Fix: urcu-bp: Bulletproof RCU arena resize bug

> From: "Milosz Tanski" <milosz@adfin.com>

> While trying to use the BP flavor of RCU I ran into random crashes. I
> tracked it down to issues with resizing of the BP RCU memory pool.
>
> The problem is in the urcu-bp.c file in the resize_arena() function.
> On successful allocation / remapping the len member of the
> registry_arena struct is never set anywhere function. On the second
> resize of the arena the code in resize_arena() still thinks the
> previous size is equal to the original mapping size. I've fixed this
> issue locally by just adding the following code at the bottom of
> resize_arena().

Good catch !!

However, I think your fix misses one case: if we happen to re-use the
same region, we want to update the length too.

Reported-by: Milosz Tanski <milosz@adfin.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu-bp.c b/urcu-bp.c
index a823659f78d4ce93d82a23b95dc05216feb9c903..93d781fcf44e5f2c9ee89928631a9d0d6cbb3e21 100644 (file)
--- a/urcu-bp.c
+++ b/urcu-bp.c
@@ -305,26 +305,37 @@ int rcu_read_ongoing(void)
  */
 static void resize_arena(struct registry_arena *arena, size_t len)
 {
-       void *new_arena;
+       void *new_p;
+       size_t old_len;
+
+       old_len = arena->len;
 
        if (!arena->p)
-               new_arena = mmap(arena->p, len,
-                                PROT_READ | PROT_WRITE,
-                                MAP_ANONYMOUS | MAP_PRIVATE,
-                                -1, 0);
+               new_p = mmap(arena->p, len,
+                       PROT_READ | PROT_WRITE,
+                       MAP_ANONYMOUS | MAP_PRIVATE,
+                       -1, 0);
        else
-               new_arena = mremap_wrapper(arena->p, arena->len,
-                                       len, MREMAP_MAYMOVE);
-       assert(new_arena != MAP_FAILED);
+               new_p = mremap_wrapper(arena->p, old_len,
+                       len, MREMAP_MAYMOVE);
+       assert(new_p != MAP_FAILED);
+
+       /*
+        * Zero the newly allocated memory. Since mmap() does not
+        * clearly specify if memory is zeroed or not (although it is
+        * very likely that it is), be extra careful by not expecting
+        * the new range to be zeroed by mremap.
+        */
+       bzero(new_p + old_len, len - old_len);
 
        /*
-        * re-used the same region ?
+        * If we did not re-use the same region, we need to update the
+        * arena pointer.
         */
-       if (new_arena == arena->p)
-               return;
+       if (new_p != arena->p)
+               arena->p = new_p;
 
-       bzero(new_arena + arena->len, len - arena->len);
-       arena->p = new_arena;
+       arena->len = len;
 }
 
 /* Called with signals off and mutex locked */