linux-hardening - Re: [PATCH v3 1/3] lib: fix _parse_integer

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Message-ID: <aXeYsUlxibMuYflx@smile.fi.intel.com>
Date: Mon, 26 Jan 2026 18:39:13 +0200
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Dmitry Antipov <dmantipov@...dex.ru>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <kees@...nel.org>,
	"Darrick J . Wong" <djwong@...nel.org>,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3 1/3] lib: fix _parse_integer_limit() to handle overflow

On Mon, Jan 26, 2026 at 07:20:57PM +0300, Dmitry Antipov wrote:
> In '_parse_integer_limit()', replace native integer arithmetic with
> 'check_mul_overflow()' and 'check_add_overflow()' 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.

...

> -		if (unlikely(res & (~0ull << 60))) {
> -			if (res > div_u64(ULLONG_MAX - val, base))

Interestingly, but the original check was made to improve performance. We don't
need to worry about overflow unless we close to it. It also has a hint to the
compiler to take branch as a slow path.

> +		if (res != ULLONG_MAX) {
> +			/*
> +			 * tmp = res * base;
> +			 * if (overflow)
> +			 *	res = ULLONG_MAX;
> +			 * else {
> +			 *	res = tmp + val;
> +			 *	if (overflow)
> +			 *		res = ULLONG_MAX;
> +			 * }
> +			 */

This looks like a left over. Use plain English to explain what's going on
here. But I think this should be only done for the last a couple of iterations
only.

> +			if (check_mul_overflow(res, base, &tmp) ||
> +			    check_add_overflow(tmp, val, &res)) {
> +				res = ULLONG_MAX;
>  				rv |= KSTRTOX_OVERFLOW;
> +			}
>  		}
> -		res = res * base + val;
>  		rv++;
>  		s++;
>  	}

-- 
With Best Regards,
Andy Shevchenko