netdev - ndo_star_xmit() - resevered head and tailroom

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Message-ID: <20180601161432.ro2lop4tz6hc43pv@x220t>
Date:   Fri, 1 Jun 2018 12:14:52 -0400
From:   Alexander Aring <aring@...atatu.com>
To:     netdev@...r.kernel.org
Cc:     stefan@....samsung.com, linux-wpan@...r.kernel.org,
        kernel@...atatu.com
Subject: ndo_star_xmit() - resevered head and tailroom

Hi netdev community,

I am again on bug fixing in 6lowpan branch and thought that my
needed_headroom and needed_tailroom of net_device are available inside
my ndo_start_xmit() callback.

In case of a UDP socket, it was not the case. I send a fix now to use
skb_expand_copy() to make sure this space is available. (I need to
unshare the buffer anyway).

I am just curious is this suppose to work like that? I cannot believe
that I need to run a realloc() inside my ndo_start_xmit(). If possible,
when the net_device is known at socket layer it should allocate the
necessary space and I thought this is the designed transmit flow.

- Alex