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

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Message-ID: <0af126ad-1a74-e4c7-d74f-658a46757b9d@oracle.com>
Date:   Tue, 27 Nov 2018 15:57:39 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        PanBian <bianpan2016@....com>
Cc:     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 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).


-boris


>
>>> -boris
>>>
>>>> +	free_page((unsigned long)map->active.ring);
>>>>  	return ret;
>>>>  }
>>>>