[bpf-next,V6,2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx

Message ID 160571337537.2801246.15228178384451037535.stgit@firesoul
State New
Headers show
Series
  • bpf: New approach for BPF MTU handling
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Commit Message

Jesper Dangaard Brouer Nov. 18, 2020, 3:29 p.m.
BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use
bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,
by adjusting fib_params 'tot_len' with the packet length plus the
expected encap size. (Just like the bpf_check_mtu helper supports). He
discovered that for SKB ctx the param->tot_len was not used, instead
skb->len was used (via MTU check in is_skb_forwardable()).

Fix this by using fib_params 'tot_len' for MTU check.  If not provided
(e.g. zero) then keep existing behaviour intact.

Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
Reported-by: Carlo Carraro <colrack@gmail.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/filter.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

Comments

Jesper Dangaard Brouer Nov. 20, 2020, 8:26 a.m. | #1
On Wed, 18 Nov 2020 16:29:35 +0100
Jesper Dangaard Brouer <brouer@redhat.com> wrote:

> BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use

> bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,

> by adjusting fib_params 'tot_len' with the packet length plus the

> expected encap size. (Just like the bpf_check_mtu helper supports). He

> discovered that for SKB ctx the param->tot_len was not used, instead

> skb->len was used (via MTU check in is_skb_forwardable()).

> 

> Fix this by using fib_params 'tot_len' for MTU check.  If not provided

> (e.g. zero) then keep existing behaviour intact.


Carlo pointed out (in slack) that the logic is not correctly
implemented in this patch.

I will send a V7.


> Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")

> Reported-by: Carlo Carraro <colrack@gmail.com>

> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

> ---

>  net/core/filter.c |   12 +++++++++++-

>  1 file changed, 11 insertions(+), 1 deletion(-)

> 

> diff --git a/net/core/filter.c b/net/core/filter.c

> index 1ee97fdeea64..ae1fe8e6069a 100644

> --- a/net/core/filter.c

> +++ b/net/core/filter.c

> @@ -5567,10 +5567,20 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,

>  

>  	if (!rc) {

>  		struct net_device *dev;

> +		u32 mtu;

>  

>  		dev = dev_get_by_index_rcu(net, params->ifindex);

> -		if (!is_skb_forwardable(dev, skb))

> +		mtu = dev->mtu;

> +

> +		/* Using tot_len for L3 MTU check if provided by user. Notice at

> +		 * this TC cls_bpf level skb->len contains L2 size, but

> +		 * is_skb_forwardable takes that into account.

> +		 */

> +		if (params->tot_len > mtu) {

>  			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;

> +		} else if (!is_skb_forwardable(dev, skb)) {

> +			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;

> +		}

>  	}

>  

>  	return rc;


-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer
Carlo Carraro Nov. 20, 2020, 1:15 p.m. | #2
Hi

I report here the issue with the previous patch.
The code is now checking against params->tot_len but then it is still
using is_skb_forwardable.
Consider this case where I shrink the packet:
skb->len == 1520
dev->mtu == 1500
params->tot_len == 1480
So the incoming pkt has len 1520, and the out interface has mtu 1500.
In this case fragmentation is not needed because params->tot_len < dev->mtu.
However the code calls is_skb_forwardable and may return false because
skb->len > dev->mtu, resulting in BPF_FIB_LKUP_RET_FRAG_NEEDED.
What I propose is using params->tot_len only if provided, without
falling back to use is_skb_forwardable when provided.
Something like this:

if (params->tot_len > 0) {
  if (params->tot_len > mtu)
    rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
} else if (!is_skb_forwardable(dev, skb)) {
  rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
}

However, doing so we are skipping more relaxed MTU checks inside
is_skb_forwardable, so I'm not sure about this.
Please comment

Il giorno ven 20 nov 2020 alle ore 09:26 Jesper Dangaard Brouer
<brouer@redhat.com> ha scritto:
>

> On Wed, 18 Nov 2020 16:29:35 +0100

> Jesper Dangaard Brouer <brouer@redhat.com> wrote:

>

> > BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use

> > bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,

> > by adjusting fib_params 'tot_len' with the packet length plus the

> > expected encap size. (Just like the bpf_check_mtu helper supports). He

> > discovered that for SKB ctx the param->tot_len was not used, instead

> > skb->len was used (via MTU check in is_skb_forwardable()).

> >

> > Fix this by using fib_params 'tot_len' for MTU check.  If not provided

> > (e.g. zero) then keep existing behaviour intact.

>

> Carlo pointed out (in slack) that the logic is not correctly

> implemented in this patch.

>

> I will send a V7.

>

>

> > Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")

> > Reported-by: Carlo Carraro <colrack@gmail.com>

> > Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

> > ---

> >  net/core/filter.c |   12 +++++++++++-

> >  1 file changed, 11 insertions(+), 1 deletion(-)

> >

> > diff --git a/net/core/filter.c b/net/core/filter.c

> > index 1ee97fdeea64..ae1fe8e6069a 100644

> > --- a/net/core/filter.c

> > +++ b/net/core/filter.c

> > @@ -5567,10 +5567,20 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,

> >

> >       if (!rc) {

> >               struct net_device *dev;

> > +             u32 mtu;

> >

> >               dev = dev_get_by_index_rcu(net, params->ifindex);

> > -             if (!is_skb_forwardable(dev, skb))

> > +             mtu = dev->mtu;

> > +

> > +             /* Using tot_len for L3 MTU check if provided by user. Notice at

> > +              * this TC cls_bpf level skb->len contains L2 size, but

> > +              * is_skb_forwardable takes that into account.

> > +              */

> > +             if (params->tot_len > mtu) {

> >                       rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;

> > +             } else if (!is_skb_forwardable(dev, skb)) {

> > +                     rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;

> > +             }

> >       }

> >

> >       return rc;

>

> --

> Best regards,

>   Jesper Dangaard Brouer

>   MSc.CS, Principal Kernel Engineer at Red Hat

>   LinkedIn: http://www.linkedin.com/in/brouer

>
David Ahern Nov. 20, 2020, 3:53 p.m. | #3
On 11/20/20 6:15 AM, Carlo Carraro wrote:
> I report here the issue with the previous patch.

> The code is now checking against params->tot_len but then it is still

> using is_skb_forwardable.

> Consider this case where I shrink the packet:

> skb->len == 1520

> dev->mtu == 1500

> params->tot_len == 1480

> So the incoming pkt has len 1520, and the out interface has mtu 1500.

> In this case fragmentation is not needed because params->tot_len < dev->mtu.

> However the code calls is_skb_forwardable and may return false because

> skb->len > dev->mtu, resulting in BPF_FIB_LKUP_RET_FRAG_NEEDED.

> What I propose is using params->tot_len only if provided, without

> falling back to use is_skb_forwardable when provided.

> Something like this:

> 

> if (params->tot_len > 0) {

>   if (params->tot_len > mtu)

>     rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;

> } else if (!is_skb_forwardable(dev, skb)) {

>   rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;

> }

> 

> However, doing so we are skipping more relaxed MTU checks inside

> is_skb_forwardable, so I'm not sure about this.

> Please comment



Daniel's just proposed patch changes this again (removes the
is_skb_forwardable check). Jesper: you might want to hold off until that
happens.

Patch

diff --git a/net/core/filter.c b/net/core/filter.c
index 1ee97fdeea64..ae1fe8e6069a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5567,10 +5567,20 @@  BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 
 	if (!rc) {
 		struct net_device *dev;
+		u32 mtu;
 
 		dev = dev_get_by_index_rcu(net, params->ifindex);
-		if (!is_skb_forwardable(dev, skb))
+		mtu = dev->mtu;
+
+		/* Using tot_len for L3 MTU check if provided by user. Notice at
+		 * this TC cls_bpf level skb->len contains L2 size, but
+		 * is_skb_forwardable takes that into account.
+		 */
+		if (params->tot_len > mtu) {
 			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
+		} else if (!is_skb_forwardable(dev, skb)) {
+			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
+		}
 	}
 
 	return rc;