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

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Date:   Tue, 27 Nov 2018 12:37:05 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     PanBian <bianpan2016@....com>
cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] pvcalls-front: fixes incorrect error
 handling

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?


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