netdev - RE: [PATCH bpf-next V4 1/5] bpf: Remove MTU check in __bpf_skb_max

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Message-ID: <5f9c6865b0e4e_16d42081b@john-XPS-13-9370.notmuch>
Date:   Fri, 30 Oct 2020 12:24:21 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, bpf@...r.kernel.org
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
        Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com
Subject: RE: [PATCH bpf-next V4 1/5] bpf: Remove MTU check in
 __bpf_skb_max_len

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@...hat.com>
> ---

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