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

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Date:	Sat, 20 Sep 2014 12:55:56 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	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

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.



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