Re: [PATCH net v2 2/3] selftests: net: tuntap: fix UDP checksum length in build_udp_packet_csum()

[sashiko-bot@kernel.org]

> build_udp_packet_csum() passes udph->len, which is in network byte
> order, as the number of bytes to checksum. On little-endian hosts this
> reads up to 64KB past the UDP header and computes a wrong checksum.
> 
> Existing callers did not crash only because they use packet buffers
> of MAX_VNET_TUNNEL_PACKET_SZ bytes.
> 
> Use ntohs(udph->len), as tap.c does in its own build_udp_packet().
> 
> While at it, clear udph->check before summing instead of relying on
> the caller, and send 0xffff instead of a computed zero checksum, as
> the kernel does with CSUM_MANGLED_0.
> 
> Fixes: a942fcd72e97 ("selftest: tun: Introduce tuntap_helpers.h header for TUN/TAP testing")
> Reviewed-by: Willem de Bruijn <willemb@google.com>
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>

Sashiko has reviewed this patch and found no issues.

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