[Drbd-dev] leak of rs_plan_s in drbd_attach_peer_device()

[David Butterfield]

Hello again,

Valgrind reported a memory leak of one instance of rs_plan_s, which appears to be genuine.

==23562== 104 bytes in 1 blocks are definitely lost in loss record 96 of 279 

Pointers to two instances of the structure get assigned to peer_device->rs_plan_s, both on the 
netlink receive thread from drbdsetup commands, with the second overwriting the first:

 First:   drbd_adm_new_peer --> adm_new_connection --> create_peer_device --> ... --> fifo_alloc
 Second:  drbd_adm_attach --> drbd_attach_peer_device

When drbd_attach_peer_device() allocates the second instance, it does not check for an "old"
instance in peer_device->rs_plan_s before writing the pointer to the new plan over it. 

I added a statement there (at 1586 below) to issue a stacktrace before the overwrite if the old 
peer_device->rs_plan_s isn't NULL.  That is the "Overwriting call stack" below.  The "Allocating
call stack" was issued by valgrind as its record of where the lost memory was allocated.

1586 expect_eq(rcu_dereference(peer_device->rs_plan_s), NULL, "overwriting rs_plan_s=%p with %p",
1587                                     rcu_dereference(peer_device->rs_plan_s), resync_plan);
1588 rcu_assign_pointer(peer_device->rs_plan_s, resync_plan);
1589 peer_device->resync_lru = resync_lru;
1590 err = 0;

drbd_main.c:1587: overwriting rs_plan_s=0x4e8bf40 with 0x4aeb6e00

Overwriting call stack:
==23562==    by 0x4890CC1: mte_backtrace (mte_debug.c:371)
==23562==    by 0x1E80E1: drbd_attach_peer_device (drbd_main.c:1586)
==23562==    by 0x20ACD1: drbd_adm_attach (drbd_nl.c:2979)
==23562==    by 0x2981E4: genl_rcv_msg (UMC_genl.c:566)

Allocating call stack:
==23562==    by 0x486E5E2: MEM_zalloc (mte_service.c:24)
==23562==    by 0x24EA8A: fifo_alloc (drbd_sender.c:552)
==23562==    by 0x1FD912: adjust_resync_fifo (drbd_nl.c:3551)
==23562==    by 0x20BA84: drbd_create_peer_device_default_config (drbd_nl.c:3666)
==23562==    by 0x1EBA2F: create_peer_device (drbd_main.c:3625)
==23562==    by 0x20EFE1: adm_new_connection (drbd_nl.c:3778)
==23562==    by 0x20F7E7: drbd_adm_new_peer (drbd_nl.c:4070)
==23562==    by 0x2981E4: genl_rcv_msg (UMC_genl.c:566)