Re: Issue with kfence and kmemleak

[Luis Henriques]

On Tue, Mar 16, 2021 at 06:30:00PM +0100, Marco Elver wrote:
> 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?

Yep, that seems to fix the issue.  Feel free to add my Tested-by.  Thanks!

Cheers,
--
Luís

> 
> 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: