linux-kernel - Re: [PATCH v5 2/8] iio: core: add fixed point parsing with 64-bit parts

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Message-ID: <shsikp7hinoxzj7pzxopvmvgpaak4dioekh4tyvns4kv6xp46f@z5vgnisqskco>
Date: Mon, 26 Jan 2026 15:30:44 +0000
From: Rodrigo Alencar <455.rodrigo.alencar@...il.com>
To: Rodrigo Alencar <455.rodrigo.alencar@...il.com>, 
	Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: rodrigo.alencar@...log.com, linux-kernel@...r.kernel.org, 
	linux-iio@...r.kernel.org, devicetree@...r.kernel.org, linux-doc@...r.kernel.org, 
	Jonathan Cameron <jic23@...nel.org>, David Lechner <dlechner@...libre.com>, 
	Andy Shevchenko <andy@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>, 
	Michael Hennerich <Michael.Hennerich@...log.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v5 2/8] iio: core: add fixed point parsing with 64-bit
 parts

On 26/01/26 03:20PM, Rodrigo Alencar wrote:
> On 26/01/26 04:53PM, Andy Shevchenko wrote:
> > On Mon, Jan 26, 2026 at 02:26:20PM +0000, Rodrigo Alencar wrote:
> > > On 26/01/26 03:35PM, Andy Shevchenko wrote:
> > > > On Mon, Jan 26, 2026 at 12:42:53PM +0000, Rodrigo Alencar wrote:
> > > > > On 26/01/26 01:49PM, Andy Shevchenko wrote:
> > > > > > On Fri, Jan 23, 2026 at 03:53:07PM +0000, Rodrigo Alencar via B4 Relay wrote:
> > 
> > ...
> > 
> > > > > > > +static int __iio_str_to_fixpoint64(const char *str, u64 fract_mult,
> > > > > > > +				   s64 *integer, s64 *fract, bool scale_db)
> > > > > > > +{
> > > > > > > +	u64 i = 0, f = 0;
> > > > > > > +	char *end;
> > > > > > > +	int digit_count, precision = ffs(fract_mult);
> > > > > > > +	bool negative = false;
> > > > > > > +
> > > > > > > +	if (str[0] == '-') {
> > > > > > > +		negative = true;
> > > > > > > +		str++;
> > > > > > > +	} else if (str[0] == '+') {
> > > > > > > +		str++;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	i = simple_strtoull(str, &end, 10);
> > > > > > > +	digit_count = end - str;
> > > > > > > +	if (digit_count > 20)
> > > > > > > +		return -EINVAL;
> > > > > > 
> > > > > > Not really. If we are talking about decimal (only) cases we need to also count
> > > > > > leading 0:s.
> > > > > > 
> > > > > > 0000000000000000000000000000000025 is still 25, no overflow.
> > > > > > 
> > > > > > That's why I recommend to have a helper, maybe for now locally here, like
> > > > > > 
> > > > > > int safe_strtoull(..., unsigned long long *res)
> > > > > > {
> > > > > > 	...
> > > > > > }
> > > > > 
> > > > > Are you suggesting to not use simple_strtoull then?
> > > > 
> > > > Nope, I suggest to do an additional step before checking for the range.
> > > 
> > > You mean, conditionally skip leading 0's when parsing the integer part?
> > > e.g.
> > > 
> > > /*function entry and arg check */
> > > while(*str == '\0')
> > > 	str++;
> > > /* then call simple_strtoull() */
> > 
> > Not skipping, but counting them.
> > 
> > > simple_strtoull() is not overflow-safe,
> > 
> > Yes, I know. That's why all these additional checks are required,
> > 
> > > as it does not use
> > > check_mul_overflow() or check_add_overflow(), only checking the
> > > amount of digits is not enough.
> > 
> > Why? Can you elaborate how checking amount of digits is different to
> > check_mul_overflow()?
> 
> consider U64_MAX = 18_446_744_073_709_551_615 as the limit:
> - 19_000_000_000_000_000_000 contains the same amount of digits but overflows.
> - 18_446_744_073_710_000_000 contains the same amount of digits but overflows.
> 
> to catch those cases, we need to check for the overflow, everytime we read a
> character and accumulate:
> 
> u64 acc;
> 
> while(isdigit(*str))
> 	if (check_mul_overflow(acc, 10, &acc) ||
> 	    check_add_overflow(acc, *str - '0', &acc))
> 		return -EOVERFLOW;
> 
> *res = acc;
> 
> acc can get weird results if not checked. 

Thinking about it again, that check could be done only in the last step
(20th for u64)

Kind regards,

Rodrigo Alencar