Re: [patch 1/2] mm, mempool: poison elements backed by slab allocator

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On Mon, Mar 09 2015, David Rientjes <rientjes@google.com> wrote:

> Mempools keep elements in a reserved pool for contexts in which
> allocation may not be possible.  When an element is allocated from the
> reserved pool, its memory contents is the same as when it was added to
> the reserved pool.
>
> Because of this, elements lack any free poisoning to detect
> use-after-free errors.
>
> This patch adds free poisoning for elements backed by the slab allocator.
> This is possible because the mempool layer knows the object size of each
> element.
>
> When an element is added to the reserved pool, it is poisoned with
> POISON_FREE.  When it is removed from the reserved pool, the contents are
> checked for POISON_FREE.  If there is a mismatch, a warning is emitted to
> the kernel log.
>
> +
> +static void poison_slab_element(mempool_t *pool, void *element)
> +{
> +	if (pool->alloc == mempool_alloc_slab ||
> +	    pool->alloc == mempool_kmalloc) {
> +		size_t size = ksize(element);
> +		u8 *obj = element;
> +
> +		memset(obj, POISON_FREE, size - 1);
> +		obj[size - 1] = POISON_END;
> +	}
> +}

Maybe a stupid question, but what happens if the underlying slab
allocator has non-trivial ->ctor?

Rasmus

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