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

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Date:	Sat, 20 Sep 2014 13:19:52 -0700
From:	Joe Perches <joe@...ches.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Yuchung Cheng <ycheng@...gle.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 12:55 -0700, Eric Dumazet wrote:
> 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);

OK.

I think an explicit cast to unsigned long after the min_t
to avoid the implicit downcast would be better

	return (unsigned long)min_t(etc...)

so that no warning is produced if someone does make W=3


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