Re: call_usermodehelper in containers

[Ian Kent <raven@themaw.net>]

On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote:
> On 11/15, Eric W. Biederman wrote:
> > 
> > I don't understand that one.  Having a preforked thread with the
> > proper
> > environment that can act like kthreadd in terms of spawning user
> > mode
> > helpers works and is simple.

Forgive me replying to such an old thread but ...

After realizing workqueues can't be used to pre-create threads to run
usermode helpers I've returned to look at this.

> 
> Can't we ask ->child_reaper to create the non-daemonized kernel thread
> with the "right" ->nsproxy, ->fs, etc?

Eric, do you think this approach would be sufficient too?

Probably wouldn't be quite right for user namespaces but should provide
what's needed for other cases?

It certainly has the advantage of not having to maintain a plague of
processes waiting around to execute helpers.

> 
> IOW. Please the the "patch" below. It is obviously incomplete and
> wrong,
> and it can be more clear/clean. And probably we need another API. Just
> to explain what I mean.

> 
> With this patch call_usermodehelper(..., UMH_IN_MY_NS) should do exec
> from the caller's namespace.
> 
> Oleg.
> ---
> 
> --- a/include/linux/kmod.h
> +++ b/include/linux/kmod.h
> @@ -24,6 +24,7 @@
>  #include <linux/errno.h>
>  #include <linux/compiler.h>
>  #include <linux/workqueue.h>
> +#include <linux/task_work.h>
>  #include <linux/sysctl.h>
>  
>  #define KMOD_PATH_LEN 256
> @@ -53,8 +54,14 @@ struct file;
>  #define UMH_WAIT_PROC	2	/* wait for the process to
> complete */
>  #define UMH_KILLABLE	4	/* wait for EXEC/PROC killable
> */
>  
> +// FIXME: IMH_* is not actually a mask
> +#define UMH_IN_MY_NS	8
> +
>  struct subprocess_info {
> -	struct work_struct work;
> +	union {
> +		struct work_struct work;
> +		struct callback_head twork;
> +	};
>  	struct completion *complete;
>  	char *path;
>  	char **argv;
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -541,7 +541,6 @@ struct subprocess_info
> *call_usermodehelper_setup(char *path, char **argv,
>  	if (!sub_info)
>  		goto out;
>  
> -	INIT_WORK(&sub_info->work, __call_usermodehelper);
>  	sub_info->path = path;
>  	sub_info->argv = argv;
>  	sub_info->envp = envp;
> @@ -554,6 +553,24 @@ struct subprocess_info
> *call_usermodehelper_setup(char *path, char **argv,
>  }
>  EXPORT_SYMBOL(call_usermodehelper_setup);
>  
> +static int call_call_usermodehelper(void *twork)
> +{
> +	struct subprocess_info *sub_info =
> +		container_of(twork, struct subprocess_info, twork);
> +
> +	__call_usermodehelper(&sub_info->work);
> +	do_exit(0);
> +
> +}
> +
> +static void fork_umh_helper(struct callback_head *twork)
> +{
> +	if (current->flags & PF_EXITING)
> +		return;	// WRONG, FIXME
> +
> +	kernel_thread(call_call_usermodehelper, twork, SIGCHLD);
> +}
> +
>  /**
>   * call_usermodehelper_exec - start a usermode application
>   * @sub_info: information about the subprocessa
> @@ -570,6 +587,10 @@ int call_usermodehelper_exec(struct
> subprocess_info *sub_info, int wait)
>  {
>  	DECLARE_COMPLETION_ONSTACK(done);
>  	int retval = 0;
> +	bool in_my_ns;
> +
> +	in_my_ns = wait & UMH_IN_MY_NS;
> +	wait &= ~UMH_IN_MY_NS;
>  
>  	if (!sub_info->path) {
>  		call_usermodehelper_freeinfo(sub_info);
> @@ -594,7 +615,21 @@ int call_usermodehelper_exec(struct
> subprocess_info *sub_info, int wait)
>  	sub_info->complete = &done;
>  	sub_info->wait = wait;
>  
> -	queue_work(khelper_wq, &sub_info->work);
> +	if (likely(!in_my_ns)) {
> +		INIT_WORK(&sub_info->work, __call_usermodehelper);
> +		queue_work(khelper_wq, &sub_info->work);
> +	} else {
> +		// RACY, WRONG, ETC
> +		struct task_struct *my_init =
> task_active_pid_ns(current)->child_reaper;
> +
> +		init_task_work(&sub_info->twork, fork_umh_helper);
> +		task_work_add(my_init, &sub_info->twork, false);
> +
> +		// until we have task_work_add_interruptibel()
> +		do_send_sig_info(SIGCHLD, SEND_SIG_FORCED, my_init,
> false);
> +
> +	}
> +
>  	if (wait == UMH_NO_WAIT)	/* task has freed sub_info */
>  		goto unlock;
>  
> 
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