Re: Issue with kfence and kmemleak

[Marco Elver]

On Tue, Mar 16, 2021 at 04:42PM +0000, Luis Henriques wrote:
> Hi!
> 
> This is probably a known issue, but just in case: looks like it's not
> possible to use kmemleak when kfence is enabled:

Thanks for spotting this.

> [    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);
>  }

Can you try the below patch?

Thanks,
-- Marco

------ >8 ------

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index f7106f28443d..5891019721f6 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -12,6 +12,7 @@
 #include <linux/debugfs.h>
 #include <linux/kcsan-checks.h>
 #include <linux/kfence.h>
+#include <linux/kmemleak.h>
 #include <linux/list.h>
 #include <linux/lockdep.h>
 #include <linux/memblock.h>
@@ -481,6 +482,13 @@ static bool __init kfence_init_pool(void)
 		addr += 2 * PAGE_SIZE;
 	}
 
+	/*
+	 * The pool is live and will never be deallocated from this point on;
+	 * tell kmemleak this is now free memory, so that later allocations can
+	 * correctly be tracked.
+	 */
+	kmemleak_free_part_phys(__pa(__kfence_pool), KFENCE_POOL_SIZE);
+
 	return true;
 
 err: