Re: Issue with kfence and kmemleak

[Catalin Marinas <catalin.marinas@arm.com>]

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:
> > 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);
> >  }
> 
> 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);

I presume this pool does not refer any objects that are only tracked
through pool pointers.

kmemleak_free() (or *_free_part) should work, no need for the _phys
variant (which converts it back with __va).

Since we normally use kmemleak_ignore() (or no_scan) for objects we
don't care about, I'd expand the comment that this object needs to be
removed from the kmemleak object tree as it will overlap with subsequent
allocations handled by kfence which return pointers within this range.

-- 
Catalin