Re: [PATCH v3] module: add link_flag pram in ref_module func to decide whether add usage link

[Jessica Yu]

+++ Zhiqiang Liu [20/07/19 22:40 +0800]:
> 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.
>
> Consider that add_module_usage and add usage_link may separate, the
> link_flag pram is added in ref_module func to decide whether add usage
> link after add_module_usage. If link_flag is true, it means usage link
> of a to b's holder_dir should be created immediately after add_module_usage.
>
> V2->V3:
> - add link_flag pram in ref_module func to decide whether add usage link
>
> 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>
> ---
> include/linux/module.h |  2 +-
> kernel/module.c        | 27 ++++++++++++++++++++-------
> 2 files changed, 21 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 188998d3dca9..9ec04b9e93e8 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -632,7 +632,7 @@ static inline void __module_get(struct module *module)
> #define symbol_put_addr(p) do { } while (0)
>
> #endif /* CONFIG_MODULE_UNLOAD */
> -int ref_module(struct module *a, struct module *b);
> +int ref_module(struct module *a, struct module *b, bool link_flag);
>
> /* This is a #define so the string doesn't get put in every .o file */
> #define module_name(mod)			\
> diff --git a/kernel/module.c b/kernel/module.c
> index 80c7c09584cf..00e4862a8ef7 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)
> +int ref_module(struct module *a, struct module *b, bool link_flag)
> {
> +	struct module_use *use;
> 	int err;
>
> 	if (b == NULL || already_uses(a, b))
> @@ -866,9 +867,21 @@ 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;
> +	}
> +
> +	if (!link_flag)
> +		return 0;
> +
> +	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;
> @@ -1152,7 +1165,7 @@ static inline void module_unload_free(struct module *mod)
> {
> }
>
> -int ref_module(struct module *a, struct module *b)
> +int ref_module(struct module *a, struct module *b, bool link_flag)
> {
> 	return strong_try_module_get(b);
> }
> @@ -1407,7 +1420,7 @@ static const struct kernel_symbol *resolve_symbol(struct module *mod,
> 		goto getname;
> 	}
>
> -	err = ref_module(mod, owner);
> +	err = ref_module(mod, owner, false);

Thanks a lot for pinging me. If I'm remembering correctly, we had to
do this because sysfs intialization is being done too late in
load_module(). The last time I was tinkering with a solution, I think
I was able to avoid needing this extra link_flag param by moving the
mod_sysfs_setup() call earlier in load_module(). It just looks a lot
cleaner that way, without that extra param. Have you looked into
trying that? I would prefer that solution rather than adding another
argument to ref_module().

Thanks,

Jessica