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From: John Fastabend <john.fastabend@gmail.com>
To: Jesper Dangaard Brouer <brouer@redhat.com>, 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: RE: [PATCH bpf-next V11 1/7] bpf: Remove MTU check in __bpf_skb_max_len
Date: Thu, 14 Jan 2021 00:18:20 -0800
Message-ID: <5ffffe4ca10bd_1eeef208f5@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <161047350559.4003084.17398867215317668954.stgit@firesoul>

Jesper Dangaard Brouer wrote:
> 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>
> ---

Acked-by: John Fastabend <john.fastabend@gmail.com>

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-12 17:45 [PATCH bpf-next V11 0/7] bpf: New approach for BPF MTU handling Jesper Dangaard Brouer
2021-01-12 17:45 ` [PATCH bpf-next V11 1/7] bpf: Remove MTU check in __bpf_skb_max_len Jesper Dangaard Brouer
2021-01-14  8:18   ` John Fastabend [this message]
2021-01-12 17:45 ` [PATCH bpf-next V11 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx Jesper Dangaard Brouer
2021-01-12 17:45 ` [PATCH bpf-next V11 3/7] bpf: bpf_fib_lookup return MTU value as output when looked up Jesper Dangaard Brouer
2021-01-12 17:45 ` [PATCH bpf-next V11 4/7] bpf: add BPF-helper for MTU checking Jesper Dangaard Brouer
2021-01-12 19:23   ` Andrii Nakryiko
2021-01-14 14:52     ` Jesper Dangaard Brouer
2021-01-14 15:33       ` Yonghong Song
2021-01-13 23:07   ` Daniel Borkmann
2021-01-14 14:36     ` Jesper Dangaard Brouer
2021-01-14 22:28       ` Daniel Borkmann
2021-01-18 11:04         ` Jesper Dangaard Brouer
2021-01-12 17:45 ` [PATCH bpf-next V11 5/7] bpf: drop MTU check when doing TC-BPF redirect to ingress Jesper Dangaard Brouer
2021-01-14  9:03   ` John Fastabend
2021-01-14 16:14     ` Jesper Dangaard Brouer
2021-01-12 17:45 ` [PATCH bpf-next V11 6/7] selftests/bpf: use bpf_check_mtu in selftest test_cls_redirect Jesper Dangaard Brouer
2021-01-12 17:45 ` [PATCH bpf-next V11 7/7] bpf/selftests: tests using bpf_check_mtu BPF-helper Jesper Dangaard Brouer
2021-01-12 19:29   ` Andrii Nakryiko

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