Re: [crng-random:jd/get_random_u32_below 23/23] include/linux/random.h:64:69: sparse: sparse: cast truncates bits from constant value (1f4 becomes f4)

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Mon, Oct 10, 2022 at 05:18:40PM +0000, David Laight wrote:
> From: kernel test robot <lkp@intel.com>
> > Sent: 10 October 2022 00:32
> > To: Jason A. Donenfeld <zx2c4@kernel.org>
> ...
> 
> I'm missing the main mailing list email for this change.
> I'm guessing the non-inlined code for non-constant ceil
> is similar.

It's part of a development tree I already linked you to. It's not done
yet. This alert is just about needing a __force.

> 
> > vim +64 include/linux/random.h
> > 
> >     53
> >     54	u32 __get_random_u32_below(u32 ceil);
> >     55	/* Returns a random integer in the interval [0, ceil), with uniform distribution. */
> >     56	static inline u32 get_random_u32_below(u32 ceil)
> >     57	{
> >     58		if (!__builtin_constant_p(ceil))
> >     59			return __get_random_u32_below(ceil);
> >     60
> >     61		for (;;) {
> >     62			if (ceil <= 1U << 8) {
> >     63				u32 mult = ceil * get_random_u8();
> >   > 64				if (is_power_of_2(ceil) || (u8)mult >= -(u8)ceil % ceil)
> >     65					return mult >> 8;
> 
> If you are going to check is_power_of_2() then you might as well do:
> 		u32 val = get_random_u8();
> 		if (is_power_of_2(ceil))
> 			return ceil == 0x100 ? val : val & (ceil - 1);
> Except that you don't want to loop on zero - so !(ceil & (ceil - 1))

I thought I opted out of the Laight drive-bys?

Either way, I don't understand your comments or how they pertain to the
current code in that unfinished development branch. Please just send
real patches.

Jason