linux-kernel - Re: [PATCH 1/4] bitmap: Modify bitmap_set

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Message-ID: <X7gD7Q/63qoUuGpi@shinobu>
Date:   Fri, 20 Nov 2020 12:59:09 -0500
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Syed Nayyar Waris <syednwaris@...il.com>
Cc:     akpm@...ux-foundation.org, andriy.shevchenko@...ux.intel.com,
        arnd@...db.de, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] bitmap: Modify bitmap_set_value() to check bitmap
 length

On Fri, Nov 20, 2020 at 11:14:16PM +0530, Syed Nayyar Waris wrote:
> Add explicit check to see if the value being written into the bitmap
> does not fall outside the bitmap.
> The situation that it is falling outside would never be possible in the
> code because the boundaries are required to be correct before the function
> is called. The responsibility is on the caller for ensuring the boundaries
> are correct.
> This is just to suppress the GCC -Wtype-limits warnings.

Hi Syed,

This commit message sounds a bit strange without the context of our
earlier discussion thread. Would you be able to reword the commit
message to explain the motivation for using __builtin_unreachable()?

Thanks,

William Breathitt Gray

> 
> Cc: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Syed Nayyar Waris <syednwaris@...il.com>
> Acked-by: William Breathitt Gray <vilhelm.gray@...il.com>
> ---
>  include/linux/bitmap.h | 35 +++++++++++++++++++++--------------
>  1 file changed, 21 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 386d08777342..efb6199ea1e7 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -78,8 +78,9 @@
>   *  bitmap_get_value(map, start, nbits)		Get bit value of size
>   *                                              'nbits' from map at start
>   *  bitmap_set_value8(map, value, start)        Set 8bit value to map at start
> - *  bitmap_set_value(map, value, start, nbits)	Set bit value of size 'nbits'
> - *                                              of map at start
> + *  bitmap_set_value(map, nbits, value, value_width, start)
> + *                                              Set bit value of size value_width
> + *                                              to map at start
>   *
>   * Note, bitmap_zero() and bitmap_fill() operate over the region of
>   * unsigned longs, that is, bits behind bitmap till the unsigned long
> @@ -610,30 +611,36 @@ static inline void bitmap_set_value8(unsigned long *map, unsigned long value,
>  }
>  
>  /**
> - * bitmap_set_value - set n-bit value within a memory region
> + * bitmap_set_value - set value within a memory region
>   * @map: address to the bitmap memory region
> - * @value: value of nbits
> - * @start: bit offset of the n-bit value
> - * @nbits: size of value in bits (must be between 1 and BITS_PER_LONG inclusive).
> + * @nbits: size of map in bits
> + * @value: value of clump
> + * @value_width: size of value in bits (must be between 1 and BITS_PER_LONG inclusive)
> + * @start: bit offset of the value
>   */
> -static inline void bitmap_set_value(unsigned long *map,
> -				    unsigned long value,
> -				    unsigned long start, unsigned long nbits)
> +static inline void bitmap_set_value(unsigned long *map, unsigned long nbits,
> +				    unsigned long value, unsigned long value_width,
> +				    unsigned long start)
>  {
> -	const size_t index = BIT_WORD(start);
> +	const unsigned long index = BIT_WORD(start);
> +	const unsigned long length = BIT_WORD(nbits);
>  	const unsigned long offset = start % BITS_PER_LONG;
>  	const unsigned long ceiling = round_up(start + 1, BITS_PER_LONG);
>  	const unsigned long space = ceiling - start;
>  
> -	value &= GENMASK(nbits - 1, 0);
> +	value &= GENMASK(value_width - 1, 0);
>  
> -	if (space >= nbits) {
> -		map[index] &= ~(GENMASK(nbits - 1, 0) << offset);
> +	if (space >= value_width) {
> +		map[index] &= ~(GENMASK(value_width - 1, 0) << offset);
>  		map[index] |= value << offset;
>  	} else {
>  		map[index + 0] &= ~BITMAP_FIRST_WORD_MASK(start);
>  		map[index + 0] |= value << offset;
> -		map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> +
> +		if (index + 1 >= length)
> +			__builtin_unreachable();
> +
> +		map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + value_width);
>  		map[index + 1] |= value >> space;
>  	}
>  }
> -- 
> 2.29.0
> 

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