[bpf-next,V2,1/6] bpf: Remove MTU check in __bpf_skb_max_len

Message ID 160208776033.798237.4028465222836713720.stgit@firesoul
State Superseded
Headers show
Series
  • bpf: New approach for BPF MTU handling and enforcement
Related show

Commit Message

Jesper Dangaard Brouer Oct. 7, 2020, 4:22 p.m.
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.

Keep a sanity max limit of IP_MAX_MTU which is 64KiB.

In later patches we will enforce the MTU limitation when transmitting
packets.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
(imported from commit 37f8552786cf46588af52b77829b730dd14524d3)
---
 net/core/filter.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

Comments

Daniel Borkmann Oct. 7, 2020, 9:26 p.m. | #1
On 10/7/20 6:22 PM, Jesper Dangaard Brouer wrote:
[...]
> 

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

> (imported from commit 37f8552786cf46588af52b77829b730dd14524d3)


slipped in?
Maciej ┼╗enczykowski Oct. 7, 2020, 11:46 p.m. | #2
>  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;

> +       return IP_MAX_MTU;

>  }


Shouldn't we just delete this helper instead and replace call sites?

Patch

diff --git a/net/core/filter.c b/net/core/filter.c
index 05df73780dd3..fed239e77bdc 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3476,8 +3476,7 @@  static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
 
 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;
+	return IP_MAX_MTU;
 }
 
 BPF_CALL_4(sk_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,