Kmemleak handling is one of the reasons why kfree_nolock() cannot currently handle kmalloc() objects, because calling kmemleak_free() would involve spinning on its internal raw spinlocks.
Kmemleak is a debugging mechanism so we could simply defer all kfree_nolock() to irq_work if it's enabled, and eat the extra cost. But that would be unnecessary pessimistic. We expect kfree_nolock() will be still mostly called on objects from kmalloc_nolock() that are not registered in kmemleak so they still don't need any deferred freeing. Thus introduce kmemleak_may_need_free() that can check if the object is registered. This is done using __lookup_object() performed under a raw_spin_trylock_irqsave(), which is safe to attempt from kfree_nolock() (except from a NMI on a !CONFIG_SMP system). When that trylock fails or can't be attempted, we however must assume the object might be registered, and defer the freeing. The ordering of kmsan/kasan handling and kmemleak is also different from what kfree() is doing, but as explained in the comment, it should be OK. Signed-off-by: Vlastimil Babka (SUSE) <[email protected]> --- include/linux/kmemleak.h | 17 +++++++++++++++++ mm/kmemleak.c | 42 ++++++++++++++++++++++++++++++++++++++++++ mm/slub.c | 34 ++++++++++++++++++++++++++-------- 3 files changed, 85 insertions(+), 8 deletions(-) diff --git a/include/linux/kmemleak.h b/include/linux/kmemleak.h index fbd424b2abb1..52f75f10a9ce 100644 --- a/include/linux/kmemleak.h +++ b/include/linux/kmemleak.h @@ -22,6 +22,7 @@ extern void kmemleak_alloc_percpu(const void __percpu *ptr, size_t size, extern void kmemleak_vmalloc(const struct vm_struct *area, size_t size, gfp_t gfp) __ref; extern void kmemleak_free(const void *ptr) __ref; +bool kmemleak_may_need_free(const void *ptr) __ref; extern void kmemleak_free_part(const void *ptr, size_t size) __ref; extern void kmemleak_free_percpu(const void __percpu *ptr) __ref; extern void kmemleak_update_trace(const void *ptr) __ref; @@ -50,6 +51,14 @@ static inline void kmemleak_free_recursive(const void *ptr, slab_flags_t flags) kmemleak_free(ptr); } +static inline bool kmemleak_may_need_free_recursive(const void *ptr, slab_flags_t flags) +{ + if (!(flags & SLAB_NOLEAKTRACE)) + return kmemleak_may_need_free(ptr); + + return false; +} + static inline void kmemleak_erase(void **ptr) { *ptr = NULL; @@ -86,6 +95,14 @@ static inline void kmemleak_free_part(const void *ptr, size_t size) static inline void kmemleak_free_recursive(const void *ptr, slab_flags_t flags) { } +static inline bool kmemleak_may_need_free(const void *ptr) +{ + return false; +} +static inline bool kmemleak_may_need_free_recursive(const void *ptr, slab_flags_t flags) +{ + return false; +} static inline void kmemleak_free_percpu(const void __percpu *ptr) { } diff --git a/mm/kmemleak.c b/mm/kmemleak.c index 7c7ba17ce7af..e3560ce82632 100644 --- a/mm/kmemleak.c +++ b/mm/kmemleak.c @@ -1168,6 +1168,48 @@ void __ref kmemleak_free(const void *ptr) } EXPORT_SYMBOL_GPL(kmemleak_free); +/** + * kmemleak_may_need_free - check if object is registered + * @ptr: pointer to beginning of the object + * + * This function is called from the kernel allocator when an object should be + * freed but the caller context might be unsafe to spin on the internal locks. + * + * It will therefore only use trylock and thus might return a false positive + * if the trylock fails and the status cannot be determined. + * + * For objects that (might) need free, the allocator has to defer the actual + * freeing to a safe context. + * + * The assumption is that most objects freed from the unsafe context are also + * allocated in such context and thus are not registered in kmemleak, so it's + * unlikely the defered freeing will be necessary just because kmemleak is + * enabled. + */ +bool __ref kmemleak_may_need_free(const void *ptr) +{ + unsigned long flags; + struct kmemleak_object *object; + + pr_debug("%s(0x%px)\n", __func__, ptr); + + if (!kmemleak_free_enabled || !ptr || IS_ERR(ptr)) + return false; + + /* On UP, raw_spin_trylock() always succeeds even when it is locked */ + if (!IS_ENABLED(CONFIG_SMP) && in_nmi()) + return true; + + if (!raw_spin_trylock_irqsave(&kmemleak_lock, flags)) + return true; + + object = __lookup_object((unsigned long)ptr, 0, 0); + + raw_spin_unlock_irqrestore(&kmemleak_lock, flags); + + return !!object; +} + /** * kmemleak_free_part - partially unregister a previously registered object * @ptr: pointer to the beginning or inside the object. This also diff --git a/mm/slub.c b/mm/slub.c index 2d7648b96bfa..423b5bdb910b 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -6390,6 +6390,8 @@ static void deferred_percpu_work_fn(struct irq_work *work) /* Point 'x' back to the beginning of allocated object */ x -= s->offset; + kmemleak_free_recursive(x, s->flags); + /* * We used freepointer in 'x' to link 'x' into df->objects. * Clear it to NULL to avoid false positive detection @@ -6403,8 +6405,15 @@ static void deferred_percpu_work_fn(struct irq_work *work) llnode = llist_del_all(&dpw->objects_kfence); llist_for_each_safe(pos, t, llnode) { + struct kmem_cache *s; + struct slab *slab; void *obj = kfence_llnode_to_obj(pos); + slab = virt_to_slab(obj); + s = slab->slab_cache; + + kmemleak_free_recursive(obj, s->flags); + __kfence_free(obj); } @@ -6781,15 +6790,10 @@ EXPORT_SYMBOL(kfree); /* * Can be called while holding raw_spinlock_t or from IRQ and NMI, - * but ONLY for objects allocated by kmalloc_nolock(). - * - * In case kmemleak is enabled, + * but may defer freeing to irq_work() in some cases. * - * obj = kmalloc(); kfree_nolock(obj); - * - * will miss kmemleak book keeping and will cause false positives. - * - * large_kmalloc is not supported either. + * Intended mainly for objects allocated from kmalloc_nolock(), but can handle + * also kmem_cache_alloc() and kmalloc() objects, except large_kmalloc. */ void kfree_nolock(const void *object) { @@ -6844,9 +6848,23 @@ void kfree_nolock(const void *object) */ kasan_slab_free(s, x, false, false, /* skip quarantine */true); + /* + * with kfree() the kmemleak handling happens much sooner, but for + * defering we need to write llnode to the object's freepointer so + * we should have it in the state when it's no longer treated as + * allocated by kasan etc. + * + * defer_free will also reset the pointer tag, but it's ok to do a + * deferred kmemleak_free() using the untagged pointer, because + * __lookup_object() resets the tag anyway + */ + if (unlikely(kmemleak_may_need_free_recursive(x, s->flags))) + goto defer; + if (likely(can_free_to_pcs(slab)) && likely(free_to_pcs(s, x, false))) return; +defer: /* * __slab_free() can locklessly cmpxchg16 into a slab, but then it might * need to take spin_lock for further processing. -- 2.55.0
