sparse: annotate (and comment on) kmod.c user pointer usage

Big comment, because it wasn't clear why this cast was valid.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 10d2e8fc58db20890939829b92e8a4abecfe707b..160aa3d958ad2ba75480d0d8c051ed235a67c635 100644 (file)
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -192,10 +192,20 @@ static int wait_for_helper(void *data)
        allow_signal(SIGCHLD);
 
        pid = kernel_thread(____call_usermodehelper, sub_info, SIGCHLD);
-       if (pid < 0)
+       if (pid < 0) {
                sub_info->retval = pid;
-       else
-               sys_wait4(pid, &sub_info->retval, 0, NULL);
+       } else {
+               /*
+                * Normally it is bogus to call wait4() from in-kernel because
+                * wait4() wants to write the exit code to a userspace address.
+                * But wait_for_helper() always runs as keventd, and put_user()
+                * to a kernel address works OK for kernel threads, due to their
+                * having an mm_segment_t which spans the entire address space.
+                *
+                * Thus the __user pointer cast is valid here.
+                */
+               sys_wait4(pid, (int __user *) &sub_info->retval, 0, NULL);
+       }
 
        complete(sub_info->complete);
        return 0;