linux-kernel - Re: [PATCH v5 1/5] lib: fix _parse_integer

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Message-ID: <20260205221537.34778ff0@pumpkin>
Date: Thu, 5 Feb 2026 22:15:37 +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>, "Darrick J .
 Wong" <djwong@...nel.org>, linux-hardening@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/5] lib: fix _parse_integer_limit() to handle
 overflow

On Wed,  4 Feb 2026 16:57:13 +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>
> ---
> 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 | 39 ++++++++++++++++++++++++++-------------
>  1 file changed, 26 insertions(+), 13 deletions(-)
> 
> diff --git a/lib/kstrtox.c b/lib/kstrtox.c
> index bdde40cd69d7..8691f85cf2ce 100644
> --- a/lib/kstrtox.c
> +++ b/lib/kstrtox.c
> @@ -39,20 +39,26 @@ 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,
>  				  size_t max_chars)
>  {
> -	unsigned long long res;
> +	unsigned long long tmp, res;
>  	unsigned int rv;
>  
>  	res = 0;
> @@ -72,14 +78,21 @@ unsigned int _parse_integer_limit(const char *s, unsigned int base, unsigned lon
>  		if (val >= base)
>  			break;
>  		/*
> -		 * Check for overflow only if we are within range of
> -		 * it in the max base we support (16)
> +		 * Accumulate result if no overflow detected.
> +		 * Otherwise just consume valid characters.
>  		 */
> -		if (unlikely(res & (~0ull << 60))) {
> -			if (res > div_u64(ULLONG_MAX - val, base))
> -				rv |= KSTRTOX_OVERFLOW;
> +		if (likely(res != ULLONG_MAX)) {
> +			if (unlikely(res & (~0ull << 60))) {

Aren't those two checks in the wrong order?
The likely/unlikely really don't make that much difference
you want the main test first.

In any case what is the first check for?
I think it just stops 0xffffffffffffffff0 being treated as an error.
If you are trying to skip the rest of the digits after an overflow
you need to check 'rv'.

Although I wonder whether strtoul() (etc) should stop 'eating' input
when the value would overflow and return a pointer to the digit that
caused the error.
Code looking at the terminating character wont be expecting a digit
and will treat it as a syntax error - which is what you are trying to do.

That is a much easier API to use, and a 'drop-in' for existing code.

	David

> +				/* We're close to possible overflow. */
> +				if (check_mul_overflow(res, base, &tmp) ||
> +				    check_add_overflow(tmp, val, &res)) {
> +					res = ULLONG_MAX;
> +					rv |= KSTRTOX_OVERFLOW;
> +				}
> +			} else {
> +				res = res * base + val;
> +			}
>  		}
> -		res = res * base + val;
>  		rv++;
>  		s++;
>  	}