[RFC PATCH] kmod: add ability to swap root in usermode helper

[Stanislav Kinsbursky <skinsbursky@parallels.com>]

Usermode helper executes all binaries in global "init" root context. This
doesn't allow to call to call the binary from other root (for example in a
container).
Currently, containerized NFS server requires an ability to execute a binary in
a other context, than "init" root (UMH is used for client recovery tracking).
This patch adds root swap to ____call_usermodehelper(), if non-NULL root was
passed as a part of subprocess_info data, and a call_usermodehelper_root()
helper, which accept root as a parameter. Root path reference must be hold by
the caller, since it will be on UMH thread exit.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
---
 include/linux/kmod.h |    9 +++++++++
 kernel/kmod.c        |   41 ++++++++++++++++++++++++++++++++++-------
 2 files changed, 43 insertions(+), 7 deletions(-)

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 0555cc6..0b041c7 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -64,12 +64,21 @@ struct subprocess_info {
 	int (*init)(struct subprocess_info *info, struct cred *new);
 	void (*cleanup)(struct subprocess_info *info);
 	void *data;
+	struct path *root;
 };
 
 extern int
+call_usermodehelper_root(char *path, char **argv, char **envp, int wait,
+			 struct path *root);
+extern int
 call_usermodehelper(char *path, char **argv, char **envp, int wait);
 
 extern struct subprocess_info *
+call_usermodehelper_setup_root(char *path, char **argv, char **envp, gfp_t gfp_mask,
+			       int (*init)(struct subprocess_info *info, struct cred *new),
+			       void (*cleanup)(struct subprocess_info *), void *data,
+			       struct path *root);
+extern struct subprocess_info *
 call_usermodehelper_setup(char *path, char **argv, char **envp, gfp_t gfp_mask,
 			  int (*init)(struct subprocess_info *info, struct cred *new),
 			  void (*cleanup)(struct subprocess_info *), void *data);
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 1296e72..1b41f2c 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -39,6 +39,7 @@
 #include <linux/rwsem.h>
 #include <linux/ptrace.h>
 #include <linux/async.h>
+#include <linux/fs_struct.h>
 #include <asm/uaccess.h>
 
 #include <trace/events/module.h>
@@ -215,6 +216,9 @@ static int ____call_usermodehelper(void *data)
 	 */
 	set_user_nice(current, 0);
 
+	if (sub_info->root)
+		set_fs_root(current->fs, sub_info->root);
+
 	retval = -ENOMEM;
 	new = prepare_kernel_cred(current);
 	if (!new)
@@ -505,7 +509,7 @@ static void helper_unlock(void)
 }
 
 /**
- * call_usermodehelper_setup - prepare to call a usermode helper
+ * call_usermodehelper_setup_root - prepare to call a usermode helper
  * @path: path to usermode executable
  * @argv: arg vector for process
  * @envp: environment for process
@@ -513,6 +517,7 @@ static void helper_unlock(void)
  * @cleanup: a cleanup function
  * @init: an init function
  * @data: arbitrary context sensitive data
+ * @root: fs root to swap to before binary execution
  *
  * Returns either %NULL on allocation failure, or a subprocess_info
  * structure.  This should be passed to call_usermodehelper_exec to
@@ -527,11 +532,11 @@ static void helper_unlock(void)
  * Function must be runnable in either a process context or the
  * context in which call_usermodehelper_exec is called.
  */
-struct subprocess_info *call_usermodehelper_setup(char *path, char **argv,
+struct subprocess_info *call_usermodehelper_setup_root(char *path, char **argv,
 		char **envp, gfp_t gfp_mask,
 		int (*init)(struct subprocess_info *info, struct cred *new),
 		void (*cleanup)(struct subprocess_info *info),
-		void *data)
+		void *data, struct path *root)
 {
 	struct subprocess_info *sub_info;
 	sub_info = kzalloc(sizeof(struct subprocess_info), gfp_mask);
@@ -546,9 +551,22 @@ struct subprocess_info *call_usermodehelper_setup(char *path, char **argv,
 	sub_info->cleanup = cleanup;
 	sub_info->init = init;
 	sub_info->data = data;
+
+	sub_info->root = root;
   out:
 	return sub_info;
 }
+EXPORT_SYMBOL(call_usermodehelper_setup_root);
+
+struct subprocess_info *call_usermodehelper_setup(char *path, char **argv,
+		char **envp, gfp_t gfp_mask,
+		int (*init)(struct subprocess_info *info, struct cred *new),
+		void (*cleanup)(struct subprocess_info *info),
+		void *data)
+{
+	return call_usermodehelper_setup_root(path, argv, envp, gfp_mask, init,
+					      cleanup, data, NULL);
+}
 EXPORT_SYMBOL(call_usermodehelper_setup);
 
 /**
@@ -617,7 +635,7 @@ unlock:
 EXPORT_SYMBOL(call_usermodehelper_exec);
 
 /**
- * call_usermodehelper() - prepare and start a usermode application
+ * call_usermodehelper_root() - prepare and start a usermode application
  * @path: path to usermode executable
  * @argv: arg vector for process
  * @envp: environment for process
@@ -625,24 +643,33 @@ EXPORT_SYMBOL(call_usermodehelper_exec);
  *        when UMH_NO_WAIT don't wait at all, but you get no useful error back
  *        when the program couldn't be exec'ed. This makes it safe to call
  *        from interrupt context.
+ * @root: fs root to swap to before binary execution
  *
  * This function is the equivalent to use call_usermodehelper_setup() and
  * call_usermodehelper_exec().
  */
-int call_usermodehelper(char *path, char **argv, char **envp, int wait)
+int call_usermodehelper_root(char *path, char **argv, char **envp, int wait,
+			     struct path *root)
 {
 	struct subprocess_info *info;
 	gfp_t gfp_mask = (wait == UMH_NO_WAIT) ? GFP_ATOMIC : GFP_KERNEL;
 
-	info = call_usermodehelper_setup(path, argv, envp, gfp_mask,
-					 NULL, NULL, NULL);
+	info = call_usermodehelper_setup_root(path, argv, envp, gfp_mask,
+					      NULL, NULL, NULL, root);
 	if (info == NULL)
 		return -ENOMEM;
 
 	return call_usermodehelper_exec(info, wait);
 }
+EXPORT_SYMBOL(call_usermodehelper_root);
+
+int call_usermodehelper(char *path, char **argv, char **envp, int wait)
+{
+	return call_usermodehelper_root(path, argv, envp, wait, NULL);
+}
 EXPORT_SYMBOL(call_usermodehelper);
 
+
 static int proc_cap_handler(struct ctl_table *table, int write,
 			 void __user *buffer, size_t *lenp, loff_t *ppos)
 {