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

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Date:	Fri, 23 Dec 2011 09:30:29 -0500
From:	Xi Wang <xi.wang@...il.com>
To:	Eric Dumazet <eric.dumazet@...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()

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.

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.

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

- xi
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