linux-hardening - Re: [PATCH v7 1/5] lib: fix _parse_integer

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Message-ID: <20260212135129.358c2462@pumpkin>
Date: Thu, 12 Feb 2026 13:51:29 +0000
From: David Laight <david.laight.linux@...il.com>
To: Dmitry Antipov <dmantipov@...dex.ru>
Cc: Andy Shevchenko <andriy.shevchenko@...el.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Kees Cook <kees@...nel.org>,
 <djwong@...nel.org>, linux-hardening@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 1/5] lib: fix _parse_integer_limit() to handle
 overflow

On Thu, 12 Feb 2026 15:56:24 +0300
Dmitry Antipov <dmantipov@...dex.ru> wrote:

> In '_parse_integer_limit()', adjust native integer arithmetic
> with near-to-overflow branch where 'check_mul_overflow()' and
> 'check_add_overflow()' are used to check whether an intermediate
> result goes out of range, and denote such a case with ULLONG_MAX,
> thus making the function more similar to standard C library's
> 'strtoull()'. Adjust comment to kernel-doc style as well.
> 
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@...el.com>
> Signed-off-by: Dmitry Antipov <dmantipov@...dex.ru>
> ---
> v7: drop redundant check against ULLONG_MAX and restore
>     original comment
> v6: more compact for-loop and minor style adjustments again
> v5: minor brace style adjustment
> v4: restore plain integer arithmetic and use check_xxx_overflow()
>     on near-to-overflow branch only
> v3: adjust commit message and comments as suggested by Andy
> v2: initial version to join the series
> ---
>  lib/kstrtox.c | 33 +++++++++++++++++++++------------
>  1 file changed, 21 insertions(+), 12 deletions(-)
> 
> diff --git a/lib/kstrtox.c b/lib/kstrtox.c
> index bdde40cd69d7..ffcf0219b1f1 100644
> --- a/lib/kstrtox.c
> +++ b/lib/kstrtox.c
> @@ -39,14 +39,20 @@ const char *_parse_integer_fixup_radix(const char *s, unsigned int *base)
>  	return s;
>  }
>  
> -/*
> - * Convert non-negative integer string representation in explicitly given radix
> - * to an integer. A maximum of max_chars characters will be converted.
> +/**
> + * _parse_integer_limit - Convert integer string representation to an integer
> + * @s: Integer string representation
> + * @base: Radix
> + * @p: Where to store result
> + * @max_chars: Maximum amount of characters to convert
> + *
> + * Convert non-negative integer string representation in explicitly given
> + * radix to an integer. If overflow occurs, value at @p is set to ULLONG_MAX.
>   *
> - * Return number of characters consumed maybe or-ed with overflow bit.
> - * If overflow occurs, result integer (incorrect) is still returned.
> + * This function is the workhorse of other string conversion functions and it
> + * is discouraged to use it explicitly. Consider kstrto*() family instead.
>   *
> - * Don't you dare use this function.
> + * Return: Number of characters consumed, maybe ORed with overflow bit
>   */
>  noinline
>  unsigned int _parse_integer_limit(const char *s, unsigned int base, unsigned long long *p,
> @@ -56,8 +62,7 @@ unsigned int _parse_integer_limit(const char *s, unsigned int base, unsigned lon
>  	unsigned int rv;
>  
>  	res = 0;
> -	rv = 0;
> -	while (max_chars--) {
> +	for (rv = 0; max_chars--; rv++, s++) {
>  		unsigned int c = *s;

I think it would be better to use a separate variable for OVERFLOW.
Then the above would be the much more readable:
	overflow = 0;
	for (rv = 0; rv < max_chars; rv++) {
		unsigned int c = s[rv];
with a final:
	return rv | overflow;
('rv' is probably not a good name any more...)
I'd guess the code comes out a bit smaller, rather depends on how the
compiler pessimises it.

>  		unsigned int lc = _tolower(c);
>  		unsigned int val;
> @@ -76,12 +81,16 @@ unsigned int _parse_integer_limit(const char *s, unsigned int base, unsigned lon
>  		 * it in the max base we support (16)
>  		 */
>  		if (unlikely(res & (~0ull << 60))) {
> -			if (res > div_u64(ULLONG_MAX - val, base))
> +			unsigned long long tmp;
> +
> +			if (check_mul_overflow(res, base, &tmp) ||
> +			    check_add_overflow(tmp, val, &res)) {

Do you need 'tmp' at all?
I think you can just use 'res'.

	David

> +				res = ULLONG_MAX;
>  				rv |= KSTRTOX_OVERFLOW;
> +			}
> +		} else {
> +			res = res * base + val;
>  		}
> -		res = res * base + val;
> -		rv++;
> -		s++;
>  	}
>  	*p = res;
>  	return rv;