Subject: Re: [V2 PATCH net-next 1/2] xdp: implement xdp_redirect_map for generic XDP

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 8 Sep 2017 10:36:01 +0200

On Thu, 07 Sep 2017 16:09:56 +0200
Daniel Borkmann <[email-masked]> wrote:

> On 09/07/2017 02:33 PM, Jesper Dangaard Brouer wrote:
> > Using bpf_redirect_map is allowed for generic XDP programs, but the
> > appropriate map lookup was never performed in xdp_do_generic_redirect().
> >
> > Instead the map-index is directly used as the ifindex.  For the
> > xdp_redirect_map sample in SKB-mode '-S', this resulted in trying
> > sending on ifindex 0 which isn't valid, resulting in getting SKB
> > packets dropped.  Thus, the reported performance numbers are wrong in
> > commit 24251c264798 ("samples/bpf: add option for native and skb mode
> > for redirect apps") for the 'xdp_redirect_map -S' case.
> >
> > It might seem innocent this was lacking, but it can actually crash the
> > kernel.  The potential crash is caused by not consuming redirect_info->map.
> > The bpf_redirect_map helper will set this_cpu_ptr(&redirect_info)->map
> > pointer, which will survive even after unloading the xdp bpf_prog and
> > deallocating the devmap data-structure.  This leaves a dead map
> > pointer around.  The kernel will crash when loading the xdp_redirect
> > sample (in native XDP mode) as it doesn't reset map (via bpf_redirect)
> > and returns XDP_REDIRECT, which will cause it to dereference the map
> > pointer.
> >
> > Fixes: 6103aa96ec07 ("net: implement XDP_REDIRECT for xdp generic")
> > Fixes: 24251c264798 ("samples/bpf: add option for native and skb mode for redirect apps")
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> >   include/trace/events/xdp.h |    4 ++--
> >   net/core/filter.c          |   14 +++++++++++---
> >   2 files changed, 13 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/trace/events/xdp.h b/include/trace/events/xdp.h
> > index 862575ac8da9..4e16c43fba10 100644
> > --- a/include/trace/events/xdp.h
> > +++ b/include/trace/events/xdp.h
> > @@ -138,11 +138,11 @@ DEFINE_EVENT_PRINT(xdp_redirect_template, xdp_redirect_map_err,
> >
> >   #define _trace_xdp_redirect_map(dev, xdp, fwd, map, idx)		\
> >   	 trace_xdp_redirect_map(dev, xdp, fwd ? fwd->ifindex : 0,	\
> > -				0, map, idx);
> > +				0, map, idx)
> >
> >   #define _trace_xdp_redirect_map_err(dev, xdp, fwd, map, idx, err)	\
> >   	 trace_xdp_redirect_map_err(dev, xdp, fwd ? fwd->ifindex : 0,	\
> > -				    err, map, idx);
> > +				    err, map, idx)
> >
> >   #endif /* _TRACE_XDP_H */
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 5912c738a7b2..3767470cab6c 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -2566,13 +2566,19 @@ int xdp_do_generic_redirect(struct net_device *dev, struct sk_buff *skb,
> >   			    struct bpf_prog *xdp_prog)
> >   {
> >   	struct redirect_info *ri = this_cpu_ptr(&redirect_info);
> > +	struct bpf_map *map = ri->map;
> >   	u32 index = ri->ifindex;
> >   	struct net_device *fwd;
> >   	unsigned int len;
> >   	int err = 0;
> >
> > -	fwd = dev_get_by_index_rcu(dev_net(dev), index);
> >   	ri->ifindex = 0;
> > +	ri->map = NULL;
> > +
> > +	if (map)
> > +		fwd = __dev_map_lookup_elem(map, index);
> > +	else
> > +		fwd = dev_get_by_index_rcu(dev_net(dev), index);
> >   	if (unlikely(!fwd)) {
> >   		err = -EINVAL;
> >   		goto err;
> > @@ -2590,10 +2596,12 @@ int xdp_do_generic_redirect(struct net_device *dev, struct sk_buff *skb,
> >   	}
> >
> >   	skb->dev = fwd;  
> 
> Looks much better above, thanks!
> 
> > -	_trace_xdp_redirect(dev, xdp_prog, index);
> > +	map ? _trace_xdp_redirect_map(dev, xdp_prog, fwd, map, index)
> > +		: _trace_xdp_redirect(dev, xdp_prog, index);  
> 
> Could we rather make this in a way such that when the two
> tracepoints are disabled and thus patched out, that we can
> also omit the extra conditional which has no purpose then?

First of all I don't think it make much of a difference, I measured the
impact of the full patch to "cost" 1.62 nanosec (which is arguably
below the accuracy level of the system under test)

Secondly, I plan to optimize the map case for generic XDP later, where
I would naturally split this into two functions (as V1, and as
native-XDP), thus this extra conditional would go away.  As I've shown
offlist (to you, John and Andy) I demonstrated a 24% speedup via a
xmit_more hack for generic XDP.

> Perhaps just a consolidated _trace_xdp_generic_redirect_map()
> would be better to avoid this altogether given we have twice
> the same anyway, here and in err path.

I do want separate tracepoints for xdp_redirect and xdp_redirect_map,
as it makes it more clear for users of the tracepoint (and attached
bpf_prog's can be faster, knowing the context).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer