Re: [PATCH v10 1/4] bitops: Introduce the for_each_set_clump macro

[Andy Shevchenko <andy.shevchenko@gmail.com>]

On Sat, Oct 3, 2020 at 2:51 AM Syed Nayyar Waris <syednwaris@gmail.com> wrote:

Now I remember...
This needs to be revisited.

> This macro iterates for each group of bits (clump) with set bits,
> within a bitmap memory region. For each iteration, "start" is set to
> the bit offset of the found clump, while the respective clump value is
> stored to the location pointed by "clump". Additionally, the
> bitmap_get_value and bitmap_set_value functions are introduced to

Mark functions like func() in the text as well.

> respectively get and set a value of n-bits in a bitmap memory region.
> The n-bits can have any size less than or equal to BITS_PER_LONG.
> Moreover, during setting value of n-bit in bitmap, if a situation arise
> that the width of next n-bit is exceeding the word boundary, then it
> will divide itself such that some portion of it is stored in that word,
> while the remaining portion is stored in the next higher word. Similar
> situation occurs while retrieving value of n-bits from bitmap.

retrieving the value
from a bitmap

...

> +/**
> + * bitmap_get_value - get a value of n-bits from the memory region
> + * @map: address to the bitmap memory region
> + * @start: bit offset of the n-bit value
> + * @nbits: size of value in bits
> + *
> + * Returns value of nbits located at the @start bit offset within the @map
> + * memory region.
> + */
> +static inline unsigned long bitmap_get_value(const unsigned long *map,
> +                                             unsigned long start,
> +                                             unsigned long nbits)
> +{
> +       const size_t index = BIT_WORD(start);
> +       const unsigned long offset = start % BITS_PER_LONG;
> +       const unsigned long ceiling = roundup(start + 1, BITS_PER_LONG);
> +       const unsigned long space = ceiling - start;
> +       unsigned long value_low, value_high;
> +
> +       if (space >= nbits)
> +               return (map[index] >> offset) & GENMASK(nbits - 1, 0);

This is UB in GENMASK() when nbits == 0.

> +       else {
> +               value_low = map[index] & BITMAP_FIRST_WORD_MASK(start);
> +               value_high = map[index + 1] & BITMAP_LAST_WORD_MASK(start + nbits);
> +               return (value_low >> offset) | (value_high << space);
> +       }
> +}

...

> +/**
> + * bitmap_set_value - set n-bit 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
> + */
> +static inline void bitmap_set_value(unsigned long *map,
> +                                   unsigned long value,
> +                                   unsigned long start, unsigned long nbits)
> +{
> +       const size_t index = BIT_WORD(start);
> +       const unsigned long offset = start % BITS_PER_LONG;
> +       const unsigned long ceiling = roundup(start + 1, BITS_PER_LONG);
> +       const unsigned long space = ceiling - start;

> +       value &= GENMASK(nbits - 1, 0);

This is UB when nbits == 0.

> +       if (space >= nbits) {
> +               map[index] &= ~(GENMASK(nbits + offset - 1, offset));

UB when nbits == 0 and start == 0.

> +               map[index] |= value << offset;
> +       } else {
> +               map[index] &= ~BITMAP_FIRST_WORD_MASK(start);
> +               map[index] |= value << offset;
> +               map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> +               map[index + 1] |= (value >> space);

And another LKP finding was among these lines, but I don't remember the details.

> +       }
> +}
> +
>  #endif /* __ASSEMBLY__ */
>
>  #endif /* __LINUX_BITMAP_H */
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index 99f2ac30b1d9..36a445e4a7cc 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -62,6 +62,19 @@ extern unsigned long __sw_hweight64(__u64 w);
>              (start) < (size); \
>              (start) = find_next_clump8(&(clump), (bits), (size), (start) + 8))
>
> +/**
> + * for_each_set_clump - iterate over bitmap for each clump with set bits
> + * @start: bit offset to start search and to store the current iteration offset
> + * @clump: location to store copy of current 8-bit clump
> + * @bits: bitmap address to base the search on
> + * @size: bitmap size in number of bits
> + * @clump_size: clump size in bits
> + */
> +#define for_each_set_clump(start, clump, bits, size, clump_size) \
> +       for ((start) = find_first_clump(&(clump), (bits), (size), (clump_size)); \
> +            (start) < (size); \
> +            (start) = find_next_clump(&(clump), (bits), (size), (start) + (clump_size), (clump_size)))
> +
>  static inline int get_bitmask_order(unsigned int count)
>  {
>         int order;
> diff --git a/lib/find_bit.c b/lib/find_bit.c
> index 49f875f1baf7..1341bd39b32a 100644
> --- a/lib/find_bit.c
> +++ b/lib/find_bit.c
> @@ -190,3 +190,17 @@ unsigned long find_next_clump8(unsigned long *clump, const unsigned long *addr,
>         return offset;
>  }
>  EXPORT_SYMBOL(find_next_clump8);
> +
> +unsigned long find_next_clump(unsigned long *clump, const unsigned long *addr,
> +                              unsigned long size, unsigned long offset,
> +                              unsigned long clump_size)
> +{
> +       offset = find_next_bit(addr, size, offset);
> +       if (offset == size)
> +               return size;
> +
> +       offset = rounddown(offset, clump_size);
> +       *clump = bitmap_get_value(addr, offset, clump_size);
> +       return offset;
> +}
> +EXPORT_SYMBOL(find_next_clump);
> --
> 2.26.2
>


-- 
With Best Regards,
Andy Shevchenko