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    SubjectRe: [PATCH v2 0/5] slab cleanups
    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 :-)

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