From f5baaf48e3e82b1caf9f5cd1207d4d6feba3a2e5 Mon Sep 17 00:00:00 2001 From: Thomas Bertschinger Date: Mon, 15 Jan 2024 23:41:02 -0700 Subject: move Rust sources to top level, C sources into c_src This moves the Rust sources out of rust_src/ and into the top level. Running the bcachefs executable out of the development tree is now: $ ./target/release/bcachefs command or $ cargo run --profile release -- command instead of "./bcachefs command". Building and installing is still: $ make && make install Signed-off-by: Thomas Bertschinger Signed-off-by: Kent Overstreet --- c_src/include/linux/refcount.h | 352 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 352 insertions(+) create mode 100644 c_src/include/linux/refcount.h (limited to 'c_src/include/linux/refcount.h') diff --git a/c_src/include/linux/refcount.h b/c_src/include/linux/refcount.h new file mode 100644 index 00000000..ddeec986 --- /dev/null +++ b/c_src/include/linux/refcount.h @@ -0,0 +1,352 @@ +/* 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 +#include +#include +#include + +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, +}; + +/** + * 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_acquire(&r->refs, &old, old + i)); + + if (oldp) + *oldp = old; + + 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_add_return(i, &r->refs); + + if (oldp) + *oldp = old; +} + +/** + * 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_sub_return_release(i, &r->refs); + + if (oldp) + *oldp = old; + + if (old == i) { + smp_acquire__after_ctrl_dep(); + return true; + } + + 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_sub_return_release(1, &r->refs); + + if (oldp) + *oldp = old; +} + +/** + * 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 */ -- cgit v1.2.3