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Date: Tue, 9 Jan 2018 12:00:14 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: Jan Kara <jack@...e.cz>
CC: "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
"tytso@....edu" <tytso@....edu>,
"peterz@...radead.org" <peterz@...radead.org>,
"keescook@...omium.org" <keescook@...omium.org>
Subject: RE: [PATCH] mbcache: convert mb_cache_entry.e_refcnt from atomic_t
to refcount_t
> On Wed 29-11-17 13:22:20, Elena Reshetova wrote:
> > atomic_t variables are currently used to implement reference
> > counters with the following properties:
> > - counter is initialized to 1 using atomic_set()
> > - a resource is freed upon counter reaching zero
> > - once counter reaches zero, its further
> > increments aren't allowed
> > - counter schema uses basic atomic operations
> > (set, inc, inc_not_zero, dec_and_test, etc.)
> >
> > Such atomic variables should be converted to a newly provided
> > refcount_t type and API that prevents accidental counter overflows
> > and underflows. This is important since overflows and underflows
> > can lead to use-after-free situation and be exploitable.
> >
> > The variable mb_cache_entry.e_refcnt is used as pure reference counter.
> > Convert it to refcount_t and fix up the operations.
> >
> > **Important note for maintainers:
> >
> > Some functions from refcount_t API defined in lib/refcount.c
> > have different memory ordering guarantees than their atomic
> > counterparts.
> > The full comparison can be seen in
> > https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
> > in state to be merged to the documentation tree.
> > Normally the differences should not matter since refcount_t provides
> > enough guarantees to satisfy the refcounting use cases, but in
> > some rare cases it might matter.
> > Please double check that you don't have some undocumented
> > memory guarantees for this variable usage.
> >
> > For the mb_cache_entry.e_refcnt it might make a difference
> > in following places:
> > - mb_cache_destroy(), mb_cache_shrink() and mb_cache_entry_delete():
> > decrement in refcount_dec() provides RELEASE ordering vs. fully
> > unordered atomic counterpart. Since the change is for better, it
> > should not matter for these cases.
> > - mb_cache_entry_put(): decrement in refcount_dec_and_test() only
> > provides RELEASE ordering and control dependency on success
> > vs. fully ordered atomic counterpart
> >
> > Suggested-by: Kees Cook <keescook@...omium.org>
> > Reviewed-by: David Windsor <dwindsor@...il.com>
> > Reviewed-by: Hans Liljestrand <ishkamiel@...il.com>
> > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
>
> The patch looks good to me. You can add:
>
> Reviewed-by: Jan Kara <jack@...e.cz>
Thank you Jan!
Would you be able to propagate the patch further into the respective tree?
These patches are best to merge using usual paths.
Best Regards,
Elena.
>
>
> Honza
>
> > ---
> > fs/mbcache.c | 16 ++++++++--------
> > include/linux/mbcache.h | 6 +++---
> > 2 files changed, 11 insertions(+), 11 deletions(-)
> >
> > diff --git a/fs/mbcache.c b/fs/mbcache.c
> > index d818fd2..8bdb52b 100644
> > --- a/fs/mbcache.c
> > +++ b/fs/mbcache.c
> > @@ -90,7 +90,7 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t
> mask, u32 key,
> >
> > INIT_LIST_HEAD(&entry->e_list);
> > /* One ref for hash, one ref returned */
> > - atomic_set(&entry->e_refcnt, 1);
> > + refcount_set(&entry->e_refcnt, 1);
> > entry->e_key = key;
> > entry->e_value = value;
> > entry->e_reusable = reusable;
> > @@ -109,7 +109,7 @@ int mb_cache_entry_create(struct mb_cache *cache,
> gfp_t mask, u32 key,
> > spin_lock(&cache->c_list_lock);
> > list_add_tail(&entry->e_list, &cache->c_list);
> > /* Grab ref for LRU list */
> > - atomic_inc(&entry->e_refcnt);
> > + refcount_inc(&entry->e_refcnt);
> > cache->c_entry_count++;
> > spin_unlock(&cache->c_list_lock);
> >
> > @@ -141,7 +141,7 @@ static struct mb_cache_entry *__entry_find(struct
> mb_cache *cache,
> > entry = hlist_bl_entry(node, struct mb_cache_entry,
> > e_hash_list);
> > if (entry->e_key == key && entry->e_reusable) {
> > - atomic_inc(&entry->e_refcnt);
> > + refcount_inc(&entry->e_refcnt);
> > goto out;
> > }
> > node = node->next;
> > @@ -204,7 +204,7 @@ struct mb_cache_entry *mb_cache_entry_get(struct
> mb_cache *cache, u32 key,
> > hlist_bl_lock(head);
> > hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
> > if (entry->e_key == key && entry->e_value == value) {
> > - atomic_inc(&entry->e_refcnt);
> > + refcount_inc(&entry->e_refcnt);
> > goto out;
> > }
> > }
> > @@ -239,7 +239,7 @@ void mb_cache_entry_delete(struct mb_cache *cache,
> u32 key, u64 value)
> > if (!list_empty(&entry->e_list)) {
> > list_del_init(&entry-
> >e_list);
> > cache->c_entry_count--;
> > - atomic_dec(&entry-
> >e_refcnt);
> > + refcount_dec(&entry-
> >e_refcnt);
> > }
> > spin_unlock(&cache->c_list_lock);
> > mb_cache_entry_put(cache, entry);
> > @@ -300,7 +300,7 @@ static unsigned long mb_cache_shrink(struct mb_cache
> *cache,
> > hlist_bl_lock(head);
> > if (!hlist_bl_unhashed(&entry->e_hash_list)) {
> > hlist_bl_del_init(&entry->e_hash_list);
> > - atomic_dec(&entry->e_refcnt);
> > + refcount_dec(&entry->e_refcnt);
> > }
> > hlist_bl_unlock(head);
> > if (mb_cache_entry_put(cache, entry))
> > @@ -397,11 +397,11 @@ void mb_cache_destroy(struct mb_cache *cache)
> > list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
> > if (!hlist_bl_unhashed(&entry->e_hash_list)) {
> > hlist_bl_del_init(&entry->e_hash_list);
> > - atomic_dec(&entry->e_refcnt);
> > + refcount_dec(&entry->e_refcnt);
> > } else
> > WARN_ON(1);
> > list_del(&entry->e_list);
> > - WARN_ON(atomic_read(&entry->e_refcnt) != 1);
> > + WARN_ON(refcount_read(&entry->e_refcnt) != 1);
> > mb_cache_entry_put(cache, entry);
> > }
> > kfree(cache->c_hash);
> > diff --git a/include/linux/mbcache.h b/include/linux/mbcache.h
> > index 20f1e3f..0bf2af6 100644
> > --- a/include/linux/mbcache.h
> > +++ b/include/linux/mbcache.h
> > @@ -5,7 +5,7 @@
> > #include <linux/hash.h>
> > #include <linux/list_bl.h>
> > #include <linux/list.h>
> > -#include <linux/atomic.h>
> > +#include <linux/refcount.h>
> > #include <linux/fs.h>
> >
> > struct mb_cache;
> > @@ -15,7 +15,7 @@ struct mb_cache_entry {
> > struct list_head e_list;
> > /* Hash table list - protected by hash chain bitlock */
> > struct hlist_bl_node e_hash_list;
> > - atomic_t e_refcnt;
> > + refcount_t e_refcnt;
> > /* Key in hash - stable during lifetime of the entry */
> > u32 e_key;
> > u32 e_referenced:1;
> > @@ -33,7 +33,7 @@ void __mb_cache_entry_free(struct mb_cache_entry
> *entry);
> > static inline int mb_cache_entry_put(struct mb_cache *cache,
> > struct
> mb_cache_entry *entry)
> > {
> > - if (!atomic_dec_and_test(&entry->e_refcnt))
> > + if (!refcount_dec_and_test(&entry->e_refcnt))
> > return 0;
> > __mb_cache_entry_free(entry);
> > return 1;
> > --
> > 2.7.4
> >
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
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