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 /* SPDX-License-Identifier: GPL-2.0 */
 /*
  * 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.
  *
  * 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.
  * Linux limits the maximum number of tasks to PID_MAX_LIMIT, which is currently
  * 0x400000 (and can't easily be raised in the future beyond FUTEX_TID_MASK).
  * With the current PID limit, if no batched refcounting operations are used and
  * the attacker can't repeatedly trigger kernel oopses in the middle of refcount
  * operations, this makes it impossible for a saturated refcount to leave the
  * saturation range, even if it is possible for multiple uses of the same
  * refcount to nest in the context of a single task:
  *
  *     (UINT_MAX+1-REFCOUNT_SATURATED) / PID_MAX_LIMIT =
  *     0x40000000 / 0x400000 = 0x100 = 256
  *
  * If hundreds of references are added/removed with a single refcounting
  * operation, it may potentially be possible to leave the saturation range; but
  * 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.
  *
  * 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.
  *
  */
 
 #ifndef _LINUX_REFCOUNT_H
 #define _LINUX_REFCOUNT_H
 
 #include <linux/atomic.h>
 #include <linux/bug.h>
 #include <linux/compiler.h>
 #include <linux/limits.h>
 #include <linux/spinlock_types.h>
 
 struct mutex;
 
 /**
  * typedef refcount_t - variant of atomic_t specialized for reference counts
  * @refs: atomic_t counter field
  *
  * The counter saturates at REFCOUNT_SATURATED and will not move once
  * there. This avoids wrapping the counter and causing 'spurious'
  * use-after-free bugs.
  */
 typedef struct refcount_struct {
     atomic_t refs;
 } 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,
     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
  * @n: value to which the refcount will be set
  */
 static inline void refcount_set(refcount_t *r, int n)
 {
     atomic_set(&r->refs, n);
 }
 
 /**
  * refcount_read - get a refcount's value
  * @r: the refcount
  *
  * Return: the refcount's value
  */
 static inline unsigned int refcount_read(const refcount_t *r)
 {
     return atomic_read(&r->refs);
 }
 
 static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
 {
     int old = refcount_read(r);
 
     do {
         if (!old)
             break;
     } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
 
     if (oldp)
         *oldp = old;
 
     if (unlikely(old < 0 || old + i < 0))
         refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
 
     return old;
 }
 
 /**
  * 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)
 {
     return __refcount_add_not_zero(i, r, NULL);
 }
 
 static inline void __refcount_add(int i, refcount_t *r, int *oldp)
 {
     int old = atomic_fetch_add_relaxed(i, &r->refs);
 
     if (oldp)
         *oldp = old;
 
     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);
 }
 
 /**
  * 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)
 {
     __refcount_add(i, r, NULL);
 }
 
 static inline __must_check bool __refcount_inc_not_zero(refcount_t *r, int *oldp)
 {
     return __refcount_add_not_zero(1, r, oldp);
 }
 
 /**
  * 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)
 {
     return __refcount_inc_not_zero(r, NULL);
 }
 
 static inline void __refcount_inc(refcount_t *r, int *oldp)
 {
     __refcount_add(1, r, oldp);
 }
 
 /**
  * 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)
 {
     __refcount_inc(r, NULL);
 }
 
 static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
 {
     int old = atomic_fetch_sub_release(i, &r->refs);
 
     if (oldp)
         *oldp = old;
 
     if (old == i) {
         smp_acquire__after_ctrl_dep();
         return true;
     }
 
     if (unlikely(old < 0 || old - i < 0))
         refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
 
     return false;
 }
 
 /**
  * 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)
 {
     return __refcount_sub_and_test(i, r, NULL);
 }
 
 static inline __must_check bool __refcount_dec_and_test(refcount_t *r, int *oldp)
 {
     return __refcount_sub_and_test(1, r, oldp);
 }
 
 /**
  * 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_dec_and_test(r, NULL);
 }
 
 static inline void __refcount_dec(refcount_t *r, int *oldp)
 {
     int old = atomic_fetch_sub_release(1, &r->refs);
 
     if (oldp)
         *oldp = old;
 
     if (unlikely(old <= 1))
         refcount_warn_saturate(r, REFCOUNT_DEC_LEAK);
 }
 
 /**
  * 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)
 {
     __refcount_dec(r, NULL);
 }
 
 extern __must_check bool refcount_dec_if_one(refcount_t *r);
 extern __must_check bool refcount_dec_not_one(refcount_t *r);
 extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) __cond_acquires(lock);
 extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) __cond_acquires(lock);
 extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
                                spinlock_t *lock,
                                unsigned long *flags) __cond_acquires(lock);
 #endif /* _LINUX_REFCOUNT_H */

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