linux-kernel - Re: [PATCH v4] mm: SLAB freelist randomization

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Message-ID: <20160429071810.GD19896@js1304-P5Q-DELUXE>
Date:	Fri, 29 Apr 2016 16:18:10 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Garnier <thgarnie@...gle.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Kees Cook <keescook@...omium.org>, gthelen@...gle.com,
	labbott@...oraproject.org, kernel-hardening@...ts.openwall.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4] mm: SLAB freelist randomization

On Wed, Apr 27, 2016 at 10:39:29AM -0500, Christoph Lameter wrote:
> On Tue, 26 Apr 2016, Andrew Morton wrote:
> 
> > : CONFIG_FREELIST_RANDOM bugs me a bit - "freelist" is so vague.
> > : CONFIG_SLAB_FREELIST_RANDOM would be better.  I mean, what Kconfig
> > : identifier could be used for implementing randomisation in
> > : slub/slob/etc once CONFIG_FREELIST_RANDOM is used up?
> >
> > but this pearl appeared to pass unnoticed.
> 
> Ok. lets add SLAB here and then use this option for the other allocators
> as well.
> 
> > > +	/* If it fails, we will just use the global lists */
> > > +	cachep->random_seq = kcalloc(count, sizeof(freelist_idx_t), GFP_KERNEL);
> > > +	if (!cachep->random_seq)
> > > +		return -ENOMEM;
> >
> > OK, no BUG.  If this happens, kmem_cache_init_late() will go BUG
> > instead ;)
> >
> > Questions for slab maintainers:
> >
> > What's going on with the gfp_flags in there?  kmem_cache_init_late()
> > passes GFP_NOWAIT into enable_cpucache().
> >
> > a) why the heck does it do that?  It's __init code!
> 
> enable_cpucache() was called when a slab cache was reconfigured by writing to /proc/slabinfo.
> That was changed awhile back when the memcg changes were made ot slab. So
> now its ok to be made init code.
> 
> > Finally, all callers of enable_cpucache() (and hence of
> > cache_random_seq_create()) are __init, so we're unnecessarily bloating
> > up vmlinux.  Could someone please take a look at this as a separate
> > thing?
> 
> Hmmm. Well if that is the case then lots of stuff could be straightened
> out. Joonsoo?
> 

As I mentioned in other thread, enable_cpucache() can be called
whenever kmem_cache is created. It should not be __init.

Thanks.