[PATCH] fork: fix kmemleak false positive due to thread stacks caching

[Luis Henriques <lhenriques@suse.com>]

kmemleak has been reporting memory leaks since commit ac496bf48d97 ("fork:
Optimize task creation by caching two thread stacks per CPU if
CONFIG_VMAP_STACK=y"):

unreferenced object 0xffffc900002b0000 (size 16384):
  comm "init", pid 147, jiffies 4294893306 (age 11.292s)
  hex dump (first 32 bytes):
    9d 6e ac 57 00 00 00 00 00 00 00 00 00 00 00 00  .n.W............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff815b481e>] kmemleak_alloc+0x4e/0xb0
    [<ffffffff8112a8b0>] __vmalloc_node_range+0x160/0x240
    [<ffffffff8104a328>] copy_process.part.8+0x478/0x1630
    [<ffffffff8104b69a>] _do_fork+0xca/0x330
    [<ffffffff8104b9a9>] SyS_clone+0x19/0x20
    [<ffffffff8100199c>] do_syscall_64+0x4c/0xb0
    [<ffffffff815b8d06>] return_from_SYSCALL_64+0x0/0x6a
    [<ffffffffffffffff>] 0xffffffffffffffff

This is because this commit started caching 2 thread stacks per CPU, and
kmemleak assumes its memory is never freed.  Report these stacks as not
being memory leaks using kmemleak_not_leak().

Cc: stable@vger.kernel.org
Fixes: ac496bf48d97 ("fork: Optimize task creation by caching two thread stacks per CPU if CONFIG_VMAP_STACK=y")
Signed-off-by: Luis Henriques <lhenriques@suse.com>
---
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index aa1076c5e4a9..c4d79ad0f5bc 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -255,6 +255,7 @@ static inline void free_thread_stack(struct task_struct *tsk)
 
 			this_cpu_write(cached_stacks[i], tsk->stack_vm_area);
 			local_irq_restore(flags);
+			kmemleak_not_leak(tsk->stack);
 			return;
 		}
 		local_irq_restore(flags);