linux-kernel - RE: [PATCH] kasan: improve double-free detection

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Message-ID: <20E775CA4D599049A25800DE5799F6DD1F61F1B2@G9W0752.americas.hpqcorp.net>
Date:	Tue, 3 May 2016 07:53:17 +0000
From:	"Luruo, Kuthonuzo" <kuthonuzo.luruo@....com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
CC:	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Alexander Potapenko <glider@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	kasan-dev <kasan-dev@...glegroups.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] kasan: improve double-free detection

> I missed that Alexander already landed patches that reduce header size
> to 16 bytes.
> It is not OK to increase them again. Please leave state as bitfield
> and update it with CAS (if we introduce helper functions for state
> manipulation, they will hide the CAS loop, which is nice).
> 

Available CAS primitives/compiler do not support CAS with bitfield. I propose
to change kasan_alloc_meta to:

struct kasan_alloc_meta {
        struct kasan_track track;
        u16 size_delta;         /* object_size - alloc size */
        u8 state;                    /* enum kasan_state */
        u8 reserved1;
        u32 reserved2;
}

This shrinks _used_ meta object by 1 byte wrt the original. (btw, patch v1 does
not increase overall alloc meta object size). "Alloc size", where needed, is
easily calculated as a delta from cache->object_size.

Kuthonuzo