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From:   Alexander Potapenko <glider@google.com>
Date:   Thu, 17 Sep 2020 11:47:59 +0200
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Subject: Re: [PATCH v2 04/10] mm, kfence: insert KFENCE hooks for SLAB
To:     Christopher Lameter <cl@linux.com>
Cc:     Marco Elver <elver@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dmitriy Vyukov <dvyukov@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        Thomas Gleixner <tglx@linutronix.de>,
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> >  static __always_inline void *
> > -slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid,
> > +slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid, si=
ze_t orig_size,
> >                  unsigned long caller)
> >  {
>
> The size of the object is available via a field in kmem_cache. And a
> pointer to the current kmem_cache is already passed to the function. Why
> is there a need to add an additional parameter?

That's because we want to do our best detecting bugs on
kmalloc-allocated objects.
kmalloc is using size classes, so e.g. when allocating 272 bytes the
object will be padded to 512.
As a result, placing that object at the end of the page won't really
help to detect out-of-bound accesses that are off by less than 270
bytes.

We probably need to better clarify this in the patch description.

--=20
Alexander Potapenko
Software Engineer

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