Re: Is strncpy really less secure than strscpy ?

[Randy Dunlap <rdunlap@infradead.org>]

On 10/18/23 18:49, Bagas Sanjaya wrote:
> [Disclaimer: I have little to no knowledge of C, so things may be wrong.
>  Please correct me if it is the case. Also Cc: recent people who work on
>  strscpy() conversion.]
> 

Also Cc: the STRING maintainers.

> On Thu, Oct 19, 2023 at 12:22:33AM +0100, James Dutton wrote:
>> Is strncpy really less secure than strscpy ?
>>
>> If one uses strncpy and thus put a limit on the buffer size during the
>> copy, it is safe. There are no writes outside of the buffer.
>> If one uses strscpy and thus put a limit on the buffer size during the
>> copy, it is safe. There are no writes outside of the buffer.
> 
> Well, assuming that the string is NUL-terminated, the end result should
> be the same.
> 
>> But, one can fit more characters in strncpy than strscpy because
>> strscpy enforces the final \0 on the end.
>> One could argue that strncpy is better because it might save the space
>> of one char at the end of a string array.
>> There are cases where strncpy might be unsafe. For example copying
>> between arrays of different sizes, and that is a case where strscpy
>> might be safer, but strncpy can be made safe if one ensures that the
>> size used in strncpy is the smallest of the two different array sizes.
> 
> Code example on both cases?
> 
>>
>> If one blindly replaces strncpy with strscpy across all uses, one
>> could unintentionally be truncating the results and introduce new
>> bugs.
>>
>> The real insecurity surely comes when one tries to use the string.
>> For example:
>>
>> #include <stdio.h>
>> #include <string.h>
>>
>> int main() {
>>         char a[10] = "HelloThere";
>>         char b[10];
>>         char c[10] = "Overflow";
>>         strncpy(b, a, 10);
>>         /* This overflows and so in unsafe */
>>         printf("a is  %s\n", a);
>>         /* This overflows and so in unsafe */
>>         printf("b is  %s\n", b);
>>         /* This is safe */
>>         printf("b is  %.*s\n", 10, a);
>>         /* This is safe */
>>         printf("b is  %.*s\n", 4, a);
>>         return 0;
>> }
> 
> What if printf("a is  %.*s\n", a);?
> 

>>
>>
>> So, why isn't the printk format specifier "%.*s" used more instead of
>> "%s" in the kernel?
> 
> Since basically strings are pointers.
> 
> Thanks.
> 

-- 
~Randy