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

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Date:   Thu, 3 Feb 2022 19:09:58 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Guenter Roeck <linux@...ck-us.net>, 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()

Hi Kees,

On Thu, Feb 3, 2022 at 6:15 PM Kees Cook <keescook@...omium.org> wrote:
> On Thu, Feb 03, 2022 at 09:04:22AM +0100, Geert Uytterhoeven wrote:
> > Not if -ffreestanding, which is what several architectures are
> > using nowadays, to a.o. prevent gcc from replacing calls to stdlib
> > functions to other stdlib functions (e.g. strncat() -> strlen() +
> > store, strncmp() -> strcmp()), which breaks linking if the latter is
> > only provided inline.
>
> Hah, for i386:
>
> arch/x86/Makefile
>         # temporary until string.h is fixed
>         KBUILD_CFLAGS += -ffreestanding
>
> This "temporary" is from 2006. ;)

And before that, we had it in the main Makefile.

> 6edfba1b33c7 ("[PATCH] x86_64: Don't define string functions to builtin")
>
> Removing that appears to solve it, and appears to build correctly. I'll
> continue testing.
>
> > It works after dropping -ffreestanding.
>
> I wonder if the other architectures were just copying x86?

At least on m68k it was added because gcc added new optimizations
that broke if an architecture provides some functions as inline.
As the kernel is not supposed to be linked with the standard C
library, -ffreestanding should be correct, isn't it?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds