linux-kernel - Re: [PATCH v2] lib/string.c: implement stpcpy

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Date:   Mon, 17 Aug 2020 14:45:26 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Fangrui Song <maskray@...gle.com>,
        Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Daniel Axtens <dja@...ens.net>, Ingo Molnar <mingo@...nel.org>,
        Yury Norov <yury.norov@...il.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH v2] lib/string.c: implement stpcpy

On Mon, Aug 17, 2020 at 1:13 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> On Mon, Aug 17, 2020 at 11:36:49AM -0700, Nick Desaulniers wrote:
> > > > Though I don't understand the original issue, with -ffreestanding,
> > > > sprintf shouldn't have been turned into strcpy in the first place.
> >
> > Huh? The original issue for this thread is because `-ffreestanding`
> > *isn't* being used for most targets (oh boy, actually mixed usage by
> > ARCH. Looks like MIPS, m68k, superH, xtensa, and 32b x86 use it?); and
> > I'm not suggesting it be used.
> >
>
> Sorry, I meant the issue mentioned in the commit that removed
> -ffreestanding, not the stpcpy one you're solving now. It says that
> sprintf got converted into strcpy, which caused failures because back
> then, strcpy was #define'd to __builtin_strcpy, and the default
> implementation was actually of a function called __builtin_strcpy o_O,
> not strcpy.
>
> Anyway, that's water under the bridge now.
>
> 6edfba1b33c7 ("x86_64: Don't define string functions to builtin")
>   gcc should handle this anyways, and it causes problems when
>   sprintf is turned into strcpy by gcc behind our backs and
>   the C fallback version of strcpy is actually defining __builtin_strcpy

For fun, I tried removing `-ffreestanding` from arch/x86/Makefile;
both gcc and clang can compile+boot the i386 defconfig just fine.  Why
don't I send a patch removing it with your suggested by in a series of
fixes for stpcpy and bcmp?

-- 
Thanks,
~Nick Desaulniers