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

From: Jessica Yu
Date: Tue Jul 09 2019 - 12:10:47 EST

+++ Zhiqiang Liu [03/07/19 10:09 +0800]:
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,
-> 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.

- 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);

Sigh. This ultimately doesn't work because in load_module(), we use
ref_module() in resolve_symbol(), and mod->mkobj.kobj doesn't get
initialized until mod_sysfs_init(), which happens much later in
load_module(). So what happens is that the ref_module(mod, owner) call
in resolve_symbol() returns an error because sysfs_create_link() fails here.
We could *maybe* move sysfs initialization earlier in load_module()
but that is an entirely untested idea and I would need to think about
that more.

if (err) {
+ list_del(&use->source_list);
+ list_del(&use->target_list);
+ kfree(use);
return err;
return 0;