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Subject: Re: [PATCH v3 0/3] Actually fix freelist pointer vs redzoning
To:     Christopher Lameter <cl@linux.com>,
        Kees Cook <keescook@chromium.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Waiman Long <longman@redhat.com>,
        Marco Elver <elver@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Roman Gushchin <guro@fb.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org
References: <20201015033712.1491731-1-keescook@chromium.org>
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From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <1e43fd23-e9f1-9c5d-3ee2-17171642877f@suse.cz>
Date:   Thu, 15 Oct 2020 11:44:15 +0200
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On 10/15/20 10:23 AM, Christopher Lameter wrote:
> On Wed, 14 Oct 2020, Kees Cook wrote:
> 
>> Note on patch 2: Christopher NAKed it, but I actually think this is a
>> reasonable thing to add -- the "too small" check is only made when built
>> with CONFIG_DEBUG_VM, so it *is* actually possible for someone to trip
>> over this directly, even if it would never make it into a released
>> kernel. I see no reason to just leave this foot-gun in place, though, so
>> we might as well just fix it too. (Which seems to be what Longman was
>> similarly supporting, IIUC.)
> 
> Well then remove the duplication of checks. The NAK was there because it
> seems that you were not aware of the existing checks.
> 
>> Anyway, if patch 2 stays NAKed, that's fine. It's entirely separable,
>> and the other 2 can land. :)
> 
> Just deal with the old checks too and it will be fine.

Yeah, the existing check is under CONFIG_DEBUG_VM, which means it's not active 
on some configurations. Creating a cache is not exactly fast path operation, so 
I would remove this guard.
As for the minimum size check, I would probably remove it (but watch out if 
SLAB/SLOB can handle it). It's not effective to use slab cache for 4-byte 
objects, but why make it an error.