linux-kernel - Re: [PATCH v4 2/4] lib: vsprintf: Fix handling of number field widths in vsscanf

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Message-ID: <YBr9c44Dvq1ZNrEa@smile.fi.intel.com>
Date:   Wed, 3 Feb 2021 21:45:55 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Richard Fitzgerald <rf@...nsource.cirrus.com>
Cc:     pmladek@...e.com, rostedt@...dmis.org,
        sergey.senozhatsky@...il.com, linux@...musvillemoes.dk,
        shuah@...nel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, patches@...nsource.cirrus.com
Subject: Re: [PATCH v4 2/4] lib: vsprintf: Fix handling of number field
 widths in vsscanf

On Wed, Feb 03, 2021 at 04:50:07PM +0000, Richard Fitzgerald wrote:
> The existing code attempted to handle numbers by doing a strto[u]l(),
> ignoring the field width, and then repeatedly dividing to extract the
> field out of the full converted value. If the string contains a run of
> valid digits longer than will fit in a long or long long, this would
> overflow and no amount of dividing can recover the correct value.
> 
> This patch fixes vsscanf to obey number field widths when parsing

vsscanf()

> the number.
> 
> A new _parse_integer_limit() is added that takes a limit for the number
> of characters to parse. The number field conversion in vsscanf is changed
> to use this new function.
> 
> If a number starts with a radix prefix, the field width  must be long
> enough for at last one digit after the prefix. If not, it will be handled
> like this:
> 
>  sscanf("0x4", "%1i", &i): i=0, scanning continues with the 'x'
>  sscanf("0x4", "%2i", &i): i=0, scanning continues with the '4'
> 
> This is consistent with the observed behaviour of userland sscanf.
> 
> Note that this patch does NOT fix the problem of a single field value
> overflowing the target type. So for example:
> 
>   sscanf("123456789abcdef", "%x", &i);
> 
> Will not produce the correct result because the value obviously overflows
> INT_MAX. But sscanf will report a successful conversion.

...

> +	for (; max_chars > 0; max_chars--) {

Less fragile is to write

	while (max_chars--)

This allows max_char to be an unsigned type.

Moreover...

> +	return _parse_integer_limit(s, base, p, INT_MAX);

You have inconsistency with INT_MAX vs, size_t above.

...

> +unsigned int _parse_integer_limit(const char *s, unsigned int base,
> +				  unsigned long long *res, size_t max_chars);

Also, can you leave res on previous line, so it will be easier to see what's
the difference with the original one?

>  unsigned int _parse_integer(const char *s, unsigned int base, unsigned long long *res);

...

> -	unsigned long long result;
> +	const char *cp;
> +	unsigned long long result = 0ULL;
>  	unsigned int rv;
>  
> -	cp = _parse_integer_fixup_radix(cp, &base);
> -	rv = _parse_integer(cp, base, &result);

> +	if (max_chars == 0) {
> +		cp = startp;
> +		goto out;
> +	}

It's redundant if I'm not mistaken.

> +	cp = _parse_integer_fixup_radix(startp, &base);
> +	if ((cp - startp) >= max_chars) {
> +		cp = startp + max_chars;
> +		goto out;
> +	}

This will be exactly the same, no?

Moreover you will have while (max_chars--) in the _limit() variant which is
also a no-op.

...

> -

Unrelated change.

> +out:
>  	if (endp)
>  		*endp = (char *)cp;

-- 
With Best Regards,
Andy Shevchenko