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Date:   Wed, 2 Feb 2022 08:01:49 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Peter Rosin <peda@...ntia.se>,
        Andy Shevchenko <andy@...nel.org>,
        Matteo Croce <mcroce@...rosoft.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] lib/test_string.c: Add test for strlen()

On Sun, Jan 30, 2022 at 10:36:53AM -0800, Kees Cook wrote:
> Add a simple test for strlen() functionality, including using it as a
> constant expression.
> 
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Cc: Peter Rosin <peda@...ntia.se>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> I'll be taking this as part of my Clang FORTIFY_SOURCE series.
> ---
>  lib/test_string.c | 37 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
> 
> diff --git a/lib/test_string.c b/lib/test_string.c
> index 9dfd6f52de92..59994f552c48 100644
> --- a/lib/test_string.c
> +++ b/lib/test_string.c
> @@ -179,6 +179,38 @@ static __init int strnchr_selftest(void)
>  	return 0;
>  }
>  
> +/*
> + * Unlike many other string functions, strlen() can be used in
> + * static initializers when string lengths are known at compile
> + * time. (i.e. Under these conditions, strlen() is a constant
> + * expression.) Make sure it can be used this way.
> + */
> +static const int strlen_ce = strlen("tada, a constant expression");
> +

This results in:

lib/test_string.c:188:30: error: initializer element is not constant
  188 | static const int strlen_ce = strlen("tada, a constant expression");

for several of my tests. I don't think you can mandate that a compiler
implements this.

Guenter

> +static __init int strlen_selftest(void)
> +{
> +	/* String length ruler:         123456789012345 */
> +	static const char normal[]   = "I am normal";
> +	static const char *ptr       = "where do I go?";
> +	static const char trailing[] = "hidden NULLs\0\0\0";
> +	static const char leading[]  = "\0\0hidden text";
> +
> +	if (strlen(normal) != 11)
> +		return 0x100001;
> +	if (strlen(ptr++) != 14)
> +		return 0x100002;
> +	if (strlen(ptr++) != 13)
> +		return 0x100003;
> +	if (strlen(trailing) != 12)
> +		return 0x100004;
> +	if (strlen(leading) != 0)
> +		return 0x100005;
> +	if (strlen_ce != 27)
> +		return 0x100006;
> +
> +	return 0;
> +}
> +
>  static __exit void string_selftest_remove(void)
>  {
>  }
> @@ -212,6 +244,11 @@ static __init int string_selftest_init(void)
>  	if (subtest)
>  		goto fail;
>  
> +	test = 5;
> +	subtest = strlen_selftest();
> +	if (subtest)
> +		goto fail;
> +
>  	pr_info("String selftests succeeded\n");
>  	return 0;
>  fail:
> -- 
> 2.30.2
> 

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