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locking/refcounts: Out-of-line everything
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Linus asked to please make this real C code.

And since size then isn't an issue what so ever anymore, remove the
debug knob and make all WARN()s unconditional.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dwindsor@gmail.com
Cc: elena.reshetova@intel.com
Cc: gregkh@linuxfoundation.org
Cc: ishkamiel@gmail.com
Cc: keescook@chromium.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Peter Zijlstra authored and Ingo Molnar committed Feb 24, 2017
1 parent 2bfe01e commit 29dee3c
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Showing 4 changed files with 280 additions and 279 deletions.
277 changes: 12 additions & 265 deletions include/linux/refcount.h
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Original file line number Diff line number Diff line change
@@ -1,55 +1,10 @@
#ifndef _LINUX_REFCOUNT_H
#define _LINUX_REFCOUNT_H

/*
* 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 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, 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().
*
*/

#include <linux/atomic.h>
#include <linux/bug.h>
#include <linux/mutex.h>
#include <linux/spinlock.h>

#ifdef CONFIG_DEBUG_REFCOUNT
#define REFCOUNT_WARN(cond, str) WARN_ON(cond)
#define __refcount_check __must_check
#else
#define REFCOUNT_WARN(cond, str) (void)(cond)
#define __refcount_check
#endif

typedef struct refcount_struct {
atomic_t refs;
} refcount_t;
Expand All @@ -66,229 +21,21 @@ static inline unsigned int refcount_read(const refcount_t *r)
return atomic_read(&r->refs);
}

static inline __refcount_check
bool refcount_add_not_zero(unsigned int i, 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 + i;
if (new < val)
new = UINT_MAX;
old = atomic_cmpxchg_relaxed(&r->refs, val, new);
if (old == val)
break;

val = old;
}

REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

return true;
}

static inline void refcount_add(unsigned int i, refcount_t *r)
{
REFCOUNT_WARN(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\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. See the comment on top.
*/
static inline __refcount_check
bool refcount_inc_not_zero(refcount_t *r)
{
unsigned int old, new, val = atomic_read(&r->refs);

for (;;) {
new = val + 1;

if (!val)
return false;

if (unlikely(!new))
return true;

old = atomic_cmpxchg_relaxed(&r->refs, val, new);
if (old == val)
break;

val = old;
}

REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

return true;
}

/*
* 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)
{
REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
}

/*
* 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, and provides a control dependency such that free() must come after.
* See the comment on top.
*/
static inline __refcount_check
bool refcount_sub_and_test(unsigned int i, refcount_t *r)
{
unsigned int old, new, val = atomic_read(&r->refs);

for (;;) {
if (unlikely(val == UINT_MAX))
return false;

new = val - i;
if (new > val) {
REFCOUNT_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;
}

static inline __refcount_check
bool refcount_dec_and_test(refcount_t *r)
{
return refcount_sub_and_test(1, r);
}
extern __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r);
extern void refcount_add(unsigned int i, refcount_t *r);

/*
* Similar to atomic_dec(), 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.
*/
static inline
void refcount_dec(refcount_t *r)
{
REFCOUNT_WARN(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
}

/*
* No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
* success thereof.
*
* Like all decrement operations, it provides release memory order and provides
* a control dependency.
*
* It can be used like a try-delete operator; this explicit case is provided
* and not cmpxchg in generic, because that would allow implementing unsafe
* operations.
*/
static inline __refcount_check
bool refcount_dec_if_one(refcount_t *r)
{
return atomic_cmpxchg_release(&r->refs, 1, 0) == 1;
}

/*
* No atomic_t counterpart, it decrements unless the value is 1, in which case
* it will return false.
*
* Was often done like: atomic_add_unless(&var, -1, 1)
*/
static inline __refcount_check
bool refcount_dec_not_one(refcount_t *r)
{
unsigned int old, new, val = atomic_read(&r->refs);
extern __must_check bool refcount_inc_not_zero(refcount_t *r);
extern void refcount_inc(refcount_t *r);

for (;;) {
if (unlikely(val == UINT_MAX))
return true;
extern __must_check bool refcount_sub_and_test(unsigned int i, refcount_t *r);
extern void refcount_sub(unsigned int i, refcount_t *r);

if (val == 1)
return false;
extern __must_check bool refcount_dec_and_test(refcount_t *r);
extern void refcount_dec(refcount_t *r);

new = val - 1;
if (new > val) {
REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n");
return true;
}

old = atomic_cmpxchg_release(&r->refs, val, new);
if (old == val)
break;

val = old;
}

return true;
}

/*
* 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, and provides a control dependency such that free() must come after.
* See the comment on top.
*/
static inline __refcount_check
bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
{
if (refcount_dec_not_one(r))
return false;

mutex_lock(lock);
if (!refcount_dec_and_test(r)) {
mutex_unlock(lock);
return false;
}

return true;
}

/*
* 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, and provides a control dependency such that free() must come after.
* See the comment on top.
*/
static inline __refcount_check
bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
{
if (refcount_dec_not_one(r))
return false;

spin_lock(lock);
if (!refcount_dec_and_test(r)) {
spin_unlock(lock);
return false;
}

return true;
}
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);
extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock);

#endif /* _LINUX_REFCOUNT_H */
13 changes: 0 additions & 13 deletions lib/Kconfig.debug
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Original file line number Diff line number Diff line change
Expand Up @@ -716,19 +716,6 @@ source "lib/Kconfig.kmemcheck"

source "lib/Kconfig.kasan"

config DEBUG_REFCOUNT
bool "Verbose refcount checks"
help
Say Y here if you want reference counters (refcount_t and kref) to
generate WARNs on dubious usage. Without this refcount_t will still
be a saturating counter and avoid Use-After-Free by turning it into
a resource leak Denial-Of-Service.

Use of this option will increase kernel text size but will alert the
admin of potential abuse.

If in doubt, say "N".

endmenu # "Memory Debugging"

config ARCH_HAS_KCOV
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2 changes: 1 addition & 1 deletion lib/Makefile
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Original file line number Diff line number Diff line change
Expand Up @@ -36,7 +36,7 @@ obj-y += bcd.o div64.o sort.o parser.o debug_locks.o random32.o \
gcd.o lcm.o list_sort.o uuid.o flex_array.o iov_iter.o clz_ctz.o \
bsearch.o find_bit.o llist.o memweight.o kfifo.o \
percpu-refcount.o percpu_ida.o rhashtable.o reciprocal_div.o \
once.o
once.o refcount.o
obj-y += string_helpers.o
obj-$(CONFIG_TEST_STRING_HELPERS) += test-string_helpers.o
obj-y += hexdump.o
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