linux-kernel - Re: [PATCH 3/5] mm/slub: Improve redzone check and zeroing for krealloc()

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Message-ID: <dee10b40-0cf5-4560-b4e7-6b682de3f415@suse.cz>
Date: Tue, 10 Sep 2024 15:15:36 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Feng Tang <feng.tang@...el.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com>,
 Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>,
 Joonsoo Kim <iamjoonsoo.kim@....com>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, Andrey Konovalov
 <andreyknvl@...il.com>, Marco Elver <elver@...gle.com>,
 Shuah Khan <skhan@...uxfoundation.org>, David Gow <davidgow@...gle.com>,
 Danilo Krummrich <dakr@...nel.org>
Cc: linux-mm@...ck.org, kasan-dev@...glegroups.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] mm/slub: Improve redzone check and zeroing for
 krealloc()

On 9/9/24 03:29, Feng Tang wrote:
> For current krealloc(), one problem is its caller doesn't know what's
> the actual request size, say the object is 64 bytes kmalloc one, but

It's more accurate to say the caller doesn't pass the old size (it might
actually know it).

> the original caller may only requested 48 bytes. And when krealloc()
> shrinks or grows in the same object, or allocate a new bigger object,
> it lacks this 'original size' information to do accurate data preserving
> or zeroing (when __GFP_ZERO is set).

Let's describe the problem specifically by adding:

Thus with slub debug redzone and object tracking enabled, parts of the
object after krealloc() might contain redzone data instead of zeroes, which
is violating the __GFP_ZERO guarantees.

> And when some slub debug option is enabled, kmalloc caches do have this
> 'orig_size' feature. So utilize it to do more accurate data handling,
> as well as enforce the kmalloc-redzone sanity check.
> 
> The krealloc() related code is moved from slab_common.c to slub.c for
> more efficient function calling.

Agreed with Danilo that having the move as separate preparatory patch would
make review easier.

Thanks.