linux-kernel - slab: krealloc with GFP

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Date:	Thu, 29 Aug 2013 14:47:16 -0700
From:	Joe Perches <joe@...ches.com>
To:	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>, linux-mm <linux-mm@...ck.org>
Cc:	linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: slab: krealloc with GFP_ZERO defect

This sequence can return non-zeroed memory from the
padding area of the original allocation.

	ptr = kzalloc(foo, GFP_KERNEL);
	if (!ptr)
		...
	new_ptr = krealloc(ptr, foo + bar, GFP_KERNEL | __GFP_ZERO);

If the realloc size is within the first actual allocation
then the additional memory is not zeroed.

If the realloc size is not within the original allocation
size, any non-zeroed padding from the original allocation
is overwriting newly allocated zeroed memory.

Maybe someone more familiar with the alignment & padding can
add the proper memset(,0,) for the __GFP_ZERO cases and also
optimize kmalloc_track_caller to not use __GFP_ZERO, memcpy
the current (non padded) size and zero the newly returned
remainder if necessary.

from: mm/util.c
---------------------------
static __always_inline void *__do_krealloc(const void *p, size_t new_size,
					   gfp_t flags)
{
	void *ret;
	size_t ks = 0;

	if (p)
		ks = ksize(p);

	if (ks >= new_size)
		return (void *)p;

	ret = kmalloc_track_caller(new_size, flags);
	if (ret && p)
		memcpy(ret, p, ks);

	return ret;
}

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