netdev - Re: [PATCH net] Revert "net: macsec: use skb_ensure_writable_head

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Date: Tue, 16 Jan 2024 14:51:19 +0100
From: Sabrina Dubroca <sd@...asysnail.net>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Rahul Rameshbabu <rrameshbabu@...dia.com>, netdev@...r.kernel.org,
	Gal Pressman <gal@...dia.com>, Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Radu Pirea <radu-nicolae.pirea@....nxp.com>,
	"David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] Revert "net: macsec: use
 skb_ensure_writable_head_tail to expand the skb"

2024-01-16, 11:39:35 +0100, Paolo Abeni wrote:
> On Sun, 2024-01-14 at 09:42 -0800, Rahul Rameshbabu wrote:
> > This reverts commit b34ab3527b9622ca4910df24ff5beed5aa66c6b5.
> > 
> > Using skb_ensure_writable_head_tail without a call to skb_unshare causes
> > the MACsec stack to operate on the original skb rather than a copy in the
> > macsec_encrypt path. This causes the buffer to be exceeded in space, and
> > leads to warnings generated by skb_put operations. 
> 
> This part of the changelog is confusing to me. It looks like the skb
> should be uncloned under the same conditions before and after this
> patch (and/or the reverted)??!

I don't think so. The old code was doing unshare +
expand. skb_ensure_writable_head_tail calls pskb_expand_head without
unshare, which doesn't give us a fresh sk_buff, only takes care of the
headroom/tailroom. Or do I need more coffee? :/

> Possibly dev->needed_headroom/needed_tailroom values are incorrect?!?

That's also possible following commit a73d8779d61a ("net: macsec:
introduce mdo_insert_tx_tag"). Then this revert would only be hiding
the issue.

-- 
Sabrina