SCTP GSO skbs have a gso_size of GSO_BY_FRAGS, so any sort of
unconditionally mangling of that will result in nonsense value
and would corrupt the skb later on.

Therefore, i) add two helpers skb_increase_gso_size() and
skb_decrease_gso_size() that would throw a one time warning and
bail out for such skbs and ii) refuse and return early with an
error in those BPF helpers that are affected. We do need to bail
out as early as possible from there before any changes on the
skb have been performed.

Fixes: 6578171a ("bpf: add bpf_skb_change_proto helper")
Co-authored-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NDaniel Axtens <dja@axtens.net>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>

Most of this is extracted from 90017acc ("sctp: Add GSO support"),
with some extra text about GSO_BY_FRAGS and the need to check for it.

Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NDaniel Axtens <dja@axtens.net>
Acked-by: NMarcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

The doc originally called it SKB_GSO_REMCSUM. Fix it.

Fixes: f7a6272b ("Documentation: Add documentation for TSO and GSO features")
Signed-off-by: NDaniel Axtens <dja@axtens.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

UFO is deprecated except for tuntap and packet per 0c19f846,
("net: accept UFO datagrams from tuntap and packet"). Update UFO
docs to reflect this.
Signed-off-by: NDaniel Axtens <dja@axtens.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Those enum values don't exist anymore.

Fixes: 7e13318d ("net: define gso types for IPx over IPv4 and IPv6")
CC: Tom Herbert <tom@herbertland.com>
Signed-off-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This document is a starting point for defining the TSO and GSO features.
The whole thing is starting to get a bit messy so I wanted to make sure we
have notes somwhere to start describing what does and doesn't work.
Signed-off-by: NAlexander Duyck <aduyck@mirantis.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>