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From: roel kluin <roel.kluin@gmail.com>
Date: Mon, 8 Feb 2010 15:38:09 +0100
Message-ID: <25e057c01002080638g149c974r759f50365cd05233@mail.gmail.com>
Subject: Re: [PATCH] kmemcheck: Test the full object in kmemcheck_is_obj_initialized()
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-kernel@vger.kernel.org, Vegard Nossum <vegardno@ifi.uio.no>,
       Andrew Morton <akpm@linux-foundation.org>,
       Pekka Enberg <penberg@cs.helsinki.fi>,
       Christian Casteyde <casteyde.christian@free.fr>
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On Mon, Feb 8, 2010 at 12:16 PM, Catalin Marinas
<catalin.marinas@arm.com> wrote:
> This is a fix for bug #14845 (bugzilla.kernel.org). The
> update_checksum() function in mm/kmemleak.c calls
> kmemcheck_is_obj_initialised() before scanning an object. When
> KMEMCHECK_PARTIAL_OK is enabled, this function returns true. However,
> the crc32_le() reads smaller intervals (32-bit) for which
> kmemleak_is_obj_initialised() may be false leading to a kmemcheck
> warning.
>
> Note that kmemcheck_is_obj_initialized() is currently only used by
> kmemleak before scanning a memory location.

>  enum kmemcheck_shadow kmemcheck_shadow_test(void *shadow, unsigned int size)
>  {
> +#ifdef CONFIG_KMEMCHECK_PARTIAL_OK
>        uint8_t *x;
>        unsigned int i;
>
>        x = shadow;
>
> -#ifdef CONFIG_KMEMCHECK_PARTIAL_OK
>        /*
>         * Make sure _some_ bytes are initialized. Gcc frequently generates
>         * code to access neighboring bytes.
> @@ -139,13 +139,25 @@ enum kmemcheck_shadow kmemcheck_shadow_test(void *shadow, unsigned int size)
>                if (x[i] == KMEMCHECK_SHADOW_INITIALIZED)
>                        return x[i];
>        }
> +
> +       return x[0];
>  #else
> +       return kmemcheck_shadow_test_all(shadow, size);
> +#endif
> +}
> +
> +enum kmemcheck_shadow kmemcheck_shadow_test_all(void *shadow, unsigned int size)
> +{
> +       uint8_t *x;
> +       unsigned int i;
> +
> +       x = shadow;
> +
>        /* All bytes must be initialized. */
>        for (i = 0; i < size; ++i) {
>                if (x[i] != KMEMCHECK_SHADOW_INITIALIZED)
>                        return x[i];
>        }
> -#endif
>
>        return x[0];
>  }

Are we certain that size cannot be 0 in kmemcheck_shadow_test()
and kmemcheck_shadow_test_all() or other functions in
arch/x86/mm/kmemcheck/shadow.c with these unsigned
comparisons in loops?

Roel
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