Re: [PATCH] ipc/util.c: sysvipc_find_ipc() incorrectly updates position index

[Vasily Averin <vvs@virtuozzo.com>]

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