linux-kernel - Re: Is strncpy really less secure than strscpy ?

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Message-ID: <ZTCLLinnaqIILXsJ@debian.me>
Date:   Thu, 19 Oct 2023 08:49:34 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     James Dutton <james.dutton@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     Justin Stitt <justinstitt@...gle.com>,
        Calvince Otieno <calvncce@...il.com>,
        Azeem Shaikh <azeemshaikh38@...il.com>
Subject: Re: Is strncpy really less secure than strscpy ?

[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.]

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.

-- 
An old man doll... just what I always wanted! - Clara

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