linux-kernel - Re: [Xen-devel] [PATCH] pvcalls-front: fixes incorrect error handling

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Date:   Tue, 27 Nov 2018 17:47:21 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     PanBian <bianpan2016@....com>, Juergen Gross <jgross@...e.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] pvcalls-front: fixes incorrect error handling

On 11/27/18 4:08 PM, Stefano Stabellini wrote:
> On Tue, 27 Nov 2018, Boris Ostrovsky wrote:
>> On 11/27/18 3:37 PM, Stefano Stabellini wrote:
>>> On Tue, 27 Nov 2018, PanBian wrote:
>>>> On Mon, Nov 26, 2018 at 03:31:39PM -0500, Boris Ostrovsky wrote:
>>>>> On 11/21/18 9:07 PM, Pan Bian wrote:
>>>>>> kfree() is incorrectly used to release the pages allocated by
>>>>>> __get_free_page() and __get_free_pages(). Use the matching deallocators
>>>>>> i.e., free_page() and free_pages(), respectively.
>>>>>>
>>>>>> Signed-off-by: Pan Bian <bianpan2016@....com>
>>>>>> ---
>>>>>>  drivers/xen/pvcalls-front.c | 4 ++--
>>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
>>>>>> index 2f11ca7..77224d8 100644
>>>>>> --- a/drivers/xen/pvcalls-front.c
>>>>>> +++ b/drivers/xen/pvcalls-front.c
>>>>>> @@ -385,8 +385,8 @@ static int create_active(struct sock_mapping *map, int *evtchn)
>>>>>>  out_error:
>>>>>>  	if (*evtchn >= 0)
>>>>>>  		xenbus_free_evtchn(pvcalls_front_dev, *evtchn);
>>>>>> -	kfree(map->active.data.in);
>>>>>> -	kfree(map->active.ring);
>>>>>> +	free_pages((unsigned long)map->active.data.in, PVCALLS_RING_ORDER);
>>>>> Is map->active.data.in guaranteed to be NULL when entering this routine?
>>>> I am not sure yet. Sorry for that. I observed the mismatches between
>>>> __get_free_page and kfree, and submitted the patch.
>>>>
>>>> But I think your consideration is reasonable. A better solution is to
>>>> directly free bytes, a local variable that holds __get_free_pages return
>>>> value. If you agree, I will rewrite the patch.
>>> Like Boris said, map->active.ring and map->active.data.in are not
>>> guaranteed to be NULL or != NULL here. For instance,map->active.ring can
>>> be != NULL and map->active.data.in can be NULL. However, free_pages and
>>> free_page should be able to cope with it, the same way that kfree is
>>> able to cope with it?
>> If map->active.data.in can be non-NULL on entry to this routine then I
>> think this has been a problem all along. Pan's suggestion to use bytes
>> for freeing is going to solve this (assuming bytes will be initialized
>> to NULL).
> Why is it a problem? map->active.data.in and map->active.ring are only
> != NULL if they need to be freed. Otherwise, they are NULL. 

That was my question --- I wasn't sure about it, and I read your
previous message as if it was possible to be calling create_active()
with map->active.data.in pointing somewhere non-NULL.

If it is NULL *upon entry* to calling_create() then Pan's original patch
is fine.


-boris


> All structs
> are always initialized to zero. I don't think there are any issues.