[PATCH bpf-next V6 0/7] bpf: New approach for BPF MTU handling

[PATCH bpf-next V6 0/7] bpf: New approach for BPF MTU handling

[Jesper Dangaard Brouer]

This patchset drops all the MTU checks in TC BPF-helpers that limits
growing the packet size. This is done because these BPF-helpers doesn't
take redirect into account, which can result in their MTU check being done
against the wrong netdev.

The new approach is to give BPF-programs knowledge about the MTU on a
netdev (via ifindex) and fib route lookup level. Meaning some BPF-helpers
are added and extended to make it possible to do MTU checks in the
BPF-code.

If BPF-prog doesn't comply with the MTU then the packet will eventually
get dropped as some other layer. In some cases the existing kernel MTU
checks will drop the packet, but there are also cases where BPF can bypass
these checks. Specifically doing TC-redirect from ingress step
(sch_handle_ingress) into egress code path (basically calling
dev_queue_xmit()). It is left up to driver code to handle these kind of
MTU violations.

One advantage of this approach is that it ingress-to-egress BPF-prog can
send information via packet data. With the MTU checks removed in the
helpers, and also not done in skb_do_redirect() call, this allows for an
ingress BPF-prog to communicate with an egress BPF-prog via packet data,
as long as egress BPF-prog remove this prior to transmitting packet.

This patchset is primarily focused on TC-BPF, but I've made sure that the
MTU BPF-helpers also works for XDP BPF-programs.

V2: Change BPF-helper API from lookup to check.
V3: Drop enforcement of MTU in net-core, leave it to drivers.
V4: Keep sanity limit + netdev "up" checks + rename BPF-helper.
V5: Fix uninit variable + name struct output member mtu_result.
V6: Use bpf_check_mtu() in selftest

---

Jesper Dangaard Brouer (7):
      bpf: Remove MTU check in __bpf_skb_max_len
      bpf: fix bpf_fib_lookup helper MTU check for SKB ctx
      bpf: bpf_fib_lookup return MTU value as output when looked up
      bpf: add BPF-helper for MTU checking
      bpf: drop MTU check when doing TC-BPF redirect to ingress
      bpf: make it possible to identify BPF redirected SKBs
      selftests/bpf: use bpf_check_mtu in selftest test_cls_redirect


 include/linux/netdevice.h                          |   31 +++
 include/uapi/linux/bpf.h                           |   78 ++++++++-
 net/core/dev.c                                     |   21 --
 net/core/filter.c                                  |  182 ++++++++++++++++++--
 net/sched/Kconfig                                  |    1 
 tools/include/uapi/linux/bpf.h                     |   78 ++++++++-
 .../selftests/bpf/progs/test_cls_redirect.c        |    7 +
 7 files changed, 356 insertions(+), 42 deletions(-)

--

[PATCH bpf-next V6 1/7] bpf: Remove MTU check in __bpf_skb_max_len

[Jesper Dangaard Brouer]

Multiple BPF-helpers that can manipulate/increase the size of the SKB uses
__bpf_skb_max_len() as the max-length. This function limit size against
the current net_device MTU (skb->dev->mtu).

When a BPF-prog grow the packet size, then it should not be limited to the
MTU. The MTU is a transmit limitation, and software receiving this packet
should be allowed to increase the size. Further more, current MTU check in
__bpf_skb_max_len uses the MTU from ingress/current net_device, which in
case of redirects uses the wrong net_device.

This patch keeps a sanity max limit of SKB_MAX_ALLOC (16KiB). The real limit
is elsewhere in the system. Jesper's testing[1] showed it was not possible
to exceed 8KiB when expanding the SKB size via BPF-helper. The limiting
factor is the define KMALLOC_MAX_CACHE_SIZE which is 8192 for
SLUB-allocator (CONFIG_SLUB) in-case PAGE_SIZE is 4096. This define is
in-effect due to this being called from softirq context see code
__gfp_pfmemalloc_flags() and __do_kmalloc_node(). Jakub's testing showed
that frames above 16KiB can cause NICs to reset (but not crash). Keep this
sanity limit at this level as memory layer can differ based on kernel
config.

[1] https://github.com/xdp-project/bpf-examples/tree/master/MTU-tests

V3: replace __bpf_skb_max_len() with define and use IPv6 max MTU size.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/filter.c |   12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 2ca5eecebacf..1ee97fdeea64 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3552,11 +3552,7 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
 	return 0;
 }
 
-static u32 __bpf_skb_max_len(const struct sk_buff *skb)
-{
-	return skb->dev ? skb->dev->mtu + skb->dev->hard_header_len :
-			  SKB_MAX_ALLOC;
-}
+#define BPF_SKB_MAX_LEN SKB_MAX_ALLOC
 
 BPF_CALL_4(sk_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
 	   u32, mode, u64, flags)
@@ -3605,7 +3601,7 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
 {
 	u32 len_cur, len_diff_abs = abs(len_diff);
 	u32 len_min = bpf_skb_net_base_len(skb);
-	u32 len_max = __bpf_skb_max_len(skb);
+	u32 len_max = BPF_SKB_MAX_LEN;
 	__be16 proto = skb->protocol;
 	bool shrink = len_diff < 0;
 	u32 off;
@@ -3688,7 +3684,7 @@ static int bpf_skb_trim_rcsum(struct sk_buff *skb, unsigned int new_len)
 static inline int __bpf_skb_change_tail(struct sk_buff *skb, u32 new_len,
 					u64 flags)
 {
-	u32 max_len = __bpf_skb_max_len(skb);
+	u32 max_len = BPF_SKB_MAX_LEN;
 	u32 min_len = __bpf_skb_min_len(skb);
 	int ret;
 
@@ -3764,7 +3760,7 @@ static const struct bpf_func_proto sk_skb_change_tail_proto = {
 static inline int __bpf_skb_change_head(struct sk_buff *skb, u32 head_room,
 					u64 flags)
 {
-	u32 max_len = __bpf_skb_max_len(skb);
+	u32 max_len = BPF_SKB_MAX_LEN;
 	u32 new_len = skb->len + head_room;
 	int ret;
 

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

[Jesper Dangaard Brouer]

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(-)

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;

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

[Jesper Dangaard Brouer]

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

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

[Carlo Carraro]

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
>

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

[David Ahern]

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 bpf-next V6 3/7] bpf: bpf_fib_lookup return MTU value as output when looked up

[Jesper Dangaard Brouer]

The BPF-helpers for FIB lookup (bpf_xdp_fib_lookup and bpf_skb_fib_lookup)
can perform MTU check and return BPF_FIB_LKUP_RET_FRAG_NEEDED.  The BPF-prog
don't know the MTU value that caused this rejection.

If the BPF-prog wants to implement PMTU (Path MTU Discovery) (rfc1191) it
need to know this MTU value for the ICMP packet.

Patch change lookup and result struct bpf_fib_lookup, to contain this MTU
value as output via a union with 'tot_len' as this is the value used for
the MTU lookup.

V5:
 - Fixed uninit value spotted by Dan Carpenter.
 - Name struct output member mtu_result

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 include/uapi/linux/bpf.h       |   11 +++++++++--
 net/core/filter.c              |   22 +++++++++++++++-------
 tools/include/uapi/linux/bpf.h |   11 +++++++++--
 3 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 162999b12790..beacd312ea17 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2220,6 +2220,9 @@ union bpf_attr {
  *		* > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
  *		  packet is not forwarded or needs assist from full stack
  *
+ *		If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
+ *		was exceeded and output params->mtu_result contains the MTU.
+ *
  * long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
  *	Description
  *		Add an entry to, or update a sockhash *map* referencing sockets.
@@ -4923,9 +4926,13 @@ struct bpf_fib_lookup {
 	__be16	sport;
 	__be16	dport;
 
-	/* total length of packet from network header - used for MTU check */
-	__u16	tot_len;
+	union {	/* used for MTU check */
+		/* input to lookup */
+		__u16	tot_len; /* L3 length from network hdr (iph->tot_len) */
 
+		/* output: MTU value */
+		__u16	mtu_result;
+	};
 	/* input: L3 device index for lookup
 	 * output: device index from FIB lookup
 	 */
diff --git a/net/core/filter.c b/net/core/filter.c
index ae1fe8e6069a..0712b7e5d9fb 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5265,12 +5265,14 @@ static const struct bpf_func_proto bpf_skb_get_xfrm_state_proto = {
 #if IS_ENABLED(CONFIG_INET) || IS_ENABLED(CONFIG_IPV6)
 static int bpf_fib_set_fwd_params(struct bpf_fib_lookup *params,
 				  const struct neighbour *neigh,
-				  const struct net_device *dev)
+				  const struct net_device *dev, u32 mtu)
 {
 	memcpy(params->dmac, neigh->ha, ETH_ALEN);
 	memcpy(params->smac, dev->dev_addr, ETH_ALEN);
 	params->h_vlan_TCI = 0;
 	params->h_vlan_proto = 0;
+	if (mtu)
+		params->mtu_result = mtu; /* union with tot_len */
 
 	return 0;
 }
@@ -5286,8 +5288,8 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	struct net_device *dev;
 	struct fib_result res;
 	struct flowi4 fl4;
+	u32 mtu = 0;
 	int err;
-	u32 mtu;
 
 	dev = dev_get_by_index_rcu(net, params->ifindex);
 	if (unlikely(!dev))
@@ -5354,8 +5356,10 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 
 	if (check_mtu) {
 		mtu = ip_mtu_from_fib_result(&res, params->ipv4_dst);
-		if (params->tot_len > mtu)
+		if (params->tot_len > mtu) {
+			params->mtu_result = mtu; /* union with tot_len */
 			return BPF_FIB_LKUP_RET_FRAG_NEEDED;
+		}
 	}
 
 	nhc = res.nhc;
@@ -5389,7 +5393,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	if (!neigh)
 		return BPF_FIB_LKUP_RET_NO_NEIGH;
 
-	return bpf_fib_set_fwd_params(params, neigh, dev);
+	return bpf_fib_set_fwd_params(params, neigh, dev, mtu);
 }
 #endif
 
@@ -5406,7 +5410,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	struct flowi6 fl6;
 	int strict = 0;
 	int oif, err;
-	u32 mtu;
+	u32 mtu = 0;
 
 	/* link local addresses are never forwarded */
 	if (rt6_need_strict(dst) || rt6_need_strict(src))
@@ -5481,8 +5485,10 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 
 	if (check_mtu) {
 		mtu = ipv6_stub->ip6_mtu_from_fib6(&res, dst, src);
-		if (params->tot_len > mtu)
+		if (params->tot_len > mtu) {
+			params->mtu_result = mtu; /* union with tot_len */
 			return BPF_FIB_LKUP_RET_FRAG_NEEDED;
+		}
 	}
 
 	if (res.nh->fib_nh_lws)
@@ -5502,7 +5508,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	if (!neigh)
 		return BPF_FIB_LKUP_RET_NO_NEIGH;
 
-	return bpf_fib_set_fwd_params(params, neigh, dev);
+	return bpf_fib_set_fwd_params(params, neigh, dev, mtu);
 }
 #endif
 
@@ -5581,6 +5587,8 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 		} else if (!is_skb_forwardable(dev, skb)) {
 			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
 		}
+
+		params->mtu_result = dev->mtu; /* union with tot_len */
 	}
 
 	return rc;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 162999b12790..beacd312ea17 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -2220,6 +2220,9 @@ union bpf_attr {
  *		* > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
  *		  packet is not forwarded or needs assist from full stack
  *
+ *		If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
+ *		was exceeded and output params->mtu_result contains the MTU.
+ *
  * long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
  *	Description
  *		Add an entry to, or update a sockhash *map* referencing sockets.
@@ -4923,9 +4926,13 @@ struct bpf_fib_lookup {
 	__be16	sport;
 	__be16	dport;
 
-	/* total length of packet from network header - used for MTU check */
-	__u16	tot_len;
+	union {	/* used for MTU check */
+		/* input to lookup */
+		__u16	tot_len; /* L3 length from network hdr (iph->tot_len) */
 
+		/* output: MTU value */
+		__u16	mtu_result;
+	};
 	/* input: L3 device index for lookup
 	 * output: device index from FIB lookup
 	 */

[PATCH bpf-next V6 4/7] bpf: add BPF-helper for MTU checking

[Jesper Dangaard Brouer]

This BPF-helper bpf_check_mtu() works for both XDP and TC-BPF programs.

The SKB object is complex and the skb->len value (accessible from
BPF-prog) also include the length of any extra GRO/GSO segments, but
without taking into account that these GRO/GSO segments get added
transport (L4) and network (L3) headers before being transmitted. Thus,
this BPF-helper is created such that the BPF-programmer don't need to
handle these details in the BPF-prog.

The API is designed to help the BPF-programmer, that want to do packet
context size changes, which involves other helpers. These other helpers
usually does a delta size adjustment. This helper also support a delta
size (len_diff), which allow BPF-programmer to reuse arguments needed by
these other helpers, and perform the MTU check prior to doing any actual
size adjustment of the packet context.

It is on purpose, that we allow the len adjustment to become a negative
result, that will pass the MTU check. This might seem weird, but it's not
this helpers responsibility to "catch" wrong len_diff adjustments. Other
helpers will take care of these checks, if BPF-programmer chooses to do
actual size adjustment.

V6:
- Took John's advice and dropped BPF_MTU_CHK_RELAX
- Returned MTU is kept at L3-level (like fib_lookup)

V4: Lot of changes
 - ifindex 0 now use current netdev for MTU lookup
 - rename helper from bpf_mtu_check to bpf_check_mtu
 - fix bug for GSO pkt length (as skb->len is total len)
 - remove __bpf_len_adj_positive, simply allow negative len adj

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 include/uapi/linux/bpf.h       |   67 ++++++++++++++++++++++
 net/core/filter.c              |  122 ++++++++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h |   67 ++++++++++++++++++++++
 3 files changed, 256 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index beacd312ea17..2619ea8c5a08 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3790,6 +3790,61 @@ union bpf_attr {
  *		*ARG_PTR_TO_BTF_ID* of type *task_struct*.
  *	Return
  *		Pointer to the current task.
+ *
+ * int bpf_check_mtu(void *ctx, u32 ifindex, u32 *mtu_len, s32 len_diff, u64 flags)
+ *	Description
+ *		Check ctx packet size against MTU of net device (based on
+ *		*ifindex*).  This helper will likely be used in combination with
+ *		helpers that adjust/change the packet size.  The argument
+ *		*len_diff* can be used for querying with a planned size
+ *		change. This allows to check MTU prior to changing packet ctx.
+ *
+ *		Specifying *ifindex* zero means the MTU check is performed
+ *		against the current net device.  This is practical if this isn't
+ *		used prior to redirect.
+ *
+ *		The Linux kernel route table can configure MTUs on a more
+ *		specific per route level, which is not provided by this helper.
+ *		For route level MTU checks use the **bpf_fib_lookup**\ ()
+ *		helper.
+ *
+ *		*ctx* is either **struct xdp_md** for XDP programs or
+ *		**struct sk_buff** for tc cls_act programs.
+ *
+ *		The *flags* argument can be a combination of one or more of the
+ *		following values:
+ *
+ *		**BPF_MTU_CHK_SEGS**
+ *			This flag will only works for *ctx* **struct sk_buff**.
+ *			If packet context contains extra packet segment buffers
+ *			(often knows as GSO skb), then MTU check is harder to
+ *			check at this point, because in transmit path it is
+ *			possible for the skb packet to get re-segmented
+ *			(depending on net device features).  This could still be
+ *			a MTU violation, so this flag enables performing MTU
+ *			check against segments, with a different violation
+ *			return code to tell it apart. Check cannot use len_diff.
+ *
+ *		On return *mtu_len* pointer contains the MTU value of the net
+ *		device.  Remember the net device configured MTU is the L3 size,
+ *		which is returned here and XDP and TX length operate at L2.
+ *		Helper take this into account for you, but remember when using
+ *		MTU value in your BPF-code.  On input *mtu_len* must be a valid
+ *		pointer and be initialized (to zero), else verifier will reject
+ *		BPF program.
+ *
+ *	Return
+ *		* 0 on success, and populate MTU value in *mtu_len* pointer.
+ *
+ *		* < 0 if any input argument is invalid (*mtu_len* not updated)
+ *
+ *		MTU violations return positive values, but also populate MTU
+ *		value in *mtu_len* pointer, as this can be needed for
+ *		implementing PMTU handing:
+ *
+ *		* **BPF_MTU_CHK_RET_FRAG_NEEDED**
+ *		* **BPF_MTU_CHK_RET_SEGS_TOOBIG**
+ *
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3951,6 +4006,7 @@ union bpf_attr {
 	FN(task_storage_get),		\
 	FN(task_storage_delete),	\
 	FN(get_current_task_btf),	\
+	FN(check_mtu),			\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
@@ -4978,6 +5034,17 @@ struct bpf_redir_neigh {
 	};
 };
 
+/* bpf_check_mtu flags*/
+enum  bpf_check_mtu_flags {
+	BPF_MTU_CHK_SEGS  = (1U << 0),
+};
+
+enum bpf_check_mtu_ret {
+	BPF_MTU_CHK_RET_SUCCESS,      /* check and lookup successful */
+	BPF_MTU_CHK_RET_FRAG_NEEDED,  /* fragmentation required to fwd */
+	BPF_MTU_CHK_RET_SEGS_TOOBIG,  /* GSO re-segmentation needed to fwd */
+};
+
 enum bpf_task_fd_type {
 	BPF_FD_TYPE_RAW_TRACEPOINT,	/* tp name */
 	BPF_FD_TYPE_TRACEPOINT,		/* tp name */
diff --git a/net/core/filter.c b/net/core/filter.c
index 0712b7e5d9fb..7e8d2475a205 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5604,6 +5604,124 @@ static const struct bpf_func_proto bpf_skb_fib_lookup_proto = {
 	.arg4_type	= ARG_ANYTHING,
 };
 
+static struct net_device *__dev_via_ifindex(struct net_device *dev_curr,
+					    u32 ifindex)
+{
+	struct net *netns = dev_net(dev_curr);
+
+	/* Non-redirect use-cases can use ifindex=0 and save ifindex lookup */
+	if (ifindex == 0)
+		return dev_curr;
+
+	return dev_get_by_index_rcu(netns, ifindex);
+}
+
+BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
+	   u32, ifindex, u32 *, mtu_len, s32, len_diff, u64, flags)
+{
+	int ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
+	struct net_device *dev = skb->dev;
+	int len;
+	int mtu;
+
+	if (flags & ~(BPF_MTU_CHK_SEGS))
+		return -EINVAL;
+
+	dev = __dev_via_ifindex(dev, ifindex);
+	if (!dev)
+		return -ENODEV;
+
+	mtu = READ_ONCE(dev->mtu);
+
+	/* TC len is L2, remove L2-header as dev MTU is L3 size */
+	len = skb->len - ETH_HLEN;
+
+	len += len_diff; /* len_diff can be negative, minus result pass check */
+	if (len <= mtu) {
+		ret = BPF_MTU_CHK_RET_SUCCESS;
+		goto out;
+	}
+	/* At this point, skb->len exceed MTU, but as it include length of all
+	 * segments, it can still be below MTU.  The SKB can possibly get
+	 * re-segmented in transmit path (see validate_xmit_skb).  Thus, user
+	 * must choose if segs are to be MTU checked.  Last SKB "headlen" is
+	 * checked against MTU.
+	 */
+	if (skb_is_gso(skb)) {
+		ret = BPF_MTU_CHK_RET_SUCCESS;
+
+		if (flags & BPF_MTU_CHK_SEGS &&
+		    skb_gso_validate_network_len(skb, mtu)) {
+			ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;
+			goto out;
+		}
+
+		len = skb_headlen(skb) - ETH_HLEN + len_diff;
+		if (len > mtu) {
+			ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
+			goto out;
+		}
+	}
+out:
+	/* BPF verifier guarantees valid pointer */
+	*mtu_len = mtu;
+
+	return ret;
+}
+
+BPF_CALL_5(bpf_xdp_check_mtu, struct xdp_buff *, xdp,
+	   u32, ifindex, u32 *, mtu_len, s32, len_diff, u64, flags)
+{
+	struct net_device *dev = xdp->rxq->dev;
+	int len = xdp->data_end - xdp->data;
+	int ret = BPF_MTU_CHK_RET_SUCCESS;
+	int mtu;
+
+	/* XDP variant doesn't support multi-buffer segment check (yet) */
+	if (flags)
+		return -EINVAL;
+
+	dev = __dev_via_ifindex(dev, ifindex);
+	if (!dev)
+		return -ENODEV;
+
+	mtu = READ_ONCE(dev->mtu);
+
+	/* XDP len is L2, remove L2-header as dev MTU is L3 size */
+	len -= ETH_HLEN;
+
+	len += len_diff; /* len_diff can be negative, minus result pass check */
+	if (len > mtu)
+		ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
+
+	/* BPF verifier guarantees valid pointer */
+	*mtu_len = mtu;
+
+	return ret;
+}
+
+static const struct bpf_func_proto bpf_skb_check_mtu_proto = {
+	.func		= bpf_skb_check_mtu,
+	.gpl_only	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type      = ARG_PTR_TO_CTX,
+	.arg2_type      = ARG_ANYTHING,
+	.arg3_type      = ARG_PTR_TO_INT,
+	.arg4_type      = ARG_ANYTHING,
+	.arg5_type      = ARG_ANYTHING,
+};
+
+static const struct bpf_func_proto bpf_xdp_check_mtu_proto = {
+	.func		= bpf_xdp_check_mtu,
+	.gpl_only	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type      = ARG_PTR_TO_CTX,
+	.arg2_type      = ARG_ANYTHING,
+	.arg3_type      = ARG_PTR_TO_INT,
+	.arg4_type      = ARG_ANYTHING,
+	.arg5_type      = ARG_ANYTHING,
+};
+
 #if IS_ENABLED(CONFIG_IPV6_SEG6_BPF)
 static int bpf_push_seg6_encap(struct sk_buff *skb, u32 type, void *hdr, u32 len)
 {
@@ -7169,6 +7287,8 @@ tc_cls_act_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_get_socket_uid_proto;
 	case BPF_FUNC_fib_lookup:
 		return &bpf_skb_fib_lookup_proto;
+	case BPF_FUNC_check_mtu:
+		return &bpf_skb_check_mtu_proto;
 	case BPF_FUNC_sk_fullsock:
 		return &bpf_sk_fullsock_proto;
 	case BPF_FUNC_sk_storage_get:
@@ -7238,6 +7358,8 @@ xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_xdp_adjust_tail_proto;
 	case BPF_FUNC_fib_lookup:
 		return &bpf_xdp_fib_lookup_proto;
+	case BPF_FUNC_check_mtu:
+		return &bpf_xdp_check_mtu_proto;
 #ifdef CONFIG_INET
 	case BPF_FUNC_sk_lookup_udp:
 		return &bpf_xdp_sk_lookup_udp_proto;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index beacd312ea17..2619ea8c5a08 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -3790,6 +3790,61 @@ union bpf_attr {
  *		*ARG_PTR_TO_BTF_ID* of type *task_struct*.
  *	Return
  *		Pointer to the current task.
+ *
+ * int bpf_check_mtu(void *ctx, u32 ifindex, u32 *mtu_len, s32 len_diff, u64 flags)
+ *	Description
+ *		Check ctx packet size against MTU of net device (based on
+ *		*ifindex*).  This helper will likely be used in combination with
+ *		helpers that adjust/change the packet size.  The argument
+ *		*len_diff* can be used for querying with a planned size
+ *		change. This allows to check MTU prior to changing packet ctx.
+ *
+ *		Specifying *ifindex* zero means the MTU check is performed
+ *		against the current net device.  This is practical if this isn't
+ *		used prior to redirect.
+ *
+ *		The Linux kernel route table can configure MTUs on a more
+ *		specific per route level, which is not provided by this helper.
+ *		For route level MTU checks use the **bpf_fib_lookup**\ ()
+ *		helper.
+ *
+ *		*ctx* is either **struct xdp_md** for XDP programs or
+ *		**struct sk_buff** for tc cls_act programs.
+ *
+ *		The *flags* argument can be a combination of one or more of the
+ *		following values:
+ *
+ *		**BPF_MTU_CHK_SEGS**
+ *			This flag will only works for *ctx* **struct sk_buff**.
+ *			If packet context contains extra packet segment buffers
+ *			(often knows as GSO skb), then MTU check is harder to
+ *			check at this point, because in transmit path it is
+ *			possible for the skb packet to get re-segmented
+ *			(depending on net device features).  This could still be
+ *			a MTU violation, so this flag enables performing MTU
+ *			check against segments, with a different violation
+ *			return code to tell it apart. Check cannot use len_diff.
+ *
+ *		On return *mtu_len* pointer contains the MTU value of the net
+ *		device.  Remember the net device configured MTU is the L3 size,
+ *		which is returned here and XDP and TX length operate at L2.
+ *		Helper take this into account for you, but remember when using
+ *		MTU value in your BPF-code.  On input *mtu_len* must be a valid
+ *		pointer and be initialized (to zero), else verifier will reject
+ *		BPF program.
+ *
+ *	Return
+ *		* 0 on success, and populate MTU value in *mtu_len* pointer.
+ *
+ *		* < 0 if any input argument is invalid (*mtu_len* not updated)
+ *
+ *		MTU violations return positive values, but also populate MTU
+ *		value in *mtu_len* pointer, as this can be needed for
+ *		implementing PMTU handing:
+ *
+ *		* **BPF_MTU_CHK_RET_FRAG_NEEDED**
+ *		* **BPF_MTU_CHK_RET_SEGS_TOOBIG**
+ *
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3951,6 +4006,7 @@ union bpf_attr {
 	FN(task_storage_get),		\
 	FN(task_storage_delete),	\
 	FN(get_current_task_btf),	\
+	FN(check_mtu),			\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
@@ -4978,6 +5034,17 @@ struct bpf_redir_neigh {
 	};
 };
 
+/* bpf_check_mtu flags*/
+enum  bpf_check_mtu_flags {
+	BPF_MTU_CHK_SEGS  = (1U << 0),
+};
+
+enum bpf_check_mtu_ret {
+	BPF_MTU_CHK_RET_SUCCESS,      /* check and lookup successful */
+	BPF_MTU_CHK_RET_FRAG_NEEDED,  /* fragmentation required to fwd */
+	BPF_MTU_CHK_RET_SEGS_TOOBIG,  /* GSO re-segmentation needed to fwd */
+};
+
 enum bpf_task_fd_type {
 	BPF_FD_TYPE_RAW_TRACEPOINT,	/* tp name */
 	BPF_FD_TYPE_TRACEPOINT,		/* tp name */

[PATCH bpf-next V6 5/7] bpf: drop MTU check when doing TC-BPF redirect to ingress

[Jesper Dangaard Brouer]

The use-case for dropping the MTU check when TC-BPF does redirect to
ingress, is described by Eyal Birger in email[0]. The summary is the
ability to increase packet size (e.g. with IPv6 headers for NAT64) and
ingress redirect packet and let normal netstack fragment packet as needed.

[0] https://lore.kernel.org/netdev/CAHsH6Gug-hsLGHQ6N0wtixdOa85LDZ3HNRHVd0opR=19Qo4W4Q@mail.gmail.com/

V4:
 - Keep net_device "up" (IFF_UP) check.
 - Adjustment to handle bpf_redirect_peer() helper

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 include/linux/netdevice.h |   31 +++++++++++++++++++++++++++++--
 net/core/dev.c            |   19 ++-----------------
 net/core/filter.c         |   14 +++++++++++---
 3 files changed, 42 insertions(+), 22 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 7ce648a564f7..4a854e09e918 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3917,11 +3917,38 @@ int dev_forward_skb(struct net_device *dev, struct sk_buff *skb);
 bool is_skb_forwardable(const struct net_device *dev,
 			const struct sk_buff *skb);
 
+static __always_inline bool __is_skb_forwardable(const struct net_device *dev,
+						 const struct sk_buff *skb,
+						 const bool check_mtu)
+{
+	const u32 vlan_hdr_len = 4; /* VLAN_HLEN */
+	unsigned int len;
+
+	if (!(dev->flags & IFF_UP))
+		return false;
+
+	if (!check_mtu)
+		return true;
+
+	len = dev->mtu + dev->hard_header_len + vlan_hdr_len;
+	if (skb->len <= len)
+		return true;
+
+	/* if TSO is enabled, we don't care about the length as the packet
+	 * could be forwarded without being segmented before
+	 */
+	if (skb_is_gso(skb))
+		return true;
+
+	return false;
+}
+
 static __always_inline int ____dev_forward_skb(struct net_device *dev,
-					       struct sk_buff *skb)
+					       struct sk_buff *skb,
+					       const bool check_mtu)
 {
 	if (skb_orphan_frags(skb, GFP_ATOMIC) ||
-	    unlikely(!is_skb_forwardable(dev, skb))) {
+	    unlikely(!__is_skb_forwardable(dev, skb, check_mtu))) {
 		atomic_long_inc(&dev->rx_dropped);
 		kfree_skb(skb);
 		return NET_RX_DROP;
diff --git a/net/core/dev.c b/net/core/dev.c
index 60d325bda0d7..6ceb6412ee97 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2189,28 +2189,13 @@ static inline void net_timestamp_set(struct sk_buff *skb)
 
 bool is_skb_forwardable(const struct net_device *dev, const struct sk_buff *skb)
 {
-	unsigned int len;
-
-	if (!(dev->flags & IFF_UP))
-		return false;
-
-	len = dev->mtu + dev->hard_header_len + VLAN_HLEN;
-	if (skb->len <= len)
-		return true;
-
-	/* if TSO is enabled, we don't care about the length as the packet
-	 * could be forwarded without being segmented before
-	 */
-	if (skb_is_gso(skb))
-		return true;
-
-	return false;
+	return __is_skb_forwardable(dev, skb, true);
 }
 EXPORT_SYMBOL_GPL(is_skb_forwardable);
 
 int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
 {
-	int ret = ____dev_forward_skb(dev, skb);
+	int ret = ____dev_forward_skb(dev, skb, true);
 
 	if (likely(!ret)) {
 		skb->protocol = eth_type_trans(skb, dev);
diff --git a/net/core/filter.c b/net/core/filter.c
index 7e8d2475a205..1ece1065bd50 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2083,13 +2083,21 @@ static const struct bpf_func_proto bpf_csum_level_proto = {
 
 static inline int __bpf_rx_skb(struct net_device *dev, struct sk_buff *skb)
 {
-	return dev_forward_skb(dev, skb);
+	int ret = ____dev_forward_skb(dev, skb, false);
+
+	if (likely(!ret)) {
+		skb->protocol = eth_type_trans(skb, dev);
+		skb_postpull_rcsum(skb, eth_hdr(skb), ETH_HLEN);
+		ret = netif_rx(skb);
+	}
+
+	return ret;
 }
 
 static inline int __bpf_rx_skb_no_mac(struct net_device *dev,
 				      struct sk_buff *skb)
 {
-	int ret = ____dev_forward_skb(dev, skb);
+	int ret = ____dev_forward_skb(dev, skb, false);
 
 	if (likely(!ret)) {
 		skb->dev = dev;
@@ -2480,7 +2488,7 @@ int skb_do_redirect(struct sk_buff *skb)
 			goto out_drop;
 		dev = ops->ndo_get_peer_dev(dev);
 		if (unlikely(!dev ||
-			     !is_skb_forwardable(dev, skb) ||
+			     !__is_skb_forwardable(dev, skb, false) ||
 			     net_eq(net, dev_net(dev))))
 			goto out_drop;
 		skb->dev = dev;

[PATCH bpf-next V6 6/7] bpf: make it possible to identify BPF redirected SKBs

[Jesper Dangaard Brouer]

This change makes it possible to identify SKBs that have been redirected
by TC-BPF (cls_act). This is needed for a number of cases.

(1) For collaborating with driver ifb net_devices.
(2) For avoiding starting generic-XDP prog on TC ingress redirect.

It is most important to fix XDP case(2), because this can break userspace
when a driver gets support for native-XDP. Imagine userspace loads XDP
prog on eth0, which fallback to generic-XDP, and it process TC-redirected
packets. When kernel is updated with native-XDP support for eth0, then the
program no-longer see the TC-redirected packets. Therefore it is important
to keep the order intact; that XDP runs before TC-BPF.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/dev.c    |    2 ++
 net/sched/Kconfig |    1 +
 2 files changed, 3 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index 6ceb6412ee97..26b40f8005ae 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3872,6 +3872,7 @@ sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
 		return NULL;
 	case TC_ACT_REDIRECT:
 		/* No need to push/pop skb's mac_header here on egress! */
+		skb_set_redirected(skb, false);
 		skb_do_redirect(skb);
 		*ret = NET_XMIT_SUCCESS;
 		return NULL;
@@ -4963,6 +4964,7 @@ sch_handle_ingress(struct sk_buff *skb, struct packet_type **pt_prev, int *ret,
 		 * redirecting to another netdev
 		 */
 		__skb_push(skb, skb->mac_len);
+		skb_set_redirected(skb, true);
 		if (skb_do_redirect(skb) == -EAGAIN) {
 			__skb_pull(skb, skb->mac_len);
 			*another = true;
diff --git a/net/sched/Kconfig b/net/sched/Kconfig
index a3b37d88800e..a1bbaa8fd054 100644
--- a/net/sched/Kconfig
+++ b/net/sched/Kconfig
@@ -384,6 +384,7 @@ config NET_SCH_INGRESS
 	depends on NET_CLS_ACT
 	select NET_INGRESS
 	select NET_EGRESS
+	select NET_REDIRECT
 	help
 	  Say Y here if you want to use classifiers for incoming and/or outgoing
 	  packets. This qdisc doesn't do anything else besides running classifiers,

[PATCH bpf-next V6 7/7] selftests/bpf: use bpf_check_mtu in selftest test_cls_redirect

[Jesper Dangaard Brouer]

This demonstrate how bpf_check_mtu() helper can easily be used together
with bpf_skb_adjust_room() helper, prior to doing size adjustment, as
delta argument is already setup.

Hint: This specific test can be selected like this:
 ./test_progs -t cls_redirect

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 .../selftests/bpf/progs/test_cls_redirect.c        |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/test_cls_redirect.c b/tools/testing/selftests/bpf/progs/test_cls_redirect.c
index c9f8464996ea..3c1e042962e6 100644
--- a/tools/testing/selftests/bpf/progs/test_cls_redirect.c
+++ b/tools/testing/selftests/bpf/progs/test_cls_redirect.c
@@ -70,6 +70,7 @@ typedef struct {
 	uint64_t errors_total_encap_adjust_failed;
 	uint64_t errors_total_encap_buffer_too_small;
 	uint64_t errors_total_redirect_loop;
+	uint64_t errors_total_encap_mtu_violate;
 } metrics_t;
 
 typedef enum {
@@ -407,6 +408,7 @@ static INLINING ret_t forward_with_gre(struct __sk_buff *skb, encap_headers_t *e
 		payload_off - sizeof(struct ethhdr) - sizeof(struct iphdr);
 	int32_t delta = sizeof(struct gre_base_hdr) - encap_overhead;
 	uint16_t proto = ETH_P_IP;
+	uint32_t mtu_len = 0;
 
 	/* Loop protection: the inner packet's TTL is decremented as a safeguard
 	 * against any forwarding loop. As the only interesting field is the TTL
@@ -479,6 +481,11 @@ static INLINING ret_t forward_with_gre(struct __sk_buff *skb, encap_headers_t *e
 		}
 	}
 
+	if (bpf_check_mtu(skb, skb->ifindex, &mtu_len, delta, 0)) {
+		metrics->errors_total_encap_mtu_violate++;
+		return TC_ACT_SHOT;
+	}
+
 	if (bpf_skb_adjust_room(skb, delta, BPF_ADJ_ROOM_NET,
 				BPF_F_ADJ_ROOM_FIXED_GSO |
 				BPF_F_ADJ_ROOM_NO_CSUM_RESET) ||