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

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Date:   Wed, 2 Feb 2022 12:52:00 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Guenter Roeck <linux@...ck-us.net>
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 Wed, Feb 02, 2022 at 08:01:49AM -0800, Guenter Roeck wrote:
> 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.

Which tests?

This property of strlen() is already required by our builds (this is how
I tripped over it). For example:

drivers/firmware/xilinx/zynqmp-debug.c:

#define PM_API(id)               {id, #id, strlen(#id)}
static struct pm_api_info pm_api_list[] = {
        PM_API(PM_GET_API_VERSION),
        PM_API(PM_QUERY_DATA),
};



> 
> 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
> > 

-- 
Kees Cook