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

[Jessica Yu]

+++ 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,
> 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);

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) {
> 		module_put(b);
> +		list_del(&use->source_list);
> +		list_del(&use->target_list);
> +		kfree(use);
> 		return err;
> 	}
> 	return 0;
> --
> 2.19.1