From: Kees Cook <keescook@chromium.org>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
Peter Rosin <peda@axentia.se>, Andy Shevchenko <andy@kernel.org>,
Matteo Croce <mcroce@microsoft.com>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
llvm@lists.linux.dev, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] lib/test_string.c: Add test for strlen()
Date: Sun, 30 Jan 2022 12:34:41 -0800 [thread overview]
Message-ID: <202201301220.529465D6@keescook> (raw)
In-Reply-To: <CAHp75Vf9S8jKQGAYRrmSET7YJQNoHMzUC6VVTAOT7DbwcCcc4Q@mail.gmail.com>
On Sun, Jan 30, 2022 at 08:56:40PM +0200, Andy Shevchenko wrote:
> On Sun, Jan 30, 2022 at 8:36 PM Kees Cook <keescook@chromium.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?
Yup! See: https://godbolt.org/z/nTqPaszTh
There a few rare places in the kernel that do this, which is how
I noticed. (I broke strlen() with the recent FORTIFY changes.)
> > +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).
Correct, unless strlen() breaks.
> Same to the rest? If so, why is this not a part of the compiler tests?
I wanted to keep everything together -- this includes a macro
side-effect test as well ("ptr++").
-Kees
--
Kees Cook
next prev parent reply other threads:[~2022-01-30 20:34 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-30 18:36 [PATCH] lib/test_string.c: Add test for strlen() Kees Cook
2022-01-30 18:56 ` Andy Shevchenko
2022-01-30 20:34 ` Kees Cook [this message]
2022-01-30 20:13 ` kernel test robot
2022-01-30 20:35 ` Kees Cook
2022-02-02 16:01 ` Guenter Roeck
2022-02-02 16:16 ` Andy Shevchenko
2022-02-02 20:52 ` Kees Cook
2022-02-02 23:12 ` Guenter Roeck
2022-02-03 8:04 ` Geert Uytterhoeven
2022-02-03 16:41 ` Kees Cook
2022-02-03 17:15 ` Kees Cook
2022-02-03 18:09 ` Geert Uytterhoeven
2022-02-03 19:50 ` Nick Desaulniers
2022-02-03 20:25 ` Kees Cook
2022-02-03 20:45 ` Kees Cook
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