linux-kernel - Re: [PATCH v7 2/4] lib: vsprintf: Fix handling of number field widths in vsscanf

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Date:   Mon, 29 Mar 2021 16:36:47 +0300
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 v7 2/4] lib: vsprintf: Fix handling of number field
 widths in vsscanf

On Mon, Mar 29, 2021 at 01:08:22PM +0100, 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
> 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.
> 
> Note that where a very large number is used to mean "unlimited", the value
> INT_MAX is used for consistency with the behaviour of vsnprintf().

...

>  unsigned long simple_strtoul(const char *cp, char **endp, unsigned int base)
>  {
> -	return simple_strtoull(cp, endp, base);
> +	return simple_strntoull(cp, INT_MAX, endp, base);

Why do you need this change?


-- 
With Best Regards,
Andy Shevchenko