linux-kernel - Re: [PATCH 1/8] idr: fix overflow bug for the max-high layer

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Date:	Fri, 18 Apr 2014 12:29:30 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jean Delvare <jdelvare@...e.de>,
	Monam Agarwal <monamagarwal123@...il.com>,
	Jeff Layton <jlayton@...hat.com>,
	Andreas Gruenbacher <agruen@...bit.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH 1/8] idr: fix overflow bug for the max-high layer

Hello,

Subject: idr: fix overflow bug during maximum ID calculation at maximum height

On Fri, Apr 18, 2014 at 08:49:48PM +0800, Lai Jiangshan wrote:
> In idr_replace(), when the top layer is the max-high layer(p->layer == 3),
> The "(1 << n)" will overflow and the result is 0, it causes idr_replace()
> return -EINVAL even the id is actually valid.

  idr_replace() open-codes the logic to calculate the maximum valid ID
  given the height of the idr tree; unfortunately, the open-coded logic
  doesn't account for the fact that the top layer may have unused slots
  and over-shifts the limit to zero when the tree is at its maximum
  height.

> The following test code shows it fails to replace the value for id=((1<<27)+42):
> 
> static void test5(void)
> {
> 	int id;
> 	DEFINE_IDR(test_idr);
> #define TEST5_START ((1<<27)+42) /* use the highest layer */
> 
> 	printk(KERN_INFO "Start test5\n");
> 	id = idr_alloc(&test_idr, (void *)1, TEST5_START, 0, GFP_KERNEL);
> 	BUG_ON(id != TEST5_START);
> 	TEST_BUG_ON(idr_replace(&test_idr, (void *)2, TEST5_START) != (void *)1);
> 	idr_destroy(&test_idr);
> 	printk(KERN_INFO "End of test5\n");
> }
> 
> Fixed the bug by using idr_max() instead of the incorrect open code.

  Fix the bug by using idr_max() which correctly takes into account the
  maximum allowed shift.

> There is the same problem in sub_alloc(). The overflow causes sub_alloc()
> returns -EAGAIN unexpectedly. But the idr_get_empty_slot() will call it
> again with increased @id. So the bug is hided.

  sub_alloc() shares the same problem and may incorrectly fail with
  -EAGAIN; however, this bug doesn't affect correct operation because
  idr_get_empty_slot(), which already uses idr_max(), retries with the
  increased @id in such cases.

> CC: stable@...r.kernel.org
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>

 Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun
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