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

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Date:   Wed, 2 Feb 2022 15:12:03 -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 2/2/22 12:52, Kees Cook wrote:
> 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?
> 

Some examples:

Build reference: next-20220202
Compiler version: m68k-linux-gcc (GCC) 11.2.0

Building m68k:defconfig ... failed
--------------
Error log:
lib/test_string.c:188:30: error: initializer element is not constant
   188 | static const int strlen_ce = strlen("tada, a constant expression");

Building mips:malta_defconfig:nocd:smp:net,e1000:initrd ... failed
------------
Error log:
lib/test_string.c:188:30: error: initializer element is not constant
  static const int strlen_ce = strlen("tada, a constant expression");

Building i386:q35:Broadwell:defconfig:smp:ata:net,rtl8139:hd ... failed
------------
Error log:
lib/test_string.c:188:30: error: initializer element is not constant
   188 | static const int strlen_ce = strlen("tada, a constant expression");

i386 and is defconfig + CONFIG_STRING_SELFTEST=y; mips is
malta_defconfig + CONFIG_STRING_SELFTEST=y. All use gcc 11.2.

There may be more, but there are so many failures in -next right now
that I may be missing some.

> 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),
> };

I do not think that it is a C standard that strlen() on a constant string
must be compile-time evaluated and result in a constant.

Anyway, key difference, I think, is the presence of an architecture-specific
version of strlen(), or the maybe non-presence of __HAVE_ARCH_STRLEN,
or the definition of strlen() in include/linux/fortify-string.h.

Guenter