netdev - Re: [PATCH v2] rps: fix insufficient bounds checking in store_rps_dev_flow_table

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Date:	Fri, 23 Dec 2011 15:53:51 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Xi Wang <xi.wang@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2] rps: fix insufficient bounds checking in
 store_rps_dev_flow_table_cnt()

Le vendredi 23 décembre 2011 à 09:30 -0500, Xi Wang a écrit :
> On Dec 23, 2011, at 8:06 AM, Eric Dumazet wrote:
> 
> > I'll submit following patch for net-next, once your patch is in this
> > tree.
> 
> Thanks for doing this. ;-)
> 
> > 		count = roundup_pow_of_two(count);
> > +		if (!count ||
> > +		    count != (unsigned long)(u32)count)
> > +			return -EINVAL;
> > 		if (count > (ULONG_MAX - sizeof(struct rps_dev_flow_table))
> > 				/ sizeof(struct rps_dev_flow)) {
> > 			/* Enforce a limit to prevent overflow */
> 
> I would rather avoid undefined behavior in C.
> 

Unsigned arithmetics is well defined in C. Very well in fact.


> Given count = ULONG_MAX on 64-bit systems, roundup_pow_of_two()
> would overflow, and the overflowed result is undefined, e.g.,
> on x86-64 it gives 1, not 0.  That's why I used INT_MAX.
> 

I'll check roundup_pow_of_two(), this seems a bug to me.

Problem is : INT_MAX doesnt allow the full range of rps : 2^32 flows.

> BTW, (count > UINT_MAX) is shorter and more easier to understand
> than (count != (unsigned long)(u32)count).

You miss the point. UINT_MAX is too small for 64bit arches.

if (count != (unsigned long)(u32)count)

is pretty common stuff in kernel.

Check _kstrtoul() for an example.



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