Re: [POC 01/23] module: Allow to delete module also from inside kernel

[Julien Thierry <jthierry@redhat.com>]

Hi Petr,

On 1/17/20 3:03 PM, Petr Mladek wrote:
> Livepatch subsystems will need to automatically load and delete
> livepatch module when the livepatched one is being removed
> or when the entire livepatch is being removed.
> 
> The code stopping the kernel module is moved to separate function
> so that it can be reused.
> 
> The function always have rights to do the action. Also it does not
> need to search for struct module because it is already passed as
> a parameter.
> 
> On the other hand, it has to make sure that the given struct module
> can't be released in parallel. It is achieved by combining module_put()
> and module_delete() functionality in a single function.
> 
> This patch does not change the existing behavior of delete_module
> syscall.
> 
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> 
> module: Add module_put_and_delete()
> ---
>   include/linux/module.h |   5 +++
>   kernel/module.c        | 119 +++++++++++++++++++++++++++++++------------------
>   2 files changed, 80 insertions(+), 44 deletions(-)
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index bd165ba68617..f69f3fd72dd5 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -623,6 +623,7 @@ extern void __module_get(struct module *module);
>   extern bool try_module_get(struct module *module);
>   
>   extern void module_put(struct module *module);
> +extern int module_put_and_delete(struct module *mod);
>   
>   #else /*!CONFIG_MODULE_UNLOAD*/
>   static inline bool try_module_get(struct module *module)
> @@ -632,6 +633,10 @@ static inline bool try_module_get(struct module *module)
>   static inline void module_put(struct module *module)
>   {
>   }
> +static inline int module_put_and_delete(struct module *mod)
> +{
> +	return 0;
> +}
>   static inline void __module_get(struct module *module)
>   {
>   }
> diff --git a/kernel/module.c b/kernel/module.c
> index b56f3224b161..b3f11524f8f9 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -964,62 +964,36 @@ EXPORT_SYMBOL(module_refcount);
>   /* This exists whether we can unload or not */
>   static void free_module(struct module *mod);
>   
> -SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
> -		unsigned int, flags)
> +int stop_module(struct module *mod, int flags)
>   {
> -	struct module *mod;
> -	char name[MODULE_NAME_LEN];
> -	int ret, forced = 0;
> -
> -	if (!capable(CAP_SYS_MODULE) || modules_disabled)
> -		return -EPERM;
> -
> -	if (strncpy_from_user(name, name_user, MODULE_NAME_LEN-1) < 0)
> -		return -EFAULT;
> -	name[MODULE_NAME_LEN-1] = '\0';
> +	int forced = 0;
>   
> -	audit_log_kern_module(name);
> -
> -	if (mutex_lock_interruptible(&module_mutex) != 0)
> -		return -EINTR;
> -
> -	mod = find_module(name);
> -	if (!mod) {
> -		ret = -ENOENT;
> -		goto out;
> -	}
> -
> -	if (!list_empty(&mod->source_list)) {
> -		/* Other modules depend on us: get rid of them first. */
> -		ret = -EWOULDBLOCK;
> -		goto out;
> -	}
> +	/* Other modules depend on us: get rid of them first. */
> +	if (!list_empty(&mod->source_list))
> +		return -EWOULDBLOCK;
>   
>   	/* Doing init or already dying? */
>   	if (mod->state != MODULE_STATE_LIVE) {
>   		/* FIXME: if (force), slam module count damn the torpedoes */
>   		pr_debug("%s already dying\n", mod->name);
> -		ret = -EBUSY;
> -		goto out;
> +		return -EBUSY;
>   	}
>   
>   	/* If it has an init func, it must have an exit func to unload */
>   	if (mod->init && !mod->exit) {
>   		forced = try_force_unload(flags);
> -		if (!forced) {
> -			/* This module can't be removed */
> -			ret = -EBUSY;
> -			goto out;
> -		}
> +		/* This module can't be removed */
> +		if (!forced)
> +			return -EBUSY;
>   	}
>   
>   	/* Stop the machine so refcounts can't move and disable module. */
> -	ret = try_stop_module(mod, flags, &forced);
> -	if (ret != 0)
> -		goto out;
> +	return try_stop_module(mod, flags, &forced);
> +}
>   
> -	mutex_unlock(&module_mutex);
> -	/* Final destruction now no one is using it. */
> +/* Final destruction now no one is using it. */

Nit: Looks like some copy/paste mixup

> +static void destruct_module(struct module *mod)
> +{
>   	if (mod->exit != NULL)
>   		mod->exit();
>   	blocking_notifier_call_chain(&module_notify_list,
> @@ -1033,8 +1007,44 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
>   	strlcpy(last_unloaded_module, mod->name, sizeof(last_unloaded_module));
>   
>   	free_module(mod);
> +
>   	/* someone could wait for the module in add_unformed_module() */
>   	wake_up_all(&module_wq);
> +}
> +
> +SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
> +		unsigned int, flags)
> +{
> +	struct module *mod;
> +	char name[MODULE_NAME_LEN];
> +	int ret;
> +
> +	if (!capable(CAP_SYS_MODULE) || modules_disabled)
> +		return -EPERM;
> +
> +	if (strncpy_from_user(name, name_user, MODULE_NAME_LEN-1) < 0)
> +		return -EFAULT;
> +	name[MODULE_NAME_LEN-1] = '\0';
> +
> +	audit_log_kern_module(name);
> +
> +	if (mutex_lock_interruptible(&module_mutex) != 0)
> +		return -EINTR;
> +
> +	mod = find_module(name);
> +	if (!mod) {
> +		ret = -ENOENT;
> +		goto out;
> +	}
> +
> +	ret = stop_module(mod, flags);
> +	if (ret)
> +		goto out;
> +
> +	mutex_unlock(&module_mutex);
> +
> +/* Final destruction now no one is using it. */
> +	destruct_module(mod);
>   	return 0;
>   out:
>   	mutex_unlock(&module_mutex);
> @@ -1138,20 +1148,41 @@ bool try_module_get(struct module *module)
>   }
>   EXPORT_SYMBOL(try_module_get);
>   
> -void module_put(struct module *module)
> +/* Must be called under module_mutex or with preemtion disabled */

Might be worth adding some asserts to check for that.

> +static void __module_put(struct module* module)
>   {
>   	int ret;
>   
> +	ret = atomic_dec_if_positive(&module->refcnt);
> +	WARN_ON(ret < 0);	/* Failed to put refcount */
> +	trace_module_put(module, _RET_IP_);
> +}
> +
> +void module_put(struct module *module)
> +{
>   	if (module) {
>   		preempt_disable();
> -		ret = atomic_dec_if_positive(&module->refcnt);
> -		WARN_ON(ret < 0);	/* Failed to put refcount */
> -		trace_module_put(module, _RET_IP_);
> +		__module_put(module);
>   		preempt_enable();
>   	}
>   }
>   EXPORT_SYMBOL(module_put);
>   
> +int module_put_and_delete(struct module *mod)
> +{
> +	int ret;
> +	mutex_lock(&module_mutex);
> +	__module_put(mod);
> +	ret = stop_module(mod, 0);
> +	mutex_unlock(&module_mutex);
> +
> +	if (ret)
> +		return ret;
> +
> +	destruct_module(mod);
> +	return 0;
> +}
> +
>   #else /* !CONFIG_MODULE_UNLOAD */
>   static inline void print_unload_info(struct seq_file *m, struct module *mod)
>   {
> 

Thanks,

-- 
Julien Thierry