Re: [PATCH] xarray: Add a BUG_ON() to ensure caller is not sibling

[Dev Jain <dev.jain@arm.com>]

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On 30/05/25 4:17 am, Zi Yan wrote:
> On 28 May 2025, at 23:17, Dev Jain wrote:
>
>> On 28/05/25 10:42 pm, Zi Yan wrote:
>>> On 28 May 2025, at 7:31, Dev Jain wrote:
>>>
>>>> Suppose xas is pointing somewhere near the end of the multi-entry batch.
>>>> Then it may happen that the computed slot already falls beyond the batch,
>>>> thus breaking the loop due to !xa_is_sibling(), and computing the wrong
>>>> order. Thus ensure that the caller is aware of this by triggering a BUG
>>>> when the entry is a sibling entry.
>>> Is it possible to add a test case in lib/test_xarray.c for this?
>>> You can compile the tests with “make -C tools/testing/radix-tree”
>>> and run “./tools/testing/radix-tree/xarray”.
>>
>> Sorry forgot to Cc you.
>> I can surely do that later, but does this patch look fine?
> I am not sure the exact situation you are describing, so I asked you
> to write a test case to demonstrate the issue. :)

I added the following:

diff--git a/lib/test_xarray.c b/lib/test_xarray.c
index 080a39d22e73..44fcc947a5b6100644
--- a/lib/test_xarray.c
+++ b/lib/test_xarray.c
@@ -2111,6+2111,8@@ staticnoinline voidcheck_xas_get_order(structxarray *xa)
rcu_read_lock();
xas_load(&xas);
XA_BUG_ON(xa, xas_get_order(&xas) != order);
+ if(xas.xa_node)
+ XA_BUG_ON(xa, xa_is_sibling(xa_entry(xas.xa, xas.xa_node, 
xas.xa_offset)));
rcu_read_unlock();
}
and the test passes.


>
>>
>>>> This patch is motivated by code inspection and not a real bug report.
>>>>
>>>> Signed-off-by: Dev Jain<dev.jain@arm.com>
>>>> ---
>>>> The patch applies on 6.15 kernel.
>>>>
>>>>    lib/xarray.c | 2 ++
>>>>    1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/lib/xarray.c b/lib/xarray.c
>>>> index 9644b18af18d..0f699766c24f 100644
>>>> --- a/lib/xarray.c
>>>> +++ b/lib/xarray.c
>>>> @@ -1917,6 +1917,8 @@ int xas_get_order(struct xa_state *xas)
>>>>    	if (!xas->xa_node)
>>>>    		return 0;
>>>>
>>>> +	XA_NODE_BUG_ON(xas->xa_node, xa_is_sibling(xa_entry(xas->xa,
>>>> +		       xas->xa_node, xas->xa_offset)));
>>>>    	for (;;) {
>>>>    		unsigned int slot = xas->xa_offset + (1 << order);
>>>>
>>>> -- 
>>>> 2.30.2
>>> Best Regards,
>>> Yan, Zi
>
> Best Regards,
> Yan, Zi

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