linux-hardening - Re: [PATCH v2] string: Introduce strtomem() and strtomem

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Date:   Fri, 2 Sep 2022 08:53:34 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     Kees Cook <keescook@...omium.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Len Baker <len.baker@....com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Francis Laniel <laniel_francis@...vacyrequired.com>,
        Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] string: Introduce strtomem() and strtomem_pad()

On 9/2/22 02:09, Kees Cook wrote:
> One of the "legitimate" uses of strncpy() is copying a NUL-terminated
> string into a fixed-size non-NUL-terminated character array. To avoid
> the weaknesses and ambiguity of intent when using strncpy(), provide
> replacement functions that explicitly distinguish between trailing
> padding and not, and require the destination buffer size be discoverable
> by the compiler.
>> For example:
> 
> struct obj {
> 	int foo;
> 	char small[4] __nonstring;
> 	char big[8] __nonstring;
> 	int bar;
> };
> 
> struct obj p;
> 
> /* This will truncate to 4 chars with no trailing NUL */
> strncpy(p.small, "hello", sizeof(p.small));
> /* p.small contains 'h', 'e', 'l', 'l' */
> 
> /* This will NUL pad to 8 chars. */
> strncpy(p.big, "hello", sizeof(p.big));
> /* p.big contains 'h', 'e', 'l', 'l', 'o', '\0', '\0', '\0' */
> 
> When the "__nonstring" attributes are missing, the intent of the
> programmer becomes ambiguous for whether the lack of a trailing NUL
> in the p.small copy is a bug. Additionally, it's not clear whether
> the trailing padding in the p.big copy is _needed_. Both cases
> become unambiguous with:
> 
> strtomem(p.small, "hello");
> strtomem_pad(p.big, "hello", 0);
> 
> See also https://github.com/KSPP/linux/issues/90
> 

Should'nt strscpy() do the job?

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