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Date:   Sat, 5 Mar 2022 04:00:12 +0000
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Marco Elver <elver@google.com>
Cc:     linux-mm@kvack.org, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Matthew WilCox <willy@infradead.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/5] slab cleanups
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On Fri, Mar 04, 2022 at 02:11:50PM +0100, Marco Elver wrote:
> On Fri, 4 Mar 2022 at 13:02, Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> >
> > On Fri, Mar 04, 2022 at 12:50:21PM +0100, Marco Elver wrote:
> > > On Fri, 4 Mar 2022 at 07:34, Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> > > >
> > > > Changes from v1:
> > > >         Now SLAB passes requests larger than order-1 page
> > > >         to page allocator.
> > > >
> > > >         Adjusted comments from Matthew, Vlastimil, Rientjes.
> > > >         Thank you for feedback!
> > > >
> > > >         BTW, I have no idea what __ksize() should return when an object that
> > > >         is not allocated from slab is passed. both 0 and folio_size()
> > > >         seems wrong to me.
> > >
> > > Didn't we say 0 would be the safer of the two options?
> > > https://lkml.kernel.org/r/0e02416f-ef43-dc8a-9e8e-50ff63dd3c61@suse.cz
> > >
> >
> > Oh sorry, I didn't understand why 0 was safer when I was reading it.
> >
> > Reading again, 0 is safer because kasan does not unpoison for
> > wrongly passed object, right?
> 
> Not quite. KASAN can tell if something is wrong, i.e. invalid object.
> Similarly, if you are able to tell if the passed pointer is not a
> valid object some other way, you can do something better - namely,
> return 0.
>
> The intuition here is that the caller has a pointer to an
> invalid object, and wants to use ksize() to determine its size, and
> most likely access all those bytes. Arguably, at that point the kernel
> is already in a degrading state. But we can try to not let things get
> worse by having ksize() return 0, in the hopes that it will stop
> corrupting more memory. It won't work in all cases, but should avoid
> things like "s = ksize(obj); touch_all_bytes(obj, s)" where the size
> bounds the memory accessed corrupting random memory.

Oh, it's to prevent to corrupt memory further in failure case,
like memset(obj, 0, s);

> The other reason is that a caller could actually check the size, and
> if 0, do something else. Few callers will do so, because nobody
> expects that their code has a bug. :-)

and making it able to check errors by caller.
Thank you so much for kind explanation.

I'll add what Vlastimil suggested in next series. Thanks!

-- 
Thank you, You are awesome!
Hyeonggon :-)