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From: Jesper Dangaard Brouer <brouer@redhat.com>
To: bpf@vger.kernel.org
Cc: Jesper Dangaard Brouer <brouer@redhat.com>,
	netdev@vger.kernel.org, Daniel Borkmann <borkmann@iogearbox.net>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	maze@google.com, lmb@cloudflare.com, shaun@tigera.io,
	Lorenzo Bianconi <lorenzo@kernel.org>,
	marek@cloudflare.com, John Fastabend <john.fastabend@gmail.com>,
	Jakub Kicinski <kuba@kernel.org>,
	eyal.birger@gmail.com, colrack@gmail.com
Subject: [PATCH bpf-next V16 1/7] bpf: Remove MTU check in __bpf_skb_max_len
Date: Tue, 09 Feb 2021 14:38:09 +0100	[thread overview]
Message-ID: <161287788936.790810.2937823995775097177.stgit@firesoul> (raw)
In-Reply-To: <161287779408.790810.15631860742170694244.stgit@firesoul>

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

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

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.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 e15d4741719a..731571762f8d 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 :
 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;

  reply	other threads:[~2021-02-09 13:40 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-09 13:38 [PATCH bpf-next V16 0/7] bpf: New approach for BPF MTU handling Jesper Dangaard Brouer
2021-02-09 13:38 ` Jesper Dangaard Brouer [this message]
2021-02-09 13:38 ` [PATCH bpf-next V16 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx Jesper Dangaard Brouer
2021-02-09 13:38 ` [PATCH bpf-next V16 3/7] bpf: bpf_fib_lookup return MTU value as output when looked up Jesper Dangaard Brouer
2021-02-09 13:38 ` [PATCH bpf-next V16 4/7] bpf: add BPF-helper for MTU checking Jesper Dangaard Brouer
2021-02-09 13:38 ` [PATCH bpf-next V16 5/7] bpf: drop MTU check when doing TC-BPF redirect to ingress Jesper Dangaard Brouer
2021-02-09 13:38 ` [PATCH bpf-next V16 6/7] selftests/bpf: use bpf_check_mtu in selftest test_cls_redirect Jesper Dangaard Brouer
2021-02-09 13:38 ` [PATCH bpf-next V16 7/7] selftests/bpf: tests using bpf_check_mtu BPF-helper Jesper Dangaard Brouer
2021-02-13  0:20 ` [PATCH bpf-next V16 0/7] bpf: New approach for BPF MTU handling patchwork-bot+netdevbpf

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