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

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Date:   Sun, 16 Aug 2020 11:02:17 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Fangrui Song <maskray@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        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>
Subject: Re: [PATCH v2] lib/string.c: implement stpcpy

On Sun, Aug 16, 2020 at 07:22:35AM +0200, Sedat Dilek wrote:
> On Sun, Aug 16, 2020 at 2:19 AM 'Fangrui Song' via Clang Built Linux
> <clang-built-linux@...glegroups.com> wrote:
> >
> > Adding a definition without a declaration for stpcpy looks good.
> > Clang LTO will work.
> >
> > (If the kernel does not want to provide these routines,
> > is http://git.kernel.org/linus/6edfba1b33c701108717f4e036320fc39abe1912
> > probably wrong? (why remove -ffreestanding from the main Makefile) )
> >
> 
> We had some many issues in arch/x86 where *FLAGS were removed or used
> differently and had to re-add them :-(.
> 
> So if -ffreestanding is used in arch/x86 and was! used in top-level
> Makefile - it makes sense to re-add it back?
> ( I cannot speak for archs other than x86. )
> 
> - Sedat -

-ffreestanding disables _all_ builtins and libcall optimizations, which
is probably not desirable. If we added it back, we'd need to also go
back to #define various string functions to the __builtin versions.

Though I don't understand the original issue, with -ffreestanding,
sprintf shouldn't have been turned into strcpy in the first place.

32-bit still has -ffreestanding -- I wonder if that's actually necessary
any more?

Why does -fno-builtin-stpcpy not work with clang LTO? Isn't that a
compiler bug?

Thanks.