usermodehelper: implement UMH_KILLABLE

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>

2 files changed, 27 insertions, 2 deletions

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 722f477c4ef7..1b5985855ffc 100644
--- 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 8ea25944ce33..f92f917c450c 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -201,7 +201,15 @@ 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. */
@@ -252,6 +260,9 @@ 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.  */
@@ -461,9 +472,21 @@ 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: