linux-kernel - Re: [RFC PATCH v2 1/8] Use atomic type for ucounts reference counting

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Message-ID: <878s8wdcib.fsf@x220.int.ebiederm.org>
Date:   Wed, 13 Jan 2021 10:31:40 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Alexey Gladkov <gladkov.alexey@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Alexey Gladkov <legion@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <christian@...uner.io>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH v2 1/8] Use atomic type for ucounts reference counting

Alexey Gladkov <gladkov.alexey@...il.com> writes:

We might want to use refcount_t instead of atomic_t.  Not a big deal
either way.

> Signed-off-by: Alexey Gladkov <gladkov.alexey@...il.com>
> ---
>  include/linux/user_namespace.h |  2 +-
>  kernel/ucount.c                | 10 +++++-----
>  2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index 64cf8ebdc4ec..84fefa9247c4 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;
> +	atomic_t count;
>  	atomic_t ucount[UCOUNT_COUNTS];
>  };
>  
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 11b1596e2542..0f2c7c11df19 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;
> +
> +		atomic_set(&new->count, 0);
>  
>  		spin_lock_irq(&ucounts_lock);
>  		ucounts = find_ucounts(ns, uid, hashent);
> @@ -152,10 +153,10 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
>  			ucounts = new;
>  		}
>  	}
> -	if (ucounts->count == INT_MAX)
> +	if (atomic_read(&ucounts->count) == INT_MAX)
>  		ucounts = NULL;
>  	else
> -		ucounts->count += 1;
> +		atomic_inc(&ucounts->count);
>  	spin_unlock_irq(&ucounts_lock);
>  	return ucounts;
>  }
> @@ -165,8 +166,7 @@ static void put_ucounts(struct ucounts *ucounts)
>  	unsigned long flags;
>  
>  	spin_lock_irqsave(&ucounts_lock, flags);
> -	ucounts->count -= 1;
> -	if (!ucounts->count)
> +	if (atomic_dec_and_test(&ucounts->count))
>  		hlist_del_init(&ucounts->node);
>  	else
>  		ucounts = NULL;


This can become:
static void put_ucounts(struct ucounts *ucounts)
{
	unsigned long flags;

        if (atomic_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, flags)) {
        	hlist_del_init(&ucounts->node);
                spin_unlock_irqrestore(&ucounts_lock);
                kfree(ucounts);
        }
}