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Message-ID: <m1eefoll6y.fsf@fess.ebiederm.org>
Date: Mon, 05 Apr 2021 12:01:41 -0500
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Alexey Gladkov <gladkov.alexey@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux Containers <containers@...ts.linux-foundation.org>, linux-mm@...ck.org, Alexey Gladkov <legion@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Christian Brauner <christian.brauner@...ntu.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: Re: [PATCH v9 3/8] Use atomic_t for ucounts reference counting
Alexey Gladkov <gladkov.alexey@...il.com> writes:
> The current implementation of the ucounts reference counter requires the
> use of spin_lock. We're going to use get_ucounts() in more performance
> critical areas like a handling of RLIMIT_SIGPENDING.
>
> Now we need to use spin_lock only if we want to change the hashtable.
>
> v9:
> * Use a negative value to check that the ucounts->count is close to
> overflow.
Overall this looks good, one small issue below.
Eric
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 50cc1dfb7d28..7bac19bb3f1e 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -11,7 +11,7 @@
> struct ucounts init_ucounts = {
> .ns = &init_user_ns,
> .uid = GLOBAL_ROOT_UID,
> - .count = 1,
> + .count = ATOMIC_INIT(1),
> };
>
> #define UCOUNTS_HASHTABLE_BITS 10
> @@ -139,6 +139,15 @@ static void hlist_add_ucounts(struct ucounts *ucounts)
> spin_unlock_irq(&ucounts_lock);
> }
>
> +struct ucounts *get_ucounts(struct ucounts *ucounts)
> +{
> + if (ucounts && atomic_add_negative(1, &ucounts->count)) {
> + atomic_dec(&ucounts->count);
^^^^^^^^^^^^^^^^^^^^^^^^^^^
To handle the pathological case of all of the other uses calling
put_ucounts after the value goes negative, the above should
be put_ucounts intead of atomic_dec.
> + ucounts = NULL;
> + }
> + return ucounts;
> +}
> +
> struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
> {
> struct hlist_head *hashent = ucounts_hashentry(ns, uid);
> @@ -155,7 +164,7 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
>
> new->ns = ns;
> new->uid = uid;
> - new->count = 0;
> + atomic_set(&new->count, 1);
>
> spin_lock_irq(&ucounts_lock);
> ucounts = find_ucounts(ns, uid, hashent);
> @@ -163,33 +172,12 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
> kfree(new);
> } else {
> hlist_add_head(&new->node, hashent);
> - ucounts = new;
> + spin_unlock_irq(&ucounts_lock);
> + return new;
> }
> }
> - if (ucounts->count == INT_MAX)
> - ucounts = NULL;
> - else
> - ucounts->count += 1;
> spin_unlock_irq(&ucounts_lock);
> - return ucounts;
> -}
> -
> -struct ucounts *get_ucounts(struct ucounts *ucounts)
> -{
> - unsigned long flags;
> -
> - if (!ucounts)
> - return NULL;
> -
> - spin_lock_irqsave(&ucounts_lock, flags);
> - if (ucounts->count == INT_MAX) {
> - WARN_ONCE(1, "ucounts: counter has reached its maximum value");
> - ucounts = NULL;
> - } else {
> - ucounts->count += 1;
> - }
> - spin_unlock_irqrestore(&ucounts_lock, flags);
> -
> + ucounts = get_ucounts(ucounts);
> return ucounts;
> }
>
> @@ -197,15 +185,12 @@ 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)) {
> + spin_lock_irqsave(&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_long_inc_below(atomic_long_t *v, int u)
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