usermodehelper: implement UMH_KILLABLE

BugLink: http://bugs.launchpad.net/bugs/963685

Implement UMH_KILLABLE, should be used along with UMH_WAIT_EXEC/PROC.
The caller must ensure that subprocess_info->path/etc can not go away
until call_usermodehelper_freeinfo().

call_usermodehelper_exec(UMH_KILLABLE) does
wait_for_completion_killable.  If it fails, it uses
xchg(&sub_info->complete, NULL) to serialize with umh_complete() which
does the same xhcg() to access sub_info->complete.

If call_usermodehelper_exec wins, it can safely return.  umh_complete()
should get NULL and call call_usermodehelper_freeinfo().

Otherwise we know that umh_complete() was already called, in this case
call_usermodehelper_exec() falls back to wait_for_completion() which
should succeed "very soon".

Note: UMH_NO_WAIT == -1 but it obviously should not be used with
UMH_KILLABLE.  We delay the neccessary cleanup to simplify the back
porting.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Tejun Heo <tj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit d0bd587a80960d7ba7e0c8396e154028c9045c54)

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 8ce70e7..3b8ffb6 100644 (file)
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -54,6 +54,8 @@ enum umh_wait {
        UMH_WAIT_PROC = 1,      /* wait for the process to complete */
 };
 
+#define UMH_KILLABLE   4       /* wait for EXEC/PROC killable */
+
 struct subprocess_info {
        struct work_struct work;
        struct completion *complete;

diff --git a/kernel/kmod.c b/kernel/kmod.c
index e5e30b6..ff1277c 100644 (file)
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -199,7 +199,15 @@ EXPORT_SYMBOL(call_usermodehelper_freeinfo);
 
 static void umh_complete(struct subprocess_info *sub_info)
 {
-       complete(sub_info->complete);
+       struct completion *comp = xchg(&sub_info->complete, NULL);
+       /*
+        * See call_usermodehelper_exec(). If xchg() returns NULL
+        * we own sub_info, the UMH_KILLABLE caller has gone away.
+        */
+       if (comp)
+               complete(comp);
+       else
+               call_usermodehelper_freeinfo(sub_info);
 }
 
 /* Keventd can't block, but this (a child) can. */
@@ -250,6 +258,9 @@ static void __call_usermodehelper(struct work_struct *work)
        enum umh_wait wait = sub_info->wait;
        pid_t pid;
 
+       if (wait != UMH_NO_WAIT)
+               wait &= ~UMH_KILLABLE;
+
        /* CLONE_VFORK: wait until the usermode helper has execve'd
         * successfully We need the data structures to stay around
         * until that is done.  */
@@ -440,9 +451,21 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info,
        queue_work(khelper_wq, &sub_info->work);
        if (wait == UMH_NO_WAIT)        /* task has freed sub_info */
                goto unlock;
+
+       if (wait & UMH_KILLABLE) {
+               retval = wait_for_completion_killable(&done);
+               if (!retval)
+                       goto wait_done;
+
+               /* umh_complete() will see NULL and free sub_info */
+               if (xchg(&sub_info->complete, NULL))
+                       goto unlock;
+               /* fallthrough, umh_complete() was already called */
+       }
+
        wait_for_completion(&done);
+wait_done:
        retval = sub_info->retval;
-
 out:
        call_usermodehelper_freeinfo(sub_info);
 unlock: