netdev - RE: [PATCH net-next] tcp: avoid possible arithmetic overflows

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Date:	Mon, 22 Sep 2014 11:13:49 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>,
	Yuchung Cheng <ycheng@...gle.com>
CC:	Joe Perches <joe@...ches.com>, David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: RE: [PATCH net-next] tcp: avoid possible arithmetic overflows

From: Eric Dumazet
> On Sat, 2014-09-20 at 12:46 -0700, Yuchung Cheng wrote:
> > On Sat, Sep 20, 2014 at 11:01 AM, Joe Perches <joe@...ches.com> wrote:
> > > On Sat, 2014-09-20 at 10:19 -0700, Eric Dumazet wrote:
> > >> From: Eric Dumazet <edumazet@...gle.com>
> > >>
> > >> icsk_rto is an 32bit field, and icsk_backoff can reach 15 by default,
> > >> or more if some sysctl (eg tcp_retries2) are changed.
> > >>
> > >> Better use 64bit to perform icsk_rto << icsk_backoff operations
> > >
> > > Maybe better to use a helper function for this?
> > >
> > > something like:
> > >
> > > static inline u64 icsk_rto_backoff(const struct inet_connection_sock *icsk)
> > > {
> > >         u64 when = (u64)icsk->icsk_rto;
> > >
> > >         return when << icsk->icsk_backoff;
> > > }
> > Thanks for the fix Eric. I second Joe's idea to use a helper function.
> >
> 
> Yep.
> 
> Given the timeout functions in the kernel use 'unsigned long', I prefer
> to keep the u64 magic private to this helper.
> 
> I will probably use
> 
> static inline unsigned long icsk_rto_backoff(const struct inet_connection_sock *icsk)
> {
> 	u64 when = (u64)icsk->icsk_rto << icsk->icsk_backoff;
> 
> 	return min_t(u64, when, ~0UL);
> }
> 
> On 64bit arches, the min_t() should be a nop.

Isn't it likely to generate a 'condition is always true/false' warning?
(that may depend on whether min_t contains < or <=)

	David