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 [thread overview]
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>
next prev parent reply other threads:[~2021-01-14 8:19 UTC|newest]
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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