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Message-Id: <59ee3289194cd97d70085cce701bc494bfcb4fd2.1615372955.git.gladkov.alexey@gmail.com>
Date: Wed, 10 Mar 2021 13:01:28 +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: [PATCH v8 3/8] Use atomic_t for ucounts reference counting
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.
Signed-off-by: Alexey Gladkov <gladkov.alexey@...il.com>
---
include/linux/user_namespace.h | 4 +--
kernel/ucount.c | 60 +++++++++++++++-------------------
2 files changed, 28 insertions(+), 36 deletions(-)
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index f71b5a4a3e74..d84cc2c0b443 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_long_t ucount[UCOUNT_COUNTS];
};
@@ -104,7 +104,7 @@ void retire_userns_sysctls(struct user_namespace *ns);
struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid, enum ucount_type type);
void dec_ucount(struct ucounts *ucounts, enum ucount_type type);
struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid);
-struct ucounts *get_ucounts(struct ucounts *ucounts);
+struct ucounts * __must_check get_ucounts(struct ucounts *ucounts);
void put_ucounts(struct ucounts *ucounts);
#ifdef CONFIG_USER_NS
diff --git a/kernel/ucount.c b/kernel/ucount.c
index 50cc1dfb7d28..bb3203039b5e 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 +139,22 @@ static void hlist_add_ucounts(struct ucounts *ucounts)
spin_unlock_irq(&ucounts_lock);
}
+/* 127: arbitrary random number, small enough to assemble well */
+#define refcount_zero_or_close_to_overflow(ucounts) \
+ ((unsigned int) atomic_read(&ucounts->count) + 127u <= 127u)
+
+struct ucounts *get_ucounts(struct ucounts *ucounts)
+{
+ if (ucounts) {
+ if (refcount_zero_or_close_to_overflow(ucounts)) {
+ WARN_ONCE(1, "ucounts: counter has reached its maximum value");
+ return NULL;
+ }
+ atomic_inc(&ucounts->count);
+ }
+ return ucounts;
+}
+
struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
{
struct hlist_head *hashent = ucounts_hashentry(ns, uid);
@@ -155,7 +171,7 @@ 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 +179,12 @@ 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 +192,12 @@ 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)
--
2.29.2
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