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Subject: Re: [PATCH] ipc/util.c: sysvipc_find_ipc() incorrectly updates
 position index
To:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, Waiman Long <longman@redhat.com>,
        Andreas Schwab <schwab@suse.de>
References: <ad24ee52-21a8-3ff0-e968-63d315472b91@virtuozzo.com>
 <4921fe9b-9385-a2b4-1dc4-1099be6d2e39@virtuozzo.com>
 <20200507170242.6cbb88ae672deed67152e221@linux-foundation.org>
 <20200508033625.GO16070@bombadil.infradead.org>
From:   Vasily Averin <vvs@virtuozzo.com>
Message-ID: <e25e5a42-159c-6bad-4f14-d70461a78b72@virtuozzo.com>
Date:   Fri, 8 May 2020 09:07:59 +0300
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On 5/8/20 6:36 AM, Matthew Wilcox wrote:
> On Thu, May 07, 2020 at 05:02:42PM -0700, Andrew Morton wrote:
>> Here's how I resolved things.  Please check?
>>
>> static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
>> 					      loff_t *new_pos)
>> {
>> 	unsigned long index = pos;
>> 	struct kern_ipc_perm *ipc;
>>
>> 	rcu_read_lock();
>> 	ipc = xa_find(&ids->ipcs, &index, ULONG_MAX, XA_PRESENT);
>> 	if (ipc)
>> 		ipc_lock_object(ipc);
>> 	else
>> 		rcu_read_unlock();
>> 	*new_pos = pos + 1;
>> 	return ipc;
>> }
> 
> Surely that should be '*new_pos = index + 1'?  Or did I misunderstand
> the reasoning behind the other patch?

I'm not sure however it looks like xa_find() can return index < pos
xa_find in our case will call xas_find_marked() that have following description

 * If no marked entry is found and the array is smaller than @max, @xas is
 * set to the bounds state and xas->xa_index is set to the smallest index
 * not yet in the array.  This allows @xas to be immediately passed to
 * xas_store().

Matthew, could you please clarify this question?

Thank you,
	Vasily Averin