Messages in this thread | | | Date | Tue, 11 Oct 2022 17:31:02 -0600 | From | "Jason A. Donenfeld" <> | Subject | 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) |
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On Tue, Oct 11, 2022 at 11:24:50PM +0000, David Laight wrote: > From: Jason A. Donenfeld > > Sent: 10 October 2022 18:54 > > > > 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. > > I keep thinking about this one. > sparse is being stupid because it is looking at code that cade be reached. > 'ceil' must be 500 so the test at line 62 is false. > But what is the cast for? > Line 64 is only executed when ceil is <= 256 and the condition > only matters when ceil <= 255 - so the cast cannot be needed.
Yes, it's sparse being stupid.
> I think you might be trying to do 8-bit arithmetic - but you > should know it gets promoted to 'int' (and then to unsigned int). > What might do what you intended is (u8)~ceil % ceil. > But that is the same as (0xff - ceil) % ceil.
Right. This already came up in a different thread and I made that change already.
> Which is the same as 0xff % ceil.
Same as 0x100 % ceil, rather. It's compile-time evaluation though, keep in mind, so it doesn't matter.
> A quick check with ceil == 5 gives the wrong answer > as all values get accepted - and one need to be excluded. > So you either need (mult & 0xff) > 0xff % ceil > or (mult & 0xff) >= 0x100 % ceil.
Yes.
Jason
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