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

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Message-ID: <5bfefab6-7a1b-6f5f-319c-8897dbb79a5b@opensource.cirrus.com>
Date:   Fri, 5 Feb 2021 11:28:15 +0000
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC:     <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 04/02/2021 16:35, Petr Mladek wrote:
> On Wed 2021-02-03 21:45:55, Andy Shevchenko wrote:
>> 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.
> 
>> ...
>>
>>> +	for (; max_chars > 0; max_chars--) {
>>
>> Less fragile is to write
>>
>> 	while (max_chars--)
> 
> Except that the original was more obvious at least for me.
> I always prefer more readable code when the compiler might do
> the optimization easily. But this is my personal taste.
> I am fine with both variants.
> 
>>
>> 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.
> 
> Ah, this was on my request. INT_MAX is already used on many other
> locations in vsnprintf() for this purpose.
> 

I originally had UINT_MAX and changed on Petr's request to be
consistent with other code. (Sorry Andy - my mistake not including
you on the earlier review versions).

But 0 < INT_MAX < UINT_MAX, so ok to pass to an unsigned. And as Petr
said on his original review, INT_MAX is "big enough".

I don't mind either way.

> An alternative is to fix all the other locations. We would need to
> check if it is really safe. Well, I do not want to force Richard
> to fix this historical mess. He already put a lot lot of effort
> into fixing this long term issue.
> 
> ...
> 
>>> -	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.
> 
> Also this is more obvious and less error prone from my POV.
> But I agree that it is redundant. I am not sure if this
> function is used in some fast paths.
> 
> Again I am fine with both variants.
> 
>>> +	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?
> 
> Best Regards,
> Petr
>