Re: [PATCH v8 02/10] lib: kstrtox: add kstrntoull() helper

[Rodrigo Alencar]

On 26/03/20 02:24PM, Andy Shevchenko wrote:
> On Fri, Mar 20, 2026 at 12:08:41PM +0000, Rodrigo Alencar wrote:
> > On 26/03/20 01:50PM, Andy Shevchenko wrote:
> > > On Fri, Mar 20, 2026 at 11:16:32AM +0000, Rodrigo Alencar wrote:
> > > > On 26/03/04 10:02AM, Rodrigo Alencar wrote:
> 
> ...
> 
> > > > could you provide more feedback here? Thanks!
> > > 
> > > I don't know what new I can add here.
> > > 
> > > My suggestion was (and still is) to have something in *_strtoull() family
> > > with additional checks added, but no limitations on the input string (i.e.
> > > no max_chars).  If you look at the printf() code the max_chars was added
> > > solely for scanf() and has no use otherwise (yes, I know about and aware
> > > of initramfs case).
> > 
> > but is it include/linux/kstrtox.h the right place for this?
> 
> Seems so, there simple_strto*() are declared.
> 
> > *_strtoull familly... then can we just expose simple_strntoull(), which is
> > private to lib/vsprintf.c, by changing its prototype to expose a error return?
> 
> Why do you need that? I'm lost, sorry, I don't understand this big desire of
> having that max_chars parameter.
> 
> > In my case the limitation on the input string is useful for the truncation of
> > decimal places when parsing the fixed point value. It would avoid a 64-bit
> > division.
> 
> How is it better than checking the returned end pointer? Just treat anything
> that parses too many digits after dot as invalid input?
> 
> 	ret = ..._strtoull(..., &end, &result);

here I would want to pass max_chars as the precision of the fixed point parsing

> 	if (ret)
> 		return ret; // overflow!
> 
> 	if (end - start > $YOUR_LIMIT)
> 		return -EINVAL; // bad input
> 
> 	...process result...

otherwise, here I would need to check the amount of parsed characters and
perform a 64-bit division if it goes beyond the desired precision.
Also, having max_chars allows for more flexible usage of the parsing function.

this is the prototype of simple_strntoull() that would be thinking on expose:

int simple_strntoull(const char *startp, char **endp,
		     unsigned long long *res, unsigned int base,
		     size_t max_chars)

that would also allow to drop the existing FIXME in simple_strntoull().

> Some (stupid) thoughts loudly. IIUC even if we implement '%g' in scanf(), it
> wont help you as you want to have more precise values. Do I get it correct?

If I am parsing 3.14159265359 with 6 decimal precision I want to stop at:

frac = 141592
int = 3

and ignore the rest.

-- 
Kind regards,

Rodrigo Alencar