Rework REFCOUNT_FULL using atomic_fetch_* operations

index 0a146b32da13..abf3b7c1f686 100644
@@ -6,14 +6,6 @@
 #include "lkdtm.h"
 #include <linux/refcount.h>
 
-#ifdef CONFIG_REFCOUNT_FULL
-#define REFCOUNT_MAX		(UINT_MAX - 1)
-#define REFCOUNT_SATURATED	UINT_MAX
-#else
-#define REFCOUNT_MAX		INT_MAX
-#define REFCOUNT_SATURATED	(INT_MIN / 2)
-#endif
-
 static void overflow_check(refcount_t *ref)
 {
 	switch (refcount_read(ref)) {

index e28cce21bad6..79f62e8d2256 100644
@@ -4,6 +4,7 @@
 
 #include <linux/atomic.h>
 #include <linux/compiler.h>
+#include <linux/limits.h>
 #include <linux/spinlock_types.h>
 
 struct mutex;

@@ -12,7 +13,7 @@ struct mutex;
  * struct refcount_t - variant of atomic_t specialized for reference counts
  * @refs: atomic_t counter field
  *
- * The counter saturates at UINT_MAX and will not move once
+ * The counter saturates at REFCOUNT_SATURATED and will not move once
  * there. This avoids wrapping the counter and causing 'spurious'
  * use-after-free bugs.
  */

@@ -56,6 +57,9 @@ extern void refcount_dec_checked(refcount_t *r);
 
 #ifdef CONFIG_REFCOUNT_FULL
 
+#define REFCOUNT_MAX		(UINT_MAX - 1)
+#define REFCOUNT_SATURATED	UINT_MAX
+
 #define refcount_add_not_zero	refcount_add_not_zero_checked
 #define refcount_add		refcount_add_checked
 

@@ -68,6 +72,10 @@ extern void refcount_dec_checked(refcount_t *r);
 #define refcount_dec		refcount_dec_checked
 
 #else
+
+#define REFCOUNT_MAX		INT_MAX
+#define REFCOUNT_SATURATED	(INT_MIN / 2)
+
 # ifdef CONFIG_ARCH_HAS_REFCOUNT
 #  include <asm/refcount.h>
 # else

index 6e904af0fb3e..48b78a423d7d 100644
@@ -5,8 +5,8 @@
  * The interface matches the atomic_t interface (to aid in porting) but only
  * provides the few functions one should use for reference counting.
  *
- * It differs in that the counter saturates at UINT_MAX and will not move once
- * there. This avoids wrapping the counter and causing 'spurious'
+ * It differs in that the counter saturates at REFCOUNT_SATURATED 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

@@ -48,7 +48,7 @@
  * @i: the value to add to the refcount
  * @r: the refcount
  *
- * Will saturate at UINT_MAX and WARN.
+ * Will saturate at REFCOUNT_SATURATED 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

@@ -69,16 +69,17 @@ bool refcount_add_not_zero_checked(unsigned int i, refcount_t *r)
 		if (!val)
 			return false;
 
-		if (unlikely(val == UINT_MAX))
+		if (unlikely(val == REFCOUNT_SATURATED))
 			return true;
 
 		new = val + i;
 		if (new < val)
-			new = UINT_MAX;
+			new = REFCOUNT_SATURATED;
 
 	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
 
-	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
+	WARN_ONCE(new == REFCOUNT_SATURATED,
+		  "refcount_t: saturated; leaking memory.\n");
 
 	return true;
 }

@@ -89,7 +90,7 @@ EXPORT_SYMBOL(refcount_add_not_zero_checked);
  * @i: the value to add to the refcount
  * @r: the refcount
  *
- * Similar to atomic_add(), but will saturate at UINT_MAX and WARN.
+ * Similar to atomic_add(), but will saturate at REFCOUNT_SATURATED 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

@@ -110,7 +111,8 @@ EXPORT_SYMBOL(refcount_add_checked);
  * refcount_inc_not_zero_checked - increment a refcount unless it is 0
  * @r: the refcount to increment
  *
- * Similar to atomic_inc_not_zero(), but will saturate at UINT_MAX and WARN.
+ * Similar to atomic_inc_not_zero(), but will saturate at REFCOUNT_SATURATED
+ * 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

@@ -133,7 +135,8 @@ bool refcount_inc_not_zero_checked(refcount_t *r)
 
 	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
 
-	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
+	WARN_ONCE(new == REFCOUNT_SATURATED,
+		  "refcount_t: saturated; leaking memory.\n");
 
 	return true;
 }

@@ -143,7 +146,7 @@ EXPORT_SYMBOL(refcount_inc_not_zero_checked);
  * refcount_inc_checked - increment a refcount
  * @r: the refcount to increment
  *
- * Similar to atomic_inc(), but will saturate at UINT_MAX and WARN.
+ * Similar to atomic_inc(), but will saturate at REFCOUNT_SATURATED and WARN.
  *
  * Provides no memory ordering, it is assumed the caller already has a
  * reference on the object.

@@ -164,7 +167,7 @@ EXPORT_SYMBOL(refcount_inc_checked);
  *
  * Similar to atomic_dec_and_test(), but it will WARN, return false and
  * ultimately leak on underflow and will fail to decrement when saturated
- * at UINT_MAX.
+ * at REFCOUNT_SATURATED.
  *
  * Provides release memory ordering, such that prior loads and stores are done
  * before, and provides an acquire ordering on success such that free()

@@ -182,7 +185,7 @@ bool refcount_sub_and_test_checked(unsigned int i, refcount_t *r)
 	unsigned int new, val = atomic_read(&r->refs);
 
 	do {
-		if (unlikely(val == UINT_MAX))
+		if (unlikely(val == REFCOUNT_SATURATED))
 			return false;
 
 		new = val - i;

@@ -207,7 +210,7 @@ EXPORT_SYMBOL(refcount_sub_and_test_checked);
  * @r: the refcount
  *
  * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
- * decrement when saturated at UINT_MAX.
+ * decrement when saturated at REFCOUNT_SATURATED.
  *
  * Provides release memory ordering, such that prior loads and stores are done
  * before, and provides an acquire ordering on success such that free()

@@ -226,7 +229,7 @@ EXPORT_SYMBOL(refcount_dec_and_test_checked);
  * @r: the refcount
  *
  * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
- * when saturated at UINT_MAX.
+ * when saturated at REFCOUNT_SATURATED.
  *
  * Provides release memory ordering, such that prior loads and stores are done
  * before.

@@ -277,7 +280,7 @@ bool refcount_dec_not_one(refcount_t *r)
 	unsigned int new, val = atomic_read(&r->refs);
 
 	do {
-		if (unlikely(val == UINT_MAX))
+		if (unlikely(val == REFCOUNT_SATURATED))
 			return true;
 
 		if (val == 1)

@@ -302,7 +305,7 @@ EXPORT_SYMBOL(refcount_dec_not_one);
  * @lock: the mutex to be locked
  *
  * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
- * to decrement when saturated at UINT_MAX.
+ * to decrement when saturated at REFCOUNT_SATURATED.
  *
  * Provides release memory ordering, such that prior loads and stores are done
  * before, and provides a control dependency such that free() must come after.

@@ -333,7 +336,7 @@ EXPORT_SYMBOL(refcount_dec_and_mutex_lock);
  * @lock: the spinlock to be locked
  *
  * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
- * decrement when saturated at UINT_MAX.
+ * decrement when saturated at REFCOUNT_SATURATED.
  *
  * Provides release memory ordering, such that prior loads and stores are done
  * before, and provides a control dependency such that free() must come after.

index 79f62e8d2256..89066a1471dd 100644
@@ -28,7 +28,7 @@ typedef struct refcount_struct {
  * @r: the refcount
  * @n: value to which the refcount will be set
  */
-static inline void refcount_set(refcount_t *r, unsigned int n)
+static inline void refcount_set(refcount_t *r, int n)
 {
 	atomic_set(&r->refs, n);
 }

@@ -44,13 +44,13 @@ static inline unsigned int refcount_read(const refcount_t *r)
 	return atomic_read(&r->refs);
 }
 
-extern __must_check bool refcount_add_not_zero_checked(unsigned int i, refcount_t *r);
-extern void refcount_add_checked(unsigned int i, refcount_t *r);
+extern __must_check bool refcount_add_not_zero_checked(int i, refcount_t *r);
+extern void refcount_add_checked(int i, refcount_t *r);
 
 extern __must_check bool refcount_inc_not_zero_checked(refcount_t *r);
 extern void refcount_inc_checked(refcount_t *r);
 
-extern __must_check bool refcount_sub_and_test_checked(unsigned int i, refcount_t *r);
+extern __must_check bool refcount_sub_and_test_checked(int i, refcount_t *r);
 
 extern __must_check bool refcount_dec_and_test_checked(refcount_t *r);
 extern void refcount_dec_checked(refcount_t *r);

@@ -79,12 +79,12 @@ extern void refcount_dec_checked(refcount_t *r);
 # ifdef CONFIG_ARCH_HAS_REFCOUNT
 #  include <asm/refcount.h>
 # else
-static inline __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r)
+static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
 {
 	return atomic_add_unless(&r->refs, i, 0);
 }
 
-static inline void refcount_add(unsigned int i, refcount_t *r)
+static inline void refcount_add(int i, refcount_t *r)
 {
 	atomic_add(i, &r->refs);
 }

@@ -99,7 +99,7 @@ static inline void refcount_inc(refcount_t *r)
 	atomic_inc(&r->refs);
 }
 
-static inline __must_check bool refcount_sub_and_test(unsigned int i, refcount_t *r)
+static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
 {
 	return atomic_sub_and_test(i, &r->refs);
 }

index 48b78a423d7d..719b0bc42ab1 100644
@@ -61,7 +61,7 @@
  *
  * Return: false if the passed refcount is 0, true otherwise
  */
-bool refcount_add_not_zero_checked(unsigned int i, refcount_t *r)
+bool refcount_add_not_zero_checked(int i, refcount_t *r)
 {
 	unsigned int new, val = atomic_read(&r->refs);
 

@@ -101,7 +101,7 @@ EXPORT_SYMBOL(refcount_add_not_zero_checked);
  * cases, refcount_inc(), or one of its variants, should instead be used to
  * increment a reference count.
  */
-void refcount_add_checked(unsigned int i, refcount_t *r)
+void refcount_add_checked(int i, refcount_t *r)
 {
 	WARN_ONCE(!refcount_add_not_zero_checked(i, r), "refcount_t: addition on 0; use-after-free.\n");
 }

@@ -180,7 +180,7 @@ EXPORT_SYMBOL(refcount_inc_checked);
  *
  * Return: true if the resulting refcount is 0, false otherwise
  */
-bool refcount_sub_and_test_checked(unsigned int i, refcount_t *r)
+bool refcount_sub_and_test_checked(int i, refcount_t *r)
 {
 	unsigned int new, val = atomic_read(&r->refs);
 

index 89066a1471dd..edd505d1a23b 100644
@@ -44,32 +44,21 @@ static inline unsigned int refcount_read(const refcount_t *r)
 	return atomic_read(&r->refs);
 }
 
-extern __must_check bool refcount_add_not_zero_checked(int i, refcount_t *r);
-extern void refcount_add_checked(int i, refcount_t *r);
-
-extern __must_check bool refcount_inc_not_zero_checked(refcount_t *r);
-extern void refcount_inc_checked(refcount_t *r);
-
-extern __must_check bool refcount_sub_and_test_checked(int i, refcount_t *r);
-
-extern __must_check bool refcount_dec_and_test_checked(refcount_t *r);
-extern void refcount_dec_checked(refcount_t *r);
-
 #ifdef CONFIG_REFCOUNT_FULL
 
 #define REFCOUNT_MAX		(UINT_MAX - 1)
 #define REFCOUNT_SATURATED	UINT_MAX
 
-#define refcount_add_not_zero	refcount_add_not_zero_checked
-#define refcount_add		refcount_add_checked
+extern __must_check bool refcount_add_not_zero(int i, refcount_t *r);
+extern void refcount_add(int i, refcount_t *r);
 
-#define refcount_inc_not_zero	refcount_inc_not_zero_checked
-#define refcount_inc		refcount_inc_checked
+extern __must_check bool refcount_inc_not_zero(refcount_t *r);
+extern void refcount_inc(refcount_t *r);
 
-#define refcount_sub_and_test	refcount_sub_and_test_checked
+extern __must_check bool refcount_sub_and_test(int i, refcount_t *r);
 
-#define refcount_dec_and_test	refcount_dec_and_test_checked
-#define refcount_dec		refcount_dec_checked
+extern __must_check bool refcount_dec_and_test(refcount_t *r);
+extern void refcount_dec(refcount_t *r);
 
 #else
 

141 lines not on this page · raw message

index edd505d1a23b..e719b5b1220e 100644
@@ -45,22 +45,241 @@ static inline unsigned int refcount_read(const refcount_t *r)
 }
 
 #ifdef CONFIG_REFCOUNT_FULL
+#include <linux/bug.h>
 
 #define REFCOUNT_MAX		(UINT_MAX - 1)
 #define REFCOUNT_SATURATED	UINT_MAX
 
-extern __must_check bool refcount_add_not_zero(int i, refcount_t *r);
-extern void refcount_add(int i, refcount_t *r);
+/*
+ * Variant of atomic_t specialized for reference counts.
+ *
+ * The interface matches the atomic_t interface (to aid in porting) but only
+ * provides the few functions one should use for reference counting.
+ *
+ * It differs in that the counter saturates at REFCOUNT_SATURATED 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, it also provides a control dependency, which
+ * will order us against the subsequent free().
+ *
+ * The control dependency is against the load of the cmpxchg (ll/sc) that
+ * succeeded. This means the stores aren't fully ordered, but this is fine
+ * because the 1->0 transition indicates no concurrency.
+ *
+ * Note that the allocator is responsible for ordering things between free()
+ * and alloc().
+ *
+ * The decrements dec_and_test() and sub_and_test() also provide acquire
+ * ordering on success.
+ *
+ */
+
+/**
+ * refcount_add_not_zero - add a value to a refcount unless it is 0
+ * @i: the value to add to the refcount
+ * @r: the refcount
+ *
+ * Will saturate at REFCOUNT_SATURATED 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. See the comment on top.
+ *
+ * Use of this function is not recommended for the normal reference counting
+ * use case in which references are taken and released one at a time.  In these
+ * cases, refcount_inc(), or one of its variants, should instead be used to
+ * increment a reference count.
+ *
+ * Return: false if the passed refcount is 0, true otherwise
+ */
+static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
+{
+	unsigned int new, val = atomic_read(&r->refs);
+
+	do {
+		if (!val)
+			return false;
+
+		if (unlikely(val == REFCOUNT_SATURATED))
+			return true;
+
+		new = val + i;
+		if (new < val)
+			new = REFCOUNT_SATURATED;
+
+	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
+
+	WARN_ONCE(new == REFCOUNT_SATURATED,
+		  "refcount_t: saturated; leaking memory.\n");
+
+	return true;
+}
+
+/**
+ * refcount_add - add a value to a refcount
+ * @i: the value to add to the refcount
+ * @r: the refcount
+ *
+ * Similar to atomic_add(), but will saturate at REFCOUNT_SATURATED 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. See the comment on top.
+ *
+ * Use of this function is not recommended for the normal reference counting
+ * use case in which references are taken and released one at a time.  In these
+ * cases, refcount_inc(), or one of its variants, should instead be used to
+ * increment a reference count.
+ */
+static inline void refcount_add(int i, refcount_t *r)
+{
+	WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
+}
+
+/**
+ * refcount_inc_not_zero - increment a refcount unless it is 0
+ * @r: the refcount to increment
+ *
+ * Similar to atomic_inc_not_zero(), but will saturate at REFCOUNT_SATURATED
+ * 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. See the comment on top.
+ *
+ * Return: true if the increment was successful, false otherwise
+ */
+static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
+{
+	unsigned int new, val = atomic_read(&r->refs);
+
+	do {
+		new = val + 1;
 
-extern __must_check bool refcount_inc_not_zero(refcount_t *r);
-extern void refcount_inc(refcount_t *r);
+		if (!val)
+			return false;
 
-extern __must_check bool refcount_sub_and_test(int i, refcount_t *r);
+		if (unlikely(!new))
+			return true;
 
-extern __must_check bool refcount_dec_and_test(refcount_t *r);
-extern void refcount_dec(refcount_t *r);
+	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
+
+	WARN_ONCE(new == REFCOUNT_SATURATED,
+		  "refcount_t: saturated; leaking memory.\n");
+
+	return true;
+}
+
+/**
+ * refcount_inc - increment a refcount
+ * @r: the refcount to increment
+ *
+ * Similar to atomic_inc(), but will saturate at REFCOUNT_SATURATED and WARN.
+ *
+ * Provides no memory ordering, it is assumed the caller already has a
+ * reference on the object.
+ *
+ * Will WARN if the refcount is 0, as this represents a possible use-after-free
+ * condition.
+ */
+static inline void refcount_inc(refcount_t *r)
+{
+	WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
+}
+
+/**
+ * refcount_sub_and_test - subtract from a refcount and test if it is 0
+ * @i: amount to subtract from the refcount
+ * @r: the refcount
+ *
+ * Similar to atomic_dec_and_test(), but it will WARN, return false and
+ * ultimately leak on underflow and will fail to decrement when saturated
+ * at REFCOUNT_SATURATED.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before, and provides an acquire ordering on success such that free()
+ * must come after.
+ *
+ * Use of this function is not recommended for the normal reference counting
+ * use case in which references are taken and released one at a time.  In these
+ * cases, refcount_dec(), or one of its variants, should instead be used to
+ * decrement a reference count.
+ *
+ * Return: true if the resulting refcount is 0, false otherwise
+ */
+static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
+{
+	unsigned int new, val = atomic_read(&r->refs);
+
+	do {
+		if (unlikely(val == REFCOUNT_SATURATED))
+			return false;
+
+		new = val - i;
+		if (new > val) {
+			WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
+			return false;
+		}
+
+	} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
+
+	if (!new) {
+		smp_acquire__after_ctrl_dep();
+		return true;
+	}
+	return false;
+
+}
+
+/**
+ * refcount_dec_and_test - decrement a refcount and test if it is 0
+ * @r: the refcount
+ *
+ * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
+ * decrement when saturated at REFCOUNT_SATURATED.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before, and provides an acquire ordering on success such that free()
+ * must come after.
+ *
+ * Return: true if the resulting refcount is 0, false otherwise
+ */
+static inline __must_check bool refcount_dec_and_test(refcount_t *r)
+{
+	return refcount_sub_and_test(1, r);
+}
+
+/**
+ * refcount_dec - decrement a refcount
+ * @r: the refcount
+ *
+ * Similar to atomic_dec(), it will WARN on underflow and fail to decrement
+ * when saturated at REFCOUNT_SATURATED.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before.
+ */
+static inline void refcount_dec(refcount_t *r)
+{
+	WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
+}
 
-#else
+#else /* CONFIG_REFCOUNT_FULL */
 
 #define REFCOUNT_MAX		INT_MAX
 #define REFCOUNT_SATURATED	(INT_MIN / 2)

@@ -103,7 +322,7 @@ static inline void refcount_dec(refcount_t *r)
 	atomic_dec(&r->refs);
 }
 # endif /* !CONFIG_ARCH_HAS_REFCOUNT */
-#endif /* CONFIG_REFCOUNT_FULL */
+#endif /* !CONFIG_REFCOUNT_FULL */
 
 extern __must_check bool refcount_dec_if_one(refcount_t *r);
 extern __must_check bool refcount_dec_not_one(refcount_t *r);

249 lines not on this page · raw message

index e719b5b1220e..e3b218d669ce 100644
@@ -47,8 +47,8 @@ static inline unsigned int refcount_read(const refcount_t *r)
 #ifdef CONFIG_REFCOUNT_FULL
 #include <linux/bug.h>
 
-#define REFCOUNT_MAX		(UINT_MAX - 1)
-#define REFCOUNT_SATURATED	UINT_MAX
+#define REFCOUNT_MAX		INT_MAX
+#define REFCOUNT_SATURATED	(INT_MIN / 2)
 
 /*
  * Variant of atomic_t specialized for reference counts.

@@ -56,9 +56,47 @@ static inline unsigned int refcount_read(const refcount_t *r)
  * The interface matches the atomic_t interface (to aid in porting) but only
  * provides the few functions one should use for reference counting.
  *
- * It differs in that the counter saturates at REFCOUNT_SATURATED and will not
- * move once there. This avoids wrapping the counter and causing 'spurious'
- * use-after-free issues.
+ * Saturation semantics
+ * ====================
+ *
+ * refcount_t differs from atomic_t in that the counter saturates at
+ * REFCOUNT_SATURATED and will not move once there. This avoids wrapping the
+ * counter and causing 'spurious' use-after-free issues. In order to avoid the
+ * cost associated with introducing cmpxchg() loops into all of the saturating
+ * operations, we temporarily allow the counter to take on an unchecked value
+ * and then explicitly set it to REFCOUNT_SATURATED on detecting that underflow
+ * or overflow has occurred. Although this is racy when multiple threads
+ * access the refcount concurrently, by placing REFCOUNT_SATURATED roughly
+ * equidistant from 0 and INT_MAX we minimise the scope for error:
+ *
+ * 	                           INT_MAX     REFCOUNT_SATURATED   UINT_MAX
+ *   0                          (0x7fff_ffff)    (0xc000_0000)    (0xffff_ffff)
+ *   +--------------------------------+----------------+----------------+
+ *                                     <---------- bad value! ---------->
+ *
+ * (in a signed view of the world, the "bad value" range corresponds to
+ * a negative counter value).
+ *
+ * As an example, consider a refcount_inc() operation that causes the counter
+ * to overflow:
+ *
+ * 	int old = atomic_fetch_add_relaxed(r);
+ *	// old is INT_MAX, refcount now INT_MIN (0x8000_0000)
+ *	if (old < 0)
+ *		atomic_set(r, REFCOUNT_SATURATED);
+ *
+ * If another thread also performs a refcount_inc() operation between the two
+ * atomic operations, then the count will continue to edge closer to 0. If it
+ * reaches a value of 1 before /any/ of the threads reset it to the saturated
+ * value, then a concurrent refcount_dec_and_test() may erroneously free the
+ * underlying object. Given the precise timing details involved with the
+ * round-robin scheduling of each thread manipulating the refcount and the need
+ * to hit the race multiple times in succession, there doesn't appear to be a
+ * practical avenue of attack even if using refcount_add() operations with
+ * larger increments.
+ *
+ * Memory ordering
+ * ===============
  *
  * Memory ordering rules are slightly relaxed wrt regular atomic_t functions
  * and provide only what is strictly required for refcounts.

@@ -109,25 +147,19 @@ static inline unsigned int refcount_read(const refcount_t *r)
  */
 static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
 {
-	unsigned int new, val = atomic_read(&r->refs);
+	int old = refcount_read(r);
 
 	do {
-		if (!val)
-			return false;
-
-		if (unlikely(val == REFCOUNT_SATURATED))
-			return true;
-
-		new = val + i;
-		if (new < val)
-			new = REFCOUNT_SATURATED;
+		if (!old)
+			break;
+	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
 
-	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
-
-	WARN_ONCE(new == REFCOUNT_SATURATED,
-		  "refcount_t: saturated; leaking memory.\n");
+	if (unlikely(old < 0 || old + i < 0)) {
+		refcount_set(r, REFCOUNT_SATURATED);
+		WARN_ONCE(1, "refcount_t: saturated; leaking memory.\n");
+	}
 
-	return true;
+	return old;
 }
 
 /**

@@ -148,7 +180,13 @@ static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
  */
 static inline void refcount_add(int i, refcount_t *r)
 {
-	WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
+	int old = atomic_fetch_add_relaxed(i, &r->refs);
+
+	WARN_ONCE(!old, "refcount_t: addition on 0; use-after-free.\n");
+	if (unlikely(old <= 0 || old + i <= 0)) {
+		refcount_set(r, REFCOUNT_SATURATED);
+		WARN_ONCE(old, "refcount_t: saturated; leaking memory.\n");
+	}
 }
 
 /**

@@ -166,23 +204,7 @@ static inline void refcount_add(int i, refcount_t *r)
  */
 static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
 {
-	unsigned int new, val = atomic_read(&r->refs);
-
-	do {
-		new = val + 1;
-
-		if (!val)
-			return false;
-
-		if (unlikely(!new))
-			return true;
-
-	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
-
-	WARN_ONCE(new == REFCOUNT_SATURATED,
-		  "refcount_t: saturated; leaking memory.\n");
-
-	return true;
+	return refcount_add_not_zero(1, r);
 }
 
 /**

@@ -199,7 +221,7 @@ static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
  */
 static inline void refcount_inc(refcount_t *r)
 {
-	WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
+	refcount_add(1, r);
 }
 
 /**

@@ -224,26 +246,19 @@ static inline void refcount_inc(refcount_t *r)
  */
 static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
 {
-	unsigned int new, val = atomic_read(&r->refs);
-
-	do {
-		if (unlikely(val == REFCOUNT_SATURATED))
-			return false;
+	int old = atomic_fetch_sub_release(i, &r->refs);
 
-		new = val - i;
-		if (new > val) {
-			WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
-			return false;
-		}
-
-	} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
-
-	if (!new) {
+	if (old == i) {
 		smp_acquire__after_ctrl_dep();
 		return true;
 	}
-	return false;
 
+	if (unlikely(old < 0 || old - i < 0)) {
+		refcount_set(r, REFCOUNT_SATURATED);
+		WARN_ONCE(1, "refcount_t: underflow; use-after-free.\n");
+	}
+
+	return false;
 }
 
 /**

@@ -276,9 +291,13 @@ static inline __must_check bool refcount_dec_and_test(refcount_t *r)
  */
 static inline void refcount_dec(refcount_t *r)
 {
-	WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
-}
+	int old = atomic_fetch_sub_release(1, &r->refs);
 
+	if (unlikely(old <= 1)) {
+		refcount_set(r, REFCOUNT_SATURATED);
+		WARN_ONCE(1, "refcount_t: decrement hit 0; leaking memory.\n");
+	}
+}
 #else /* CONFIG_REFCOUNT_FULL */
 
 #define REFCOUNT_MAX		INT_MAX

index e3b218d669ce..1cd0a876a789 100644
@@ -23,6 +23,16 @@ typedef struct refcount_struct {
 
 #define REFCOUNT_INIT(n)	{ .refs = ATOMIC_INIT(n), }
 
+enum refcount_saturation_type {
+	REFCOUNT_ADD_NOT_ZERO_OVF,
+	REFCOUNT_ADD_OVF,
+	REFCOUNT_ADD_UAF,
+	REFCOUNT_SUB_UAF,
+	REFCOUNT_DEC_LEAK,
+};
+
+void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t);
+
 /**
  * refcount_set - set a refcount's value
  * @r: the refcount

@@ -154,10 +164,8 @@ static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
 			break;
 	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
 
-	if (unlikely(old < 0 || old + i < 0)) {
-		refcount_set(r, REFCOUNT_SATURATED);
-		WARN_ONCE(1, "refcount_t: saturated; leaking memory.\n");
-	}
+	if (unlikely(old < 0 || old + i < 0))
+		refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
 
 	return old;
 }

@@ -182,11 +190,10 @@ static inline void refcount_add(int i, refcount_t *r)
 {
 	int old = atomic_fetch_add_relaxed(i, &r->refs);
 
-	WARN_ONCE(!old, "refcount_t: addition on 0; use-after-free.\n");
-	if (unlikely(old <= 0 || old + i <= 0)) {
-		refcount_set(r, REFCOUNT_SATURATED);
-		WARN_ONCE(old, "refcount_t: saturated; leaking memory.\n");
-	}
+	if (unlikely(!old))
+		refcount_warn_saturate(r, REFCOUNT_ADD_UAF);
+	else if (unlikely(old < 0 || old + i < 0))
+		refcount_warn_saturate(r, REFCOUNT_ADD_OVF);
 }
 
 /**

@@ -253,10 +260,8 @@ static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
 		return true;
 	}
 
-	if (unlikely(old < 0 || old - i < 0)) {
-		refcount_set(r, REFCOUNT_SATURATED);
-		WARN_ONCE(1, "refcount_t: underflow; use-after-free.\n");
-	}
+	if (unlikely(old < 0 || old - i < 0))
+		refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
 
 	return false;
 }

@@ -291,12 +296,8 @@ static inline __must_check bool refcount_dec_and_test(refcount_t *r)
  */
 static inline void refcount_dec(refcount_t *r)
 {
-	int old = atomic_fetch_sub_release(1, &r->refs);
-
-	if (unlikely(old <= 1)) {
-		refcount_set(r, REFCOUNT_SATURATED);
-		WARN_ONCE(1, "refcount_t: decrement hit 0; leaking memory.\n");
-	}
+	if (unlikely(atomic_fetch_sub_release(1, &r->refs) <= 1))
+		refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
 }
 #else /* CONFIG_REFCOUNT_FULL */
 

index 3a534fbebdcc..8b7e249c0e10 100644
@@ -8,6 +8,34 @@
 #include <linux/spinlock.h>
 #include <linux/bug.h>
 
+#define REFCOUNT_WARN(str)	WARN_ONCE(1, "refcount_t: " str ".\n")
+
+void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t)
+{
+	refcount_set(r, REFCOUNT_SATURATED);
+
+	switch (t) {
+	case REFCOUNT_ADD_NOT_ZERO_OVF:
+		REFCOUNT_WARN("saturated; leaking memory");
+		break;
+	case REFCOUNT_ADD_OVF:
+		REFCOUNT_WARN("saturated; leaking memory");
+		break;
+	case REFCOUNT_ADD_UAF:
+		REFCOUNT_WARN("addition on 0; use-after-free");
+		break;
+	case REFCOUNT_SUB_UAF:
+		REFCOUNT_WARN("underflow; use-after-free");
+		break;
+	case REFCOUNT_DEC_LEAK:
+		REFCOUNT_WARN("decrement hit 0; leaking memory");
+		break;
+	default:
+		REFCOUNT_WARN("unknown saturation event!?");
+	}
+}
+EXPORT_SYMBOL(refcount_warn_saturate);
+
 /**
  * refcount_dec_if_one - decrement a refcount if it is 1
  * @r: the refcount

index 1cd0a876a789..757d4630115c 100644
@@ -22,6 +22,8 @@ typedef struct refcount_struct {
 } refcount_t;
 
 #define REFCOUNT_INIT(n)	{ .refs = ATOMIC_INIT(n), }
+#define REFCOUNT_MAX		INT_MAX
+#define REFCOUNT_SATURATED	(INT_MIN / 2)
 
 enum refcount_saturation_type {
 	REFCOUNT_ADD_NOT_ZERO_OVF,

@@ -57,9 +59,6 @@ static inline unsigned int refcount_read(const refcount_t *r)
 #ifdef CONFIG_REFCOUNT_FULL
 #include <linux/bug.h>
 
-#define REFCOUNT_MAX		INT_MAX
-#define REFCOUNT_SATURATED	(INT_MIN / 2)
-
 /*
  * Variant of atomic_t specialized for reference counts.
  *

@@ -300,10 +299,6 @@ static inline void refcount_dec(refcount_t *r)
 		refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
 }
 #else /* CONFIG_REFCOUNT_FULL */
-
-#define REFCOUNT_MAX		INT_MAX
-#define REFCOUNT_SATURATED	(INT_MIN / 2)
-
 # ifdef CONFIG_ARCH_HAS_REFCOUNT
 #  include <asm/refcount.h>
 # else

index 3ff577c0b102..5a0c14ebef70 100644
@@ -139,9 +139,6 @@
 # define _ASM_EXTABLE_EX(from, to)				\
 	_ASM_EXTABLE_HANDLE(from, to, ex_handler_ext)
 
-# define _ASM_EXTABLE_REFCOUNT(from, to)			\
-	_ASM_EXTABLE_HANDLE(from, to, ex_handler_refcount)
-
 # define _ASM_NOKPROBE(entry)					\
 	.pushsection "_kprobe_blacklist","aw" ;			\
 	_ASM_ALIGN ;						\

@@ -170,9 +167,6 @@
 # define _ASM_EXTABLE_EX(from, to)				\
 	_ASM_EXTABLE_HANDLE(from, to, ex_handler_ext)
 
-# define _ASM_EXTABLE_REFCOUNT(from, to)			\
-	_ASM_EXTABLE_HANDLE(from, to, ex_handler_refcount)
-
 /* For C file, we already have NOKPROBE_SYMBOL macro */
 #endif
 

126 lines not on this page · raw message

55 lines not on this page · raw message

174 lines not on this page · raw message

index 8b7e249c0e10..ebac8b7d15a7 100644
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
- * Out-of-line refcount functions common to all refcount implementations.
+ * Out-of-line refcount functions.
  */
 
 #include <linux/mutex.h>

index d83d403dac2e..09f759228e3f 100644
@@ -328,13 +328,6 @@ extern int oops_may_print(void);
 void do_exit(long error_code) __noreturn;
 void complete_and_exit(struct completion *, long) __noreturn;
 
-#ifdef CONFIG_ARCH_HAS_REFCOUNT
-void refcount_error_report(struct pt_regs *regs, const char *err);
-#else
-static inline void refcount_error_report(struct pt_regs *regs, const char *err)
-{ }
-#endif
-
 /* Internal, do not use. */
 int __must_check _kstrtoul(const char *s, unsigned int base, unsigned long *res);
 int __must_check _kstrtol(const char *s, unsigned int base, long *res);

index 47e8ebccc22b..10d05fd4f9c3 100644
@@ -670,17 +670,6 @@ EXPORT_SYMBOL(__stack_chk_fail);
 
 #endif
 
-#ifdef CONFIG_ARCH_HAS_REFCOUNT
-void refcount_error_report(struct pt_regs *regs, const char *err)
-{
-	WARN_RATELIMIT(1, "refcount_t %s at %pB in %s[%d], uid/euid: %u/%u\n",
-		err, (void *)instruction_pointer(regs),
-		current->comm, task_pid_nr(current),
-		from_kuid_munged(&init_user_ns, current_uid()),
-		from_kuid_munged(&init_user_ns, current_euid()));
-}
-#endif
-
 core_param(panic, panic_timeout, int, 0644);
 core_param(panic_print, panic_print, ulong, 0644);
 core_param(pause_on_oops, pause_on_oops, int, 0644);

index abf3b7c1f686..de7c5ab528d9 100644
@@ -119,7 +119,7 @@ void lkdtm_REFCOUNT_DEC_ZERO(void)
 static void check_negative(refcount_t *ref, int start)
 {
 	/*
-	 * CONFIG_REFCOUNT_FULL refuses to move a refcount at all on an
+	 * refcount_t refuses to move a refcount at all on an
 	 * over-sub, so we have to track our starting position instead of
 	 * looking only at zero-pinning.
 	 */

@@ -202,7 +202,6 @@ static void check_from_zero(refcount_t *ref)
 
 /*
  * A refcount_inc() from zero should pin to zero or saturate and may WARN.
- * Only CONFIG_REFCOUNT_FULL provides this protection currently.
  */
 void lkdtm_REFCOUNT_INC_ZERO(void)
 {