linux-kernel - Re: sscanf: implement basic character sets

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Date:	Mon, 22 Feb 2016 12:51:54 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: sscanf: implement basic character sets

+++ Andy Shevchenko [22/02/16 12:13 +0200]:
>On Fri, 2016-02-19 at 20:22 -0500, Jessica Yu wrote:
>> Implement basic character sets for the '%[]' conversion specifier.
>>
>> The '%[]' conversion specifier matches a nonempty sequence of
>> characters
>> from the specified set of accepted (or with '^', rejected) characters
>> between the brackets. The substring matched is to be made up of
>> characters
>> in (or not in) the set. This implementation differs from its glibc
>> counterpart in that it does not support character ranges (e.g., 'a-z'
>> or
>> '0-9'), the hyphen '-' is *not* a special character, and the brackets
>> themselves cannot be matched.
>>
>> Signed-off-by: Jessica Yu <jeyu@...hat.com>
>> ---
>>  lib/vsprintf.c | 35 +++++++++++++++++++++++++++++++++++
>>  1 file changed, 35 insertions(+)
>>
>> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
>> index 525c8e1..6ee3e7f 100644
>> --- a/lib/vsprintf.c
>> +++ b/lib/vsprintf.c
>> @@ -2714,6 +2714,41 @@ int vsscanf(const char *buf, const char *fmt,
>> va_list args)
>>  			num++;
>>  		}
>>  		continue;
>> +		case '[':
>> +		{
>> +			char *s = (char *)va_arg(args, char *);
>> +			char set[U8_MAX] = { 0 };
>
>Hmm... 255 on stack, not the best idea.
>
>> +			size_t (*op)(const char *str, const char
>> *set);
>> +			size_t len = 0;
>> +			bool negate = (*(fmt) == '^');
>> +
>> +			if (field_width == -1)
>> +				field_width = SHRT_MAX;
>> +
>> +			op = negate ? &strcspn : &strspn;
>> +			if (negate)
>> +				fmt++;
>> +
>> +			len = strcspn(fmt, "]");
>> +			/* invalid format; stop here */
>> +			if (!len)
>> +				return num;
>> +
>> +			strncpy(set, fmt, len);
>
>Perhaps here you may allocate memory on heap and copy the given set.
>IIRC kstrndup() does this.

Thanks for the comments Andy. I did in fact use kstrndup() originally,
but I was not sure about error handling. i.e., if kstrndup() fails we
normally return -ENOMEM, but in this case I suppose sscanf() could
just fail and return num?

Thanks,
Jessica