linux-kernel - Re: [PATCH 1/2] bitmap: Fix bitmap_cut() for partial overlapping case

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Date:   Mon, 15 Jun 2020 12:42:38 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Stefano Brivio <sbrivio@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yury Norov <yury.norov@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] bitmap: Fix bitmap_cut() for partial overlapping case

On Sun, Jun 14, 2020 at 07:40:53PM +0200, Stefano Brivio wrote:
> Yury Norov reports that bitmap_cut() will not produce the right outcome
> if src and dst partially overlap, with src pointing at some location
> after dst, because the memmove() affects src before we store the bits
> that we need to keep, that is, the bits preceding the cut -- as long as
> we the beginning of the cut is not aligned to a long.
> 
> Fix this by storing those bits before the memmove().
> 
> Note that this is just a theoretical concern so far, as the only user
> of this function, pipapo_drop() from the nftables set back-end
> implemented in net/netfilter/nft_set_pipapo.c, always supplies entirely
> overlapping src and dst.

LGTM as long as test cases are passed,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> Reported-by: Yury Norov <yury.norov@...il.com>
> Fixes: 2092767168f0 ("bitmap: Introduce bitmap_cut(): cut bits and shift remaining")
> Signed-off-by: Stefano Brivio <sbrivio@...hat.com>
> ---
> v2: No changes
> 
>  lib/bitmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index 89260aa342d6..c5712e8f4c38 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -211,13 +211,13 @@ void bitmap_cut(unsigned long *dst, const unsigned long *src,
>  	unsigned long keep = 0, carry;
>  	int i;
>  
> -	memmove(dst, src, len * sizeof(*dst));
> -
>  	if (first % BITS_PER_LONG) {
>  		keep = src[first / BITS_PER_LONG] &
>  		       (~0UL >> (BITS_PER_LONG - first % BITS_PER_LONG));
>  	}
>  
> +	memmove(dst, src, len * sizeof(*dst));
> +
>  	while (cut--) {
>  		for (i = first / BITS_PER_LONG; i < len; i++) {
>  			if (i < len - 1)
> -- 
> 2.27.0
> 

-- 
With Best Regards,
Andy Shevchenko