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SubjectRe: call_usermodehelper in containers
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;



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