netdev - RE: [PATCH net] net: read sk->sk_family once in sk_mc

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Date: Mon, 28 Aug 2023 14:21:47 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Eric Dumazet' <edumazet@...gle.com>, "David S . Miller"
	<davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>, syzbot
	<syzkaller@...glegroups.com>
Subject: RE: [PATCH net] net: read sk->sk_family once in sk_mc_loop()

From: Eric Dumazet
> Sent: 28 August 2023 12:31
> 
> syzbot is playing with IPV6_ADDRFORM quite a lot these days,
> and managed to hit the WARN_ON_ONCE(1) in sk_mc_loop()
> 
> We have many more similar issues to fix.

Is it worth revisiting the use of volatile?
If all accesses to a field have to be marked READ_ONCE()
and WRITE_ONCE() then isn't that just 'volatile'?

IIRC READ_ONCE() (well ACCESS_ONCE()) was originally only
used to stop the compiler re-reading a value.
The current code also worries about the compiler generating
non-atomic read/write (even to a 32bit word).
So typically all references end up being annotated.

	David

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