参考文章：Linux内核态内存泄漏检测工具--kmemleak工具原理及应用_linux 内存泄漏检测工具-CSDN博客

细说｜Linux 内存泄漏检测实现原理与实现_内核_指针_信息 

kmemleak原理：看网上说大概原理是在通过kmalloc，vmalloc，kmem_cache_alloc等函数申请的时候，把申请的内存地址、内存大小，stack trace等信息记录下来。在内存释放的时候去将记录的信息删除掉。这样就能知道了哪些内存没有被释放掉，同时也能知道这些未是否的内存究竟是哪些人申请的。

那么如何能够确认这些未释放的内存是泄漏，而不是其他人正在使用的呢。kmemleak认为如果这个内存没有泄漏，那么内存中一定存在一个值去记录这个未释放内存块的地址。所以基于这个假设，kmemleak会定期去内存里面扫描，看内存里面是否会存在这些地址。如果不存在，则认为这个内存块泄漏了。但是当内存中恰好有个值和内存块地址相同。那按照上面的逻辑就会出现漏报，另外如何申请的内存块的地址并未直接进行保存，是通过多个数字计算得到。那么这种情况下会出现误报的情况。

下面看看代码是怎么实现的(以vmalloc为例)

内存申请记录：vmalloc->__vmalloc-->__vmalloc_node->__vmalloc_node_range->kmemleak_vmalloc

void *__vmalloc_node_range(unsigned long size, unsigned long align,
			unsigned long start, unsigned long end, gfp_t gfp_mask,
			pgprot_t prot, unsigned long vm_flags, int node,
			const void *caller)
{
....................................
	kmemleak_vmalloc(area, size, gfp_mask);
......................

fail:
	warn_alloc(gfp_mask, NULL,
			  "vmalloc: allocation failure: %lu bytes", real_size);
	return NULL;
}

void __ref kmemleak_vmalloc(const struct vm_struct *area, size_t size, gfp_t gfp)
{
........................................
	if (kmemleak_enabled) {
		create_object((unsigned long)area->addr, size, 2, gfp);
		object_set_excess_ref((unsigned long)area,
				      (unsigned long)area->addr);
	}
}

create_object：对于每个申请的内存块，创建一个节点保存相关信息，加入到红黑树中

static struct kmemleak_object *create_object(unsigned long ptr, size_t size,
					     int min_count, gfp_t gfp)
{
......................................
	/* kernel backtrace */
	object->trace_len = __save_stack_trace(object->trace);

	write_lock_irqsave(&kmemleak_lock, flags);

	untagged_ptr = (unsigned long)kasan_reset_tag((void *)ptr);
	min_addr = min(min_addr, untagged_ptr);
	max_addr = max(max_addr, untagged_ptr + size);
	link = &object_tree_root.rb_node;
	rb_parent = NULL;
	while (*link) {
		rb_parent = *link;
		parent = rb_entry(rb_parent, struct kmemleak_object, rb_node);
		if (ptr + size <= parent->pointer)
			link = &parent->rb_node.rb_left;
		else if (parent->pointer + parent->size <= ptr)
			link = &parent->rb_node.rb_right;
		else {
			kmemleak_stop("Cannot insert 0x%lx into the object search tree (overlaps existing)\n",
				      ptr);
			/*
			 * No need for parent->lock here since "parent" cannot
			 * be freed while the kmemleak_lock is held.
			 */
			dump_object_info(parent);
			kmem_cache_free(object_cache, object);
			object = NULL;
			goto out;
		}
	}
	rb_link_node(&object->rb_node, rb_parent, link);
	rb_insert_color(&object->rb_node, &object_tree_root);

	list_add_tail_rcu(&object->object_list, &object_list);
out:
	write_unlock_irqrestore(&kmemleak_lock, flags);
	return object;
}

内存记录删除：vfree->kmemleak_free->delete_object_full。从树里面把节点删除

void vfree(const void *addr)
{
	BUG_ON(in_nmi());
	kmemleak_free(addr);
	.................
}
void __ref kmemleak_free(const void *ptr)
{
	pr_debug("%s(0x%p)\n", __func__, ptr);

	if (kmemleak_free_enabled && ptr && !IS_ERR(ptr))
		delete_object_full((unsigned long)ptr);
}
static void delete_object_full(unsigned long ptr)
{
	struct kmemleak_object *object;

	object = find_and_remove_object(ptr, 0);
	if (!object) {
#ifdef DEBUG
		kmemleak_warn("Freeing unknown object at 0x%08lx\n",
			      ptr);
#endif
		return;
	}
	__delete_object(object);
}

至此，申请和释放的内存都能被记录和删除了。如果存在内存泄漏，那么泄漏的地址一定在树里面。那么何时、如何检查这颗树呢？

扫描内核线程初始化

static int __init kmemleak_late_init(void)
{
	kmemleak_initialized = 1;

.................................
	if (IS_ENABLED(CONFIG_DEBUG_KMEMLEAK_AUTO_SCAN)) {
		mutex_lock(&scan_mutex);
		start_scan_thread();
		mutex_unlock(&scan_mutex);
	}

	pr_info("Kernel memory leak detector initialized (mem pool available: %d)\n",
		mem_pool_free_count);

	return 0;
}
late_initcall(kmemleak_late_init);

static void start_scan_thread(void)
{
	if (scan_thread)
		return;
	scan_thread = kthread_run(kmemleak_scan_thread, NULL, "kmemleak");
	if (IS_ERR(scan_thread)) {
		pr_warn("Failed to create the scan thread\n");
		scan_thread = NULL;
	}
}

static int kmemleak_scan_thread(void *arg)
{
	static int first_run = IS_ENABLED(CONFIG_DEBUG_KMEMLEAK_AUTO_SCAN);

	pr_info("Automatic memory scanning thread started\n");
	set_user_nice(current, 10);

	/*
	 * Wait before the first scan to allow the system to fully initialize.
	 */
	if (first_run) {
		signed long timeout = msecs_to_jiffies(SECS_FIRST_SCAN * 1000);
		first_run = 0;
		while (timeout && !kthread_should_stop())
			timeout = schedule_timeout_interruptible(timeout);
	}

	while (!kthread_should_stop()) {
		signed long timeout = jiffies_scan_wait;//这里是10分钟

		mutex_lock(&scan_mutex);
		kmemleak_scan();
		mutex_unlock(&scan_mutex);

		/* wait before the next scan */
		while (timeout && !kthread_should_stop())
			timeout = schedule_timeout_interruptible(timeout);
	}

	pr_info("Automatic memory scanning thread ended\n");

	return 0;
}

kmemleak_scan：真正负责扫描红黑树的地方

参考文章细说｜Linux 内存泄漏检测实现原理与实现_内核_指针_信息

• 白色节点：表示此对象没有被指针引用（ count 字段少于 min_count 字段）。

• 灰色节点：表示此对象被一个或多个指针引用（ count 字段大于或等于 min_count 字段）。

• 黑色节点：表示此对象不需要被扫描（ min_count 字段等于 -1）。

static void kmemleak_scan(void)
{
	unsigned long flags;
	struct kmemleak_object *object;
	int i;
	int new_leaks = 0;

	jiffies_last_scan = jiffies;

	/* prepare the kmemleak_object's */
	rcu_read_lock();
    /* 将所有节点标记为白色节点 */
	list_for_each_entry_rcu(object, &object_list, object_list) {
		spin_lock_irqsave(&object->lock, flags);
#ifdef DEBUG
		/*
		 * With a few exceptions there should be a maximum of
		 * 1 reference to any object at this point.
		 */
		if (atomic_read(&object->use_count) > 1) {
			pr_debug("object->use_count = %d\n",
				 atomic_read(&object->use_count));
			dump_object_info(object);
		}
#endif
		/* reset the reference count (whiten the object) */
		object->count = 0;
		if (color_gray(object) && get_object(object))
			list_add_tail(&object->gray_list, &gray_list);

		spin_unlock_irqrestore(&object->lock, flags);
	}
	rcu_read_unlock();

    /* 扫描数据段 */
	/* data/bss scanning */
	scan_block(_sdata, _edata, NULL, 1);
	scan_block(__bss_start, __bss_stop, NULL, 1);

#ifdef CONFIG_SMP
	/* per-cpu sections scanning */
	for_each_possible_cpu(i)
		scan_block(__per_cpu_start + per_cpu_offset(i),
			   __per_cpu_end + per_cpu_offset(i), NULL, 1);
#endif

	/*
	 * Struct page scanning for each node.
	 */
	get_online_mems();
	for_each_online_node(i) {
		unsigned long start_pfn = node_start_pfn(i);
		unsigned long end_pfn = node_end_pfn(i);
		unsigned long pfn;

		for (pfn = start_pfn; pfn < end_pfn; pfn++) {
			struct page *page;

			if (!pfn_valid(pfn))
				continue;
			page = pfn_to_page(pfn);
			/* only scan if page is in use */
			if (page_count(page) == 0)
				continue;
			scan_block(page, page + 1, NULL, 1);
		}
	}
	put_online_mems();

	/*
	 * Scanning the task stacks (may introduce false negatives).
	 */
	if (kmemleak_stack_scan) {
		struct task_struct *p, *g;

		read_lock(&tasklist_lock);
		do_each_thread(g, p) {
			scan_block(task_stack_page(p), task_stack_page(p) +
				   THREAD_SIZE, NULL, 0);
		} while_each_thread(g, p);
		read_unlock(&tasklist_lock);
	}

	/*
	 * Scan the objects already referenced from the sections scanned
	 * above.
	 */
    /* 看网上意思是灰色节点里面可能也包含了指向内存块的指针，因此也需要扫描 */
	scan_gray_list();

	/*
	 * Check for new or unreferenced objects modified since the previous
	 * scan and color them gray until the next scan.
	 */
	rcu_read_lock();
	/* 感觉是需要重新在来一下 */
    list_for_each_entry_rcu(object, &object_list, object_list) {
		spin_lock_irqsave(&object->lock, flags);
		if (color_white(object) && (object->flags & OBJECT_ALLOCATED)
		    && update_checksum(object) && get_object(object)) {
			/* color it gray temporarily */
			object->count = object->min_count;
			list_add_tail(&object->gray_list, &gray_list);
		}
		spin_unlock_irqrestore(&object->lock, flags);
	}
	rcu_read_unlock();

	/*
	 * Re-scan the gray list for modified unreferenced objects.
	 */
	scan_gray_list();

	/*
	 * If scanning was stopped do not report any new unreferenced objects.
	 */
	if (scan_should_stop())
		return;

	/*
	 * Scanning result reporting.
	 */
    /* 最后白色节点就可能是出问题了 */
	rcu_read_lock();
	list_for_each_entry_rcu(object, &object_list, object_list) {
		spin_lock_irqsave(&object->lock, flags);
		if (unreferenced_object(object) &&
		    !(object->flags & OBJECT_REPORTED)) {
			object->flags |= OBJECT_REPORTED;
			new_leaks++;
		}
		spin_unlock_irqrestore(&object->lock, flags);
	}
	rcu_read_unlock();

	if (new_leaks) {
		kmemleak_found_leaks = true;

		pr_info("%d new suspected memory leaks (see "
			"/sys/kernel/debug/kmemleak)\n", new_leaks);
	}

}

检查功能开启后重新编译内核即可

实际效果展示：

int test_thread(void* a)
{
	int i = 0;	
	while (i < 100)
	{
		kmalloc(100, GFP_KERNEL);
		++i;
	}
	return 0;
}

这里是主动触发扫描

触发扫描

[root@arm_test ]# echo scan  > /sys/kernel/debug/kmemleak 

查看扫描结果

[root@arm_test ]# cat /sys/kernel/debug/kmemleak 

可以看到这块内存是在哪个进程，哪个函数申请的。如果它泄露了，就可以根据这些信息排查 （6b应该是开启了slub debug导致的）