Re: [PATCH 1/2] powerpc/prom_init: Convert prom_strcpy() into prom_strscpy_pad()

[Tyrel Datwyler <tyreld@linux.ibm.com>]

On 6/21/21 9:11 PM, Michael Ellerman wrote:
> Daniel Axtens <dja@axtens.net> writes:
>> Hi
>>
>>> -static char __init *prom_strcpy(char *dest, const char *src)
>>> +static ssize_t __init prom_strscpy_pad(char *dest, const char *src, size_t n)
>>>  {
>>> -	char *tmp = dest;
>>> +	ssize_t rc;
>>> +	size_t i;
>>>  
>>> -	while ((*dest++ = *src++) != '\0')
>>> -		/* nothing */;
>>> -	return tmp;
>>> +	if (n == 0 || n > INT_MAX)
>>> +		return -E2BIG;
>>> +
>>> +	// Copy up to n bytes
>>> +	for (i = 0; i < n && src[i] != '\0'; i++)
>>> +		dest[i] = src[i];
>>> +
>>> +	rc = i;
>>> +
>>> +	// If we copied all n then we have run out of space for the nul
>>> +	if (rc == n) {
>>> +		// Rewind by one character to ensure nul termination
>>> +		i--;
>>> +		rc = -E2BIG;
>>> +	}
>>> +
>>> +	for (; i < n; i++)
>>> +		dest[i] = '\0';
>>> +
>>> +	return rc;
>>>  }
>>>  
>>
>> This implementation seems good to me.
>>
>> I copied it into a new C file and added the following:
>>
>> int main() {
>> 	char longstr[255]="abcdefghijklmnopqrstuvwxyz";
>> 	char shortstr[5];
>> 	assert(prom_strscpy_pad(longstr, "", 0) == -E2BIG);
>> 	assert(prom_strscpy_pad(longstr, "hello", 255) == 5);
>> 	assert(prom_strscpy_pad(shortstr, "hello", 5) == -E2BIG);
>> 	assert(memcmp(shortstr, "hell", 5) == 0);
>> 	assert(memcmp(longstr, "hello\0\0\0\0\0\0\0\0\0", 6) == 0);
>> 	return 0;
>> }
>>
>> All the assertions pass. I believe this covers all the conditions from
>> the strscpy_pad docstring.
>>
>> Reviewed-by: Daniel Axtens <dja@axtens.net>
> 
> Thanks.
> 
> I'll also drop the explicit nul termination in patch 2, which is a
> leftover from when I was using strncpy().

I guess you can ignore my other email questioning this.

-Tyrel

> 
> cheers
>