Issue with kfence and kmemleak

[Luis Henriques]

Hi!

This is probably a known issue, but just in case: looks like it's not
possible to use kmemleak when kfence is enabled:

[    0.272136] kmemleak: Cannot insert 0xffff888236e02f00 into the object search tree (overlaps existing)
[    0.272136] CPU: 0 PID: 8 Comm: kthreadd Not tainted 5.12.0-rc3+ #92
[    0.272136] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a-rebuilt.opensuse.org 04/01/2014
[    0.272136] Call Trace:
[    0.272136]  dump_stack+0x6d/0x89
[    0.272136]  create_object.isra.0.cold+0x40/0x62
[    0.272136]  ? process_one_work+0x5a0/0x5a0
[    0.272136]  ? process_one_work+0x5a0/0x5a0
[    0.272136]  kmem_cache_alloc_trace+0x110/0x2f0
[    0.272136]  ? process_one_work+0x5a0/0x5a0
[    0.272136]  kthread+0x3f/0x150
[    0.272136]  ? lockdep_hardirqs_on_prepare+0xd4/0x170
[    0.272136]  ? __kthread_bind_mask+0x60/0x60
[    0.272136]  ret_from_fork+0x22/0x30
[    0.272136] kmemleak: Kernel memory leak detector disabled
[    0.272136] kmemleak: Object 0xffff888236e00000 (size 2097152):
[    0.272136] kmemleak:   comm "swapper", pid 0, jiffies 4294892296
[    0.272136] kmemleak:   min_count = 0
[    0.272136] kmemleak:   count = 0
[    0.272136] kmemleak:   flags = 0x1
[    0.272136] kmemleak:   checksum = 0
[    0.272136] kmemleak:   backtrace:
[    0.272136]      memblock_alloc_internal+0x6d/0xb0
[    0.272136]      memblock_alloc_try_nid+0x6c/0x8a
[    0.272136]      kfence_alloc_pool+0x26/0x3f
[    0.272136]      start_kernel+0x242/0x548
[    0.272136]      secondary_startup_64_no_verify+0xb0/0xbb

I've tried the hack below but it didn't really helped.  Obviously I don't
really understand what's going on ;-)  But I think the reason for this
patch not working as (I) expected is because kfence is initialised
*before* kmemleak.

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 3b8ec938470a..b4ffd7695268 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -631,6 +631,9 @@ void __init kfence_alloc_pool(void)
 
 	if (!__kfence_pool)
 		pr_err("failed to allocate pool\n");
+	kmemleak_no_scan(__kfence_pool);
 }


Cheers,
--
Luís