Re: [PATCH v12 02/11] lib: kstrtox: add kstrtoudec64() and kstrtodec64()

[David Laight <david.laight.linux@gmail.com>]

On Tue, 12 May 2026 18:21:44 +0300
Andy Shevchenko <andy.shevchenko@gmail.com> wrote:

...
> > I think we are going in circles here and we could look at the code instead:
> > - integer parsing with _parse_integer()
> >         - overflow check and validation of the return value
> > - fractional parsing with _parse_integer_limit()
> >         - overflow check and validation of the return value  
> 
> No, this is not fully true. That's what my whole point is about. The
> max_chars parameter limits the input check, then it skips an arbitrary
> number of digits and only *then* it checks for \n and \0. What will be
> the result of the
> 0.00000000000000000000000000000000423 in your case? Whatever scale you
> gave it will return 0 without checking on how many digits were
> supplied. All the same for 0.9999999999999999999999999999999000423. My
> point is that we should limit this by 19 digits.

Don't forget about 000000000.123
And that you also need to worry about leading spaces affecting the length.

To me, the easy way to parse it is to know how many digits are valid
after the '.' and just carry on parsing digits after a '.' until the
limit is hit.
If you really want one function, pass zero to indicate that '.' is invalid.

> 
> On top of that, what about -0.9(19 times) ? the fraction should be u64
> in this case and it's fine. The sign applies to the combined value.
> 
> >         - extra scaling and truncation happening outside if needed.  
> 
> Right, but the given input may be way too long and still needs more validation.
> 
> > - check for input termination
> > - combination of integer and fractional parts with check_mul_overflow() and check_add_overflow()

A lot of the time overflow can be ignored because the digit string is short.
The check_mul_overflow() code is likely to measurably slow things down.
(Especially on 32bit where even a compare against 2**64/10 isn't cheap.)

-- David

> >  
> > > > > Maybe I'm missing these checks already performed?
> > > > >  
> > > > > > > Having the test cases is a big benefit, and that part I like the most.  
> 
> 
>