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

[Andy Shevchenko]

On Sat, Oct 3, 2020 at 2:37 PM Syed Nayyar Waris <syednwaris@xxxxxxxxx> wrote:
> On Sat, Oct 3, 2020 at 2:14 PM Andy Shevchenko
> <andy.shevchenko@xxxxxxxxx> wrote:
> > On Sat, Oct 3, 2020 at 2:51 AM Syed Nayyar Waris <syednwaris@xxxxxxxxx> wrote:

...

> > > +/**
> > > + * 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.
> > > + */

...

> > > +               return (map[index] >> offset) & GENMASK(nbits - 1, 0);
> >
> > This is UB in GENMASK() when nbits == 0.
>
> 'nbits' actually specifies the width of clump value. Basically 'nbits'
> denotes how-many-bits wide the clump value is.
> 'nbits' having a value of '0' means zero-width-sized clump, meaning
> nothing. 'nbits' can take valid values from '1' to BITS_PER_LONG.
> The minimum value the 'nbits' can have is 1 because the smallest sized
> clump can be 1-bit-wide. It can't be smaller than that.
>
> Let me know if I have misunderstood something?

It's still possible to call with an nbits parameter be equal to 0.
If code is optimized to allow it, it should be documented that 0
parameter is not valid and behaviour is undefined.

...

> > > +/**
> > > + * 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
> > > + */

...

> > > +       value &= GENMASK(nbits - 1, 0);
> >
> > This is UB when nbits == 0.
>
> Same as above.
> 'nbits' actually specifies the width of clump value. Basically 'nbits'
> denotes how-many-bits wide the clump value is.
> 'nbits' having a value of '0' means zero-width-sized clump, meaning
> nothing. 'nbits' can take valid values from '1' to BITS_PER_LONG.
> The minimum value the 'nbits' can have is 1 because the smallest sized
> clump can be 1-bit-wide. It can't be smaller than that.

Same as above.

...

> > > +               map[index] &= ~BITMAP_FIRST_WORD_MASK(start);
> > > +               map[index] |= value << offset;

Side note: I would prefer + 0 here and there, but it's up to you.

> > > +               map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> > > +               map[index + 1] |= (value >> space);

By the way, what about this in the case of start=0, nbits > 64?
space == 64 -> UB.

(And btw parentheses are redundant here)

> > And another LKP finding was among these lines, but I don't remember the details.
>
> Yes you are right. There was sparse warning reported for this.
> sparse: shift too big (64) for type unsigned long
> The warning was reported in patch [4/4] referring to this patch [1/4].
>
> Later it was clarified by the sparse-check maintainer that this
> warning is to be ignored and no code fix is required.
>
> https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg2202377.html

Ah, okay!
--
With Best Regards,
Andy Shevchenko