linux-kernel - [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting

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Date:   Fri, 15 Jan 2021 15:57:22 +0100
From:   Alexey Gladkov <gladkov.alexey@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>, io-uring@...r.kernel.org,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-mm@...ck.org
Cc:     Alexey Gladkov <legion@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting

Signed-off-by: Alexey Gladkov <gladkov.alexey@...il.com>
---
 include/linux/user_namespace.h |  2 +-
 kernel/ucount.c                | 20 +++++++-------------
 2 files changed, 8 insertions(+), 14 deletions(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 64cf8ebdc4ec..f84fc2d9ce20 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -92,7 +92,7 @@ struct ucounts {
 	struct hlist_node node;
 	struct user_namespace *ns;
 	kuid_t uid;
-	int count;
+	refcount_t count;
 	atomic_t ucount[UCOUNT_COUNTS];
 };
 
diff --git a/kernel/ucount.c b/kernel/ucount.c
index 11b1596e2542..82acd2226460 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -141,7 +141,8 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
 
 		new->ns = ns;
 		new->uid = uid;
-		new->count = 0;
+
+		refcount_set(&new->count, 0);
 
 		spin_lock_irq(&ucounts_lock);
 		ucounts = find_ucounts(ns, uid, hashent);
@@ -152,10 +153,7 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
 			ucounts = new;
 		}
 	}
-	if (ucounts->count == INT_MAX)
-		ucounts = NULL;
-	else
-		ucounts->count += 1;
+	refcount_inc(&ucounts->count);
 	spin_unlock_irq(&ucounts_lock);
 	return ucounts;
 }
@@ -164,15 +162,11 @@ static void put_ucounts(struct ucounts *ucounts)
 {
 	unsigned long flags;
 
-	spin_lock_irqsave(&ucounts_lock, flags);
-	ucounts->count -= 1;
-	if (!ucounts->count)
+	if (refcount_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, &flags)) {
 		hlist_del_init(&ucounts->node);
-	else
-		ucounts = NULL;
-	spin_unlock_irqrestore(&ucounts_lock, flags);
-
-	kfree(ucounts);
+		spin_unlock_irqrestore(&ucounts_lock, flags);
+		kfree(ucounts);
+	}
 }
 
 static inline bool atomic_inc_below(atomic_t *v, int u)
-- 
2.29.2