSBM-57921
> > > 
> > > CC: Andrey Vagin <avagin at openvz.org>
> > > Signed-off-by: Cyrill Gorcunov <gorcunov at openvz.org>
> > > ---
> > >  net/netlink/af_netlink.c |    4 ++++
> > >  1 file changed, 4 insertions(+)
> > > 
> > > Index: linux-pcs7.git/net/netlink/af_netlink.c
> > > ===================================================================
> > > --- linux-pcs7.git.orig/net/netlink/af_netlink.c
> > > +++ linux-pcs7.git/net/netlink/af_netlink.c
> > > @@ -2473,6 +2473,10 @@ static int netlink_recvmsg(struct kiocb
> > >  
> > >  	skb_reset_transport_header(data_skb);
> > >  	err = skb_copy_datagram_iovec(data_skb, skip, msg->msg_iov, copied);
> > > +	if (err) {
> > > +		skb_free_datagram(sk, skb);
> > 
> > hmmm, why do we need to call skb_free_datagram?
> 
> It should be ok, skb_free_datagram() decriments a reference counter
> 
> Acked-by: Andrey Vagin <avagin at virtuozzo.com>

Actually, this patch breaks the old behaviour even when MSG_PEEK isn't set.

I was thinking a bit more and I don't understand why it is a problem. If
we can't fill a buffer, an error will be returned and a user will be able
to set peek_offset to get these data again.

> 
> > > +		goto out;
> > > +	}
> > >  
> > >  	if (flags & MSG_PEEK)
> > >  		sk_peek_offset_fwd(sk, copied);
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