syzbot rcu/debugobjects warning

[Joel Fernandes]

Hi Paul, Thomas,

I received a crash report from syzbot on the android 4.9 kernel and I
am looking into it, it seems the debugobjects subsystem is warning
that a certain RCU structure is not allocated on the stack, but is
annotated to be.

------------[ cut here ]------------
WARNING: CPU: 1 PID: 0 at lib/debugobjects.c:300
debug_object_is_on_stack lib/debugobjects.c:300 [inline]
WARNING: CPU: 1 PID: 0 at lib/debugobjects.c:300
__debug_object_init+0x526/0xc40 lib/debugobjects.c:326
[...]
 <IRQ> [  150.631700]  [<ffffffff81d96069>] dump_stack+0xc1/0x128
lib/dump_stack.c:51
 [<ffffffff8142fbd1>] panic+0x1bc/0x3a8 kernel/panic.c:179
 [<ffffffff81131894>] __warn+0x1c4/0x1e0 kernel/panic.c:542
 [<ffffffff81131afc>] warn_slowpath_null+0x2c/0x40 kernel/panic.c:585
 [<ffffffff81dfefb6>] debug_object_is_on_stack lib/debugobjects.c:300 [inline]
 [<ffffffff81dfefb6>] __debug_object_init+0x526/0xc40 lib/debugobjects.c:326
 [<ffffffff81dff709>] debug_object_init_on_stack+0x19/0x20
lib/debugobjects.c:378
 [<ffffffff81287a93>] init_rcu_head_on_stack kernel/rcu/update.c:403 [inline]
 [<ffffffff81287a93>] __wait_rcu_gp+0x93/0x1b0 kernel/rcu/update.c:358
 [<ffffffff81290251>] synchronize_rcu.part.65+0x101/0x110
kernel/rcu/tree_plugin.h:678
 [<ffffffff81290287>] synchronize_rcu+0x27/0x90 kernel/rcu/tree_plugin.h:679
 [<ffffffff83588b35>] __l2tp_session_unhash+0x3d5/0x550
net/l2tp/l2tp_core.c:1792

The full report is here:
https://syzkaller.appspot.com/bug?extid=e6a19b585ab2dba3eee8

It seems as per the code that the structure is on the stack so its
weird why debugobjects thinks its not.
The object in question is allocated on the stack by the __wait_rcu_gp
macro when its called from synchronize_rcu:

#define _wait_rcu_gp(checktiny, ...) \
do {                                                                    \
        call_rcu_func_t __crcu_array[] = { __VA_ARGS__ };               \
        struct rcu_synchronize __rs_array[ARRAY_SIZE(__crcu_array)];    \
        __wait_rcu_gp(checktiny, ARRAY_SIZE(__crcu_array),              \
                        __crcu_array, __rs_array);                      \
} while (0)


Any debug ideas or thoughts about it?

thanks for any help,

- Joel