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
Subject: RE: [PATCH bpf-next V4 1/5] bpf: Remove MTU check in __bpf_skb_max_len
Date: Fri, 30 Oct 2020 12:24:21 -0700 [thread overview]
Message-ID: <5f9c6865b0e4e_16d42081b@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <160381601014.1435097.12501509708690649646.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.
>
> Patch V4 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:[~2020-10-30 19:24 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-27 16:26 [PATCH bpf-next V4 0/5] bpf: New approach for BPF MTU handling Jesper Dangaard Brouer
2020-10-27 16:26 ` [PATCH bpf-next V4 1/5] bpf: Remove MTU check in __bpf_skb_max_len Jesper Dangaard Brouer
2020-10-30 19:24 ` John Fastabend [this message]
2020-10-27 16:26 ` [PATCH bpf-next V4 2/5] bpf: bpf_fib_lookup return MTU value as output when looked up Jesper Dangaard Brouer
2020-10-27 17:15 ` David Ahern
2020-10-30 17:01 ` Jesper Dangaard Brouer
2020-10-28 12:49 ` Dan Carpenter
2020-10-30 14:35 ` Jesper Dangaard Brouer
2020-10-27 16:27 ` [PATCH bpf-next V4 3/5] bpf: add BPF-helper for MTU checking Jesper Dangaard Brouer
2020-10-27 16:27 ` [PATCH bpf-next V4 4/5] bpf: drop MTU check when doing TC-BPF redirect to ingress Jesper Dangaard Brouer
2020-10-27 16:27 ` [PATCH bpf-next V4 5/5] bpf: make it possible to identify BPF redirected SKBs Jesper Dangaard Brouer
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