linux-kernel - Re: [PATCH v3] mm/hugetlb: Fix unsigned overflow in __nr_hugepages_store

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Message-ID: <9258d571-26d0-f9ad-a60e-0449ac8dd5f9@oracle.com>
Date:   Fri, 22 Feb 2019 10:56:36 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Jing Xiangfeng <jingxiangfeng@...wei.com>, mhocko@...nel.org,
        akpm@...ux-foundation.org
Cc:     hughd@...gle.com, linux-mm@...ck.org, n-horiguchi@...jp.nec.com,
        aarcange@...hat.com, kirill.shutemov@...ux.intel.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/hugetlb: Fix unsigned overflow in
 __nr_hugepages_store_common()

On 2/22/19 6:01 AM, Jing Xiangfeng wrote:
Thanks, just a couple small changes.

> User can change a node specific hugetlb count. i.e.
> /sys/devices/system/node/node1/hugepages/hugepages-2048kB

Please make that,
/sys/devices/system/node/node1/hugepages/hugepages-2048kB/nr_hugepages

> the calculated value of count is a total number of huge pages. It could
> be overflow when a user entering a crazy high value. If so, the total
> number of huge pages could be a small value which is not user expect.
> We can simply fix it by setting count to ULONG_MAX, then it goes on. This
> may be more in line with user's intention of allocating as many huge pages
> as possible.
> 
> Signed-off-by: Jing Xiangfeng <jingxiangfeng@...wei.com>
> ---
>  mm/hugetlb.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index afef616..18fa7d7 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2423,7 +2423,10 @@ static ssize_t __nr_hugepages_store_common(bool obey_mempolicy,
>  		 * per node hstate attribute: adjust count to global,
>  		 * but restrict alloc/free to the specified node.
>  		 */
> +		unsigned long old_count = count;
>  		count += h->nr_huge_pages - h->nr_huge_pages_node[nid];

Also, adding a comment here about checking for overflow would help people
reading the code.  Something like,

		/*
		 * If user specified count causes overflow, set to
		 * largest possible value.
		 */

-- 
Mike Kravetz

> +		if (count < old_count)
> +			count = ULONG_MAX;
>  		init_nodemask_of_node(nodes_allowed, nid);
>  	} else
>  		nodes_allowed = &node_states[N_MEMORY];
>