linux-hardening - Re: [PATCH] lib/test_string.c: Add test for strlen()

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Date:   Sun, 30 Jan 2022 20:56:40 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
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 Mailing List <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 8:36 PM Kees Cook <keescook@...omium.org> wrote:
>
> Add a simple test for strlen() functionality, including using it as a
> constant expression.

...

> +/*
> + * 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");

So, the compiler will replace this by a constant and then eliminate
the condition completely from the code. Did I understand this
correctly?

> +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;

...so this part won't ever appear in the assembly (assuming -O2).

Same to the rest? If so, why is this not a part of the compiler tests?

> +       return 0;
> +}


-- 
With Best Regards,
Andy Shevchenko