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Date:   Fri, 23 Sep 2022 11:07:59 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Kees Cook <keescook@...omium.org>
Cc:     Christian König <christian.koenig@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Feng Tang <feng.tang@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Alex Elder <elder@...nel.org>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Daniel Micay <danielmicay@...il.com>,
        Yonghong Song <yhs@...com>, Marco Elver <elver@...gle.com>,
        Miguel Ojeda <ojeda@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, netdev@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        linux-fsdevel@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
        dev@...nvswitch.org, x86@...nel.org,
        linux-wireless@...r.kernel.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: Re: [PATCH 00/12] slab: Introduce kmalloc_size_roundup()

On 9/22/22 23:49, Kees Cook wrote:
> On Thu, Sep 22, 2022 at 11:05:47PM +0200, Vlastimil Babka wrote:
>> On 9/22/22 17:55, Kees Cook wrote:
>> > On Thu, Sep 22, 2022 at 09:10:56AM +0200, Christian König wrote:
>> > [...]
>> > > So when this patch set is about to clean up this use case it should probably
>> > > also take care to remove ksize() or at least limit it so that it won't be
>> > > used for this use case in the future.
>> > 
>> > Yeah, my goal would be to eliminate ksize(), and it seems possible if
>> > other cases are satisfied with tracking their allocation sizes directly.
>> 
>> I think we could leave ksize() to determine the size without a need for
>> external tracking, but from now on forbid callers from using that hint to
>> overflow the allocation size they actually requested? Once we remove the
>> kasan/kfence hooks in ksize() that make the current kinds of usage possible,
>> we should be able to catch any offenders of the new semantics that would appear?
> 
> That's correct. I spent the morning working my way through the rest of
> the ksize() users I didn't clean up yesterday, and in several places I
> just swapped in __ksize(). But that wouldn't even be needed if we just
> removed the kasan unpoisoning from ksize(), etc.
> 
> I am tempted to leave it __ksize(), though, just to reinforce that it's
> not supposed to be used "normally". What do you think?

Sounds good. Note in linux-next there's now a series in slab.git planned for
6.1 that moves __ksize() declaration to mm/slab.h to make it more private.
But we don't want random users outside mm and related kasan/kfence
subsystems to include mm/slab.h, so we'll have to expose it again instead of
ksize().

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