[PATCH v2] module: add usage links when calling ref_module func

[Zhiqiang Liu]

From: Zhiqiang Liu <liuzhiqiang26@xxxxxxxxxx>

Users can call ref_module func in their modules to construct
relationships with other modules. However, the holders
'/sys/module/<mod-name>/holders' of the target module donot include
the users` module. So lsmod command misses detailed info of 'Used by'.

When load module, the process is given as follows,
load_module()
	-> mod_sysfs_setup()
		-> add_usage_links
	-> do_init_module
		-> mod->init()

add_usage_links func creates holders of target modules linking to
this module. If ref_module is called in mod->init() func, the usage
links cannot be added.

Here, we will add usage link of a to b's holder_dir.

V1->V2:
- remove incorrect Fixes tag
- fix error handling of sysfs_create_link as suggested by Jessica Yu

Signed-off-by: Zhiqiang Liu <liuzhiqiang26@xxxxxxxxxx>
Suggested-by: Jessica Yu <jeyu@xxxxxxxxxx>
Reviewed-by: Kang Zhou <zhoukang7@xxxxxxxxxx>
---
 kernel/module.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 80c7c09584cf..672abce2222c 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -837,25 +837,26 @@ static int already_uses(struct module *a, struct module *b)
  *    'b' can walk the list to see who sourced them), and of 'a'
  *    targets (so 'a' can see what modules it targets).
  */
-static int add_module_usage(struct module *a, struct module *b)
+static struct module_use *add_module_usage(struct module *a, struct module *b)
 {
 	struct module_use *use;

 	pr_debug("Allocating new usage for %s.\n", a->name);
 	use = kmalloc(sizeof(*use), GFP_ATOMIC);
 	if (!use)
-		return -ENOMEM;
+		return NULL;

 	use->source = a;
 	use->target = b;
 	list_add(&use->source_list, &b->source_list);
 	list_add(&use->target_list, &a->target_list);
-	return 0;
+	return use;
 }

 /* Module a uses b: caller needs module_mutex() */
 int ref_module(struct module *a, struct module *b)
 {
+	struct module_use *use;
 	int err;

 	if (b == NULL || already_uses(a, b))
@@ -866,9 +867,18 @@ int ref_module(struct module *a, struct module *b)
 	if (err)
 		return err;

-	err = add_module_usage(a, b);
+	use = add_module_usage(a, b);
+	if (!use) {
+		module_put(b);
+		return -ENOMEM;
+	}
+
+	err = sysfs_create_link(b->holders_dir, &a->mkobj.kobj, a->name);
 	if (err) {
 		module_put(b);
+		list_del(&use->source_list);
+		list_del(&use->target_list);
+		kfree(use);
 		return err;
 	}
 	return 0;
-- 
2.19.1