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

[Stefano Stabellini]

On Tue, 27 Nov 2018, Boris Ostrovsky wrote:
> 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@xxxxxxx>
> >>>>>> ---
> >>>>>>  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.

Right, I think it is fine too.

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

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