[Devel] call_usermodehelper in containers

[Oleg Nesterov]

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.

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

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;