netdev - RE: [PATCH net] tcp: gso: really support BIG TCP

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Message-ID: <b0ecec17f9c1409ba476cf370730278a@AcuMS.aculab.com>
Date: Mon, 5 Jun 2023 13:58:10 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Alexander Duyck' <alexander.duyck@...il.com>, Eric Dumazet
	<edumazet@...gle.com>
CC: Alexander Duyck <alexanderduyck@...a.com>, "David S . Miller"
	<davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "Xin
 Long" <lucien.xin@...il.com>, David Ahern <dsahern@...nel.org>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>
Subject: RE: [PATCH net] tcp: gso: really support BIG TCP

From: Alexander Duyck
> Sent: 02 June 2023 16:39
...
> I wouldn't even bother with any of these changes.
> 
> > @@ -78,7 +78,7 @@ struct sk_buff *tcp_gso_segment(struct sk_buff *skb,
> >         if (!pskb_may_pull(skb, thlen))
> >                 goto out;
> >
> > -       oldlen = (u16)~skb->len;
> > +       oldlen = skb->len;
> >         __skb_pull(skb, thlen);
> >
> >         mss = skb_shinfo(skb)->gso_size;
> 
> As I stated before I would just drop the "(u16)". We are expanding the
> oldlen to a u32, but don't have to worry about it overflowing because
> skb->len should always be greater than or equal to our segmented
> length. So the most it could reach is ~0 if skb->len == segmented
> length.

Have you missed the ~ ?
The (u16) makes it equivalent to skb->len ^ 0xffff.
Otherwise all the high bit are set and any arithmetic will generate
a carry and the wrong value.

FWIW this C version of csum_fold() (eg from arch/arc)
generates better code than the existing versions on pretty
much every architecture.

	return (~sum - ror32(sum, 16)) >> 16;

The main exception will be sparc (which has a carry flag but
no rotate) and arm with its barrel shifter.

	David

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