LinuxLists.cc - [PATCH 3/3] xen/blkback: Use refcount

2019-08-08 13:13:08

Subject: [PATCH 3/3] xen/blkback: Use refcount_t for refcount

Reference counters are preferred to use refcount_t instead of
atomic_t.
This is because the implementation of refcount_t can prevent
overflows and detect possible use-after-free.
So convert atomic_t ref counters to refcount_t.

Signed-off-by: Chuhong Yuan <[email protected]>
---
drivers/block/xen-blkback/common.h | 7 ++++---
drivers/block/xen-blkback/xenbus.c | 2 +-
2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 1d3002d773f7..9db5f3586fb4 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -35,6 +35,7 @@
#include <linux/wait.h>
#include <linux/io.h>
#include <linux/rbtree.h>
+#include <linux/refcount.h>
#include <asm/setup.h>
#include <asm/pgalloc.h>
#include <asm/hypervisor.h>
@@ -309,7 +310,7 @@ struct xen_blkif {
struct xen_vbd vbd;
/* Back pointer to the backend_info. */
struct backend_info *be;
- atomic_t refcnt;
+ refcount_t refcnt;
/* for barrier (drain) requests */
struct completion drain_complete;
atomic_t drain;
@@ -362,10 +363,10 @@ struct pending_req {
(_v)->bdev->bd_part->nr_sects : \
get_capacity((_v)->bdev->bd_disk))

-#define xen_blkif_get(_b) (atomic_inc(&(_b)->refcnt))
+#define xen_blkif_get(_b) (refcount_inc(&(_b)->refcnt))
#define xen_blkif_put(_b) \
do { \
- if (atomic_dec_and_test(&(_b)->refcnt)) \
+ if (refcount_dec_and_test(&(_b)->refcnt)) \
schedule_work(&(_b)->free_work);\
} while (0)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 3ac6a5d18071..ecc5f9c5bf3f 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -169,7 +169,7 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
return ERR_PTR(-ENOMEM);

blkif->domid = domid;
- atomic_set(&blkif->refcnt, 1);
+ refcount_set(&blkif->refcnt, 1);
init_completion(&blkif->drain_complete);
INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);

--
2.20.1

2019-08-08 13:43:22

by Roger Pau Monne

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Subject: Re: [PATCH 3/3] xen/blkback: Use refcount_t for refcount

On Thu, Aug 08, 2019 at 09:11:00PM +0800, Chuhong Yuan wrote:
> Reference counters are preferred to use refcount_t instead of
> atomic_t.
> This is because the implementation of refcount_t can prevent
> overflows and detect possible use-after-free.
> So convert atomic_t ref counters to refcount_t.

Thanks!

I think there are more reference counters in blkback than
the one you fixed. There's also an inflight field in xen_blkif_ring,
and a pendcnt in pending_req which look like possible candidates to
switch to use refcount_t, have you looked into switching those two
also?

Roger.

2019-08-09 01:26:15

by Chuhong Yuan

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Subject: Re: [PATCH 3/3] xen/blkback: Use refcount_t for refcount

On Thu, Aug 8, 2019 at 9:35 PM Roger Pau Monné <[email protected]> wrote:
>
> On Thu, Aug 08, 2019 at 09:11:00PM +0800, Chuhong Yuan wrote:
> > Reference counters are preferred to use refcount_t instead of
> > atomic_t.
> > This is because the implementation of refcount_t can prevent
> > overflows and detect possible use-after-free.
> > So convert atomic_t ref counters to refcount_t.
>
> Thanks!
>
> I think there are more reference counters in blkback than
> the one you fixed. There's also an inflight field in xen_blkif_ring,
> and a pendcnt in pending_req which look like possible candidates to
> switch to use refcount_t, have you looked into switching those two
> also?
>

I will switch those two in next version.

> Roger.

2019-08-13 08:43:21

by Chuhong Yuan

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