bcache: safeguard a dangerous addressing in closure_queue

The use of the union reduces the size of closure struct by taking advantage
of the current size of its members. The offset of func in work_struct
equals the size of the first three members, so that work.work_func will
just reference the forth member - fn.

This is smart but dangerous. It can be broken if work_struct or the other
structs get changed, and can be a bit difficult to debug.

Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/md/bcache/closure.h b/drivers/md/bcache/closure.h
index 295b7e43f92c233c0d1e0cf0575dc9f0e99481cf..00fb314cce57c4e910601eb87085f3d58bce321b 100644 (file)
--- a/drivers/md/bcache/closure.h
+++ b/drivers/md/bcache/closure.h
@@ -251,6 +251,12 @@ static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
 static inline void closure_queue(struct closure *cl)
 {
        struct workqueue_struct *wq = cl->wq;
+       /**
+        * Changes made to closure, work_struct, or a couple of other structs
+        * may cause work.func not pointing to the right location.
+        */
+       BUILD_BUG_ON(offsetof(struct closure, fn)
+                    != offsetof(struct work_struct, func));
        if (wq) {
                INIT_WORK(&cl->work, cl->work.func);
                BUG_ON(!queue_work(wq, &cl->work));