From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: gregkh@linuxfoundation.org, keescook@chromium.org,
will.deacon@arm.com, elena.reshetova@intel.com, arnd@arndb.de,
tglx@linutronix.de, hpa@zytor.com, dave@progbits.org,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH 7/7] kref: Implement using refcount_t
Date: Tue, 15 Nov 2016 11:46:08 +0100 [thread overview]
Message-ID: <20161115104608.GH3142@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20161115100359.GA7757@gmail.com>
On Tue, Nov 15, 2016 at 11:03:59AM +0100, Ingo Molnar wrote:
> > Should I also make a CONFIG knob that implements refcount_t with the
> > 'normal' atomic_t primitives?
>
> I'd suggest doing the saturation/safe-wrap semantics only for now (i.e. the
> current patch, split into two perhaps), and reconsider if there's any complaints?
>
> > And possibly another knob to toggle the BUG()s into WARN()s. With the
> > full saturation semantics WARN() is a lot safer and will not corrupt
> > kernel state as much.
>
> I'd suggest changing it to a WARN() straight away, no extra knobs.
OK, a little like so then? Note that the overflow tests went away
because increments guarantee we saturate before we overflow.
---
Subject: refcount_t: A special purpose refcount type
From: Peter Zijlstra <peterz@infradead.org>
Date: Mon Nov 14 18:06:19 CET 2016
Provide refcount_t, an atomic_t like primitive built just for
refcounting.
It provides saturation semantics such that overflow becomes impossible
and thereby 'spurious' use-after-free is avoided.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/refcount.h | 229 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 229 insertions(+)
--- /dev/null
+++ b/include/linux/refcount.h
@@ -0,0 +1,229 @@
+#ifndef _LINUX_REFCOUNT_H
+#define _LINUX_REFCOUNT_H
+
+/*
+ * Variant of atomic_t specialized for refcounting.
+ *
+ * The interface matches the atomic_t interface (to aid in porting) but only
+ * provides the few functions one should use for refcounting.
+ *
+ * It differs in that the counter saturates at UINT_MAX and will not move once
+ * there. This avoids wrapping the counter and causing 'spurious'
+ * use-after-free issues.
+ *
+ * Memory ordering rules are slightly relaxed wrt regular atomic_t functions
+ * and provide only what is strictly required for refcounts.
+ *
+ * The increments are fully relaxed; these will not provide ordering. The
+ * rationale is that whatever is used to obtain the object we're increasing the
+ * reference count on will provide the ordering. For locked data structures,
+ * its the lock acquire, for RCU/lockless data structures its the dependent
+ * load.
+ *
+ * Do note that inc_not_zero() provides a control dependency which will order
+ * future stores against the inc, this ensures we'll never modify the object
+ * if we did not in fact acquire a reference.
+ *
+ * The decrements will provide release order, such that all the prior loads and
+ * stores will be issued before we proceed with freeing the object.
+ *
+ * Note: the implementation hard relies on increments, bigger than 1 additions
+ * need explicit overflow -> saturation logic.
+ *
+ */
+
+#include <linux/atomic.h>
+#include <linux/bug.h>
+#include <linux/mutex.h>
+#include <linux/spinlock.h>
+
+typedef struct refcount_struct {
+ atomic_t refs;
+} refcount_t;
+
+#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
+
+static inline void refcount_set(refcount_t *r, int n)
+{
+ atomic_set(&r->refs, n);
+}
+
+static inline unsigned int refcount_read(const refcount_t *r)
+{
+ return atomic_read(&r->refs);
+}
+
+/*
+ * Similar to atomic_inc(), will saturate at UINT_MAX and WARN.
+ *
+ * Provides no memory ordering, it is assumed the caller already has a
+ * reference on the object, will WARN when this is not so.
+ */
+static inline void refcount_inc(refcount_t *r)
+{
+ unsigned int old, new, val = atomic_read(&r->refs);
+
+ for (;;) {
+ WARN(!val, "refcount_t: increment on 0; use-after-free.\n");
+
+ if (unlikely(val == UINT_MAX))
+ return;
+
+ new = val + 1;
+ old = atomic_cmpxchg_relaxed(&r->refs, val, new);
+ if (old == val)
+ break;
+
+ val = old;
+ }
+
+ WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
+}
+
+/*
+ * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN.
+ *
+ * Provides no memory ordering, it is assumed the caller has guaranteed the
+ * object memory to be stable (RCU, etc.). It does provide a control dependency
+ * and thereby orders future stores.
+ */
+static inline __must_check
+bool refcount_inc_not_zero(refcount_t *r)
+{
+ unsigned int old, new, val = atomic_read(&r->refs);
+
+ for (;;) {
+ if (!val)
+ return false;
+
+ if (unlikely(val == UINT_MAX))
+ return true;
+
+ new = val + 1;
+ old = atomic_cmpxchg_relaxed(&r->refs, val, new);
+ if (old == val)
+ break;
+
+ val = old;
+ }
+
+ WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
+
+ return true;
+}
+
+/*
+ * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
+ * decrement when saturated at UINT_MAX.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before a subsequent free.
+ */
+static inline __must_check
+bool refcount_dec_and_test(refcount_t *r)
+{
+ unsigned int old, new, val = atomic_read(&r->refs);
+
+ for (;;) {
+ if (val == UINT_MAX)
+ return false;
+
+ new = val - 1;
+ if (WARN(new > val, "refcount_t: underflow; use-after-free.\n"))
+ return false;
+
+ old = atomic_cmpxchg_release(&r->refs, val, new);
+ if (old == val)
+ break;
+
+ val = old;
+ }
+
+ return !new;
+}
+
+/*
+ * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
+ * to decrement when saturated at UINT_MAX.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before a subsequent free. This allows free() while holding the mutex.
+ */
+static inline __must_check
+bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
+{
+ unsigned int old, new, val = atomic_read(&r->refs);
+ bool locked = false;
+
+ for (;;) {
+ if (val == UINT_MAX)
+ return false;
+
+ if (val == 1 && !locked) {
+ locked = true;
+ mutex_lock(lock);
+ }
+
+ new = val - 1;
+ if (WARN(new > val, "refcount_t: underflow; use-after-free.\n")) {
+ if (locked)
+ mutex_unlock(lock);
+ return false;
+ }
+
+ old = atomic_cmpxchg_release(&r->refs, val, new);
+ if (old == val)
+ break;
+
+ val = old;
+ }
+
+ if (new && locked)
+ mutex_unlock(lock);
+
+ return !new;
+}
+
+/*
+ * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
+ * decrement when saturated at UINT_MAX.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before a subsequent free. This allows free() while holding the lock.
+ */
+static inline __must_check
+bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
+{
+ unsigned int old, new, val = atomic_read(&r->refs);
+ bool locked = false;
+
+ for (;;) {
+ if (val == UINT_MAX)
+ return false;
+
+ if (val == 1 && !locked) {
+ locked = true;
+ spin_lock(lock);
+ }
+
+ new = val - 1;
+ if (WARN(new > val, "refcount_t: underflow; use-after-free.\n")) {
+ if (locked)
+ mutex_unlock(lock);
+ return false;
+ }
+
+ old = atomic_cmpxchg_release(&r->refs, val, new);
+ if (old == val)
+ break;
+
+ val = old;
+ }
+
+ if (new && locked)
+ spin_unlock(lock);
+
+ return !new;
+}
+
+#endif /* _LINUX_REFCOUNT_H */
next prev parent reply other threads:[~2016-11-15 10:46 UTC|newest]
Thread overview: 96+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-14 17:39 [RFC][PATCH 0/7] kref improvements Peter Zijlstra
2016-11-14 17:39 ` [RFC][PATCH 1/7] kref: Add KREF_INIT() Peter Zijlstra
2016-11-14 17:39 ` [RFC][PATCH 2/7] kref: Add kref_read() Peter Zijlstra
2016-11-14 18:16 ` Christoph Hellwig
2016-11-15 7:28 ` Greg KH
2016-11-15 7:47 ` Peter Zijlstra
2016-11-15 8:37 ` [PATCH] printk, locking/atomics, kref: Introduce new %pAr and %pAk format string options for atomic_t and 'struct kref' Ingo Molnar
2016-11-15 8:43 ` [PATCH v2] " Ingo Molnar
2016-11-15 9:21 ` Peter Zijlstra
2016-11-15 9:41 ` [PATCH v3] printk, locking/atomics, kref: Introduce new %pAa " Ingo Molnar
2016-11-15 10:10 ` [PATCH v2] printk, locking/atomics, kref: Introduce new %pAr " kbuild test robot
2016-11-15 16:42 ` [PATCH] " Linus Torvalds
2016-11-16 8:13 ` Ingo Molnar
2016-11-15 7:33 ` [RFC][PATCH 2/7] kref: Add kref_read() Greg KH
2016-11-15 8:03 ` Peter Zijlstra
2016-11-15 20:53 ` Kees Cook
2016-11-16 8:21 ` Greg KH
2016-11-16 10:10 ` Peter Zijlstra
2016-11-16 10:18 ` Greg KH
2016-11-16 10:11 ` Daniel Borkmann
2016-11-16 10:19 ` Greg KH
2016-11-16 10:09 ` Peter Zijlstra
2016-11-16 18:58 ` Kees Cook
2016-11-17 8:34 ` Peter Zijlstra
2016-11-17 12:30 ` David Windsor
2016-11-17 12:43 ` Peter Zijlstra
2016-11-17 13:01 ` Reshetova, Elena
2016-11-17 13:22 ` Peter Zijlstra
2016-11-17 15:42 ` Reshetova, Elena
2016-11-17 18:02 ` Reshetova, Elena
2016-11-17 19:10 ` Peter Zijlstra
2016-11-17 19:29 ` Peter Zijlstra
2016-11-17 19:34 ` Kees Cook
2016-11-14 17:39 ` [RFC][PATCH 3/7] kref: Kill kref_sub() Peter Zijlstra
2016-11-14 17:39 ` [RFC][PATCH 4/7] kref: Use kref_get_unless_zero() more Peter Zijlstra
2016-11-14 17:39 ` [RFC][PATCH 5/7] kref: Implement kref_put_lock() Peter Zijlstra
2016-11-14 20:35 ` Kees Cook
2016-11-15 7:50 ` Peter Zijlstra
2016-11-14 17:39 ` [RFC][PATCH 6/7] kref: Avoid more abuse Peter Zijlstra
2016-11-14 17:39 ` [RFC][PATCH 7/7] kref: Implement using refcount_t Peter Zijlstra
2016-11-15 8:40 ` Ingo Molnar
2016-11-15 9:47 ` Peter Zijlstra
2016-11-15 10:03 ` Ingo Molnar
2016-11-15 10:46 ` Peter Zijlstra [this message]
2016-11-15 13:03 ` Ingo Molnar
2016-11-15 18:06 ` Kees Cook
2016-11-15 19:16 ` Peter Zijlstra
2016-11-15 19:23 ` Kees Cook
2016-11-16 8:31 ` Ingo Molnar
2016-11-16 8:51 ` Greg KH
2016-11-16 9:07 ` Ingo Molnar
2016-11-16 9:24 ` Greg KH
2016-11-16 10:15 ` Peter Zijlstra
2016-11-16 18:55 ` Kees Cook
2016-11-17 8:33 ` Peter Zijlstra
2016-11-17 19:50 ` Kees Cook
2016-11-16 18:41 ` Kees Cook
2016-11-15 12:33 ` Boqun Feng
2016-11-15 13:01 ` Peter Zijlstra
2016-11-15 14:19 ` Boqun Feng
2016-11-17 9:28 ` Peter Zijlstra
2016-11-17 9:48 ` Boqun Feng
2016-11-17 10:29 ` Peter Zijlstra
2016-11-17 10:39 ` Peter Zijlstra
2016-11-17 11:03 ` Greg KH
2016-11-17 12:48 ` Peter Zijlstra
[not found] ` <CAL0jBu-GnREUPSX4kUDp-Cc8ZGp6+Cb2q0HVandswcLzPRnChQ@mail.gmail.com>
2016-11-17 12:08 ` Peter Zijlstra
2016-11-17 12:08 ` Will Deacon
2016-11-17 16:11 ` Peter Zijlstra
2016-11-17 16:36 ` Will Deacon
2016-11-18 8:26 ` Boqun Feng
2016-11-18 10:16 ` Will Deacon
2016-11-18 10:07 ` Reshetova, Elena
2016-11-18 11:37 ` Peter Zijlstra
2016-11-18 17:06 ` Will Deacon
2016-11-18 18:57 ` Peter Zijlstra
2016-11-21 4:06 ` Boqun Feng
2016-11-21 7:48 ` Ingo Molnar
2016-11-21 8:38 ` Boqun Feng
2016-11-21 8:44 ` Boqun Feng
2016-11-21 9:02 ` Peter Zijlstra
2016-11-21 9:37 ` Boqun Feng
2016-11-18 10:47 ` Reshetova, Elena
2016-11-18 10:52 ` Peter Zijlstra
2016-11-18 16:58 ` Reshetova, Elena
2016-11-18 18:53 ` Peter Zijlstra
2016-11-19 7:14 ` Reshetova, Elena
2016-11-19 11:45 ` Peter Zijlstra
2017-01-26 23:14 ` Kees Cook
2017-01-27 9:58 ` Peter Zijlstra
2017-01-27 21:07 ` Kees Cook
2017-01-30 13:40 ` Peter Zijlstra
2016-11-15 7:27 ` [RFC][PATCH 0/7] kref improvements Greg KH
2016-11-15 7:42 ` Ingo Molnar
2016-11-15 15:05 ` Greg KH
2016-11-15 7:48 ` Peter Zijlstra
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