[bpf-next,V16,1/7] bpf: Remove MTU check in __bpf_skb_max_len

Message ID 161287788936.790810.2937823995775097177.stgit@firesoul
State New
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  • [bpf-next,V16,1/7] bpf: Remove MTU check in __bpf_skb_max_len
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Commit Message

Jesper Dangaard Brouer Feb. 9, 2021, 1:38 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.

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;