netdev - [PATCH bpf-next V2 1/6] bpf: Remove MTU check in __bpf_skb_max

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Date:   Wed, 07 Oct 2020 18:22:40 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     bpf@...r.kernel.org
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
        Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com
Subject: [PATCH bpf-next V2 1/6] bpf: Remove MTU check in __bpf_skb_max_len

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@...hat.com>
(imported from commit 37f8552786cf46588af52b77829b730dd14524d3)
---
 net/core/filter.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

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,