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[v7,1/4] lib: vsprintf: scanf: Negative number must have field width > 1

Message ID 20210329120824.3006-1-rf@opensource.cirrus.com
State Superseded
Headers show
Series [v7,1/4] lib: vsprintf: scanf: Negative number must have field width > 1 | expand

Commit Message

Richard Fitzgerald March 29, 2021, 12:08 p.m. UTC 
If a signed number field starts with a '-' the field width must be > 1,
or unlimited, to allow at least one digit after the '-'.

This patch adds a check for this. If a signed field starts with '-'
and field_width == 1 the scanf will quit.

It is ok for a signed number field to have a field width of 1 if it
starts with a digit. In that case the single digit can be converted.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 lib/vsprintf.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Comments

Richard Fitzgerald March 29, 2021, 4:27 p.m. UTC | #1 
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.
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Patch

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 41ddc353ebb8..f78651e9b030 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -3466,8 +3466,12 @@  int vsscanf(const char *buf, const char *fmt, va_list args)
 		str = skip_spaces(str);
 
 		digit = *str;
-		if (is_sign && digit == '-')
+		if (is_sign && digit == '-') {
+			if (field_width == 1)
+				break;
+
 			digit = *(str + 1);
+		}
 
 		if (!digit
 		    || (base == 16 && !isxdigit(digit))