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 17:27:32 +0100
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.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 29/03/2021 14:36, Andy Shevchenko wrote:
> 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?
> 

I agree it's not necessary. I changed it between V1 and V2 but I can't
remember what the reason was.