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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Fri, 14 Aug 2020 17:53:28 -0700
Message-ID: <CABCJKue4ikLs9NuxkhwxZUMZzgD10mVXnwt4=_8euXer8gzOQQ@mail.gmail.com>
Subject: Re: [PATCH] lib/string.c: implement stpcpy
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        =?UTF-8?B?RMOhdmlkIEJvbHZhbnNrw70=?= <david.bolvansky@gmail.com>,
        Eli Friedman <efriedma@quicinc.com>, stable@vger.kernel.org,
        Joe Perches <joe@perches.com>, Tony Luck <tony.luck@intel.com>,
        Yury Norov <yury.norov@gmail.com>,
        Daniel Axtens <dja@axtens.net>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Dan Williams <dan.j.williams@intel.com>,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Kees Cook <keescook@chromium.org>,
        Alexandru Ardelean <alexandru.ardelean@analog.com>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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Hi Nick,

On Fri, Aug 14, 2020 at 5:24 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> LLVM implemented a recent "libcall optimization" that lowers calls to
> `sprintf(dest, "%s", str)` where the return value is used to
> `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> in parsing format strings.
>
> `stpcpy` is just like `strcpy` except:
> 1. it returns the pointer to the new tail of `dest`. This allows you to
>    chain multiple calls to `stpcpy` in one statement.
> 2. it requires the parameters not to overlap.  Calling `sprintf` with
>    overlapping arguments was clarified in ISO C99 and POSIX.1-2001 to be
>    undefined behavior.
>
> `stpcpy` was first standardized in POSIX.1-2008.
>
> Implement this so that we don't observe linkage failures due to missing
> symbol definitions for `stpcpy`.
>
> Similar to last year's fire drill with:
> commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
>
> This optimization was introduced into clang-12.
>
> Cc: stable@vger.kernel.org
> Link: https://bugs.llvm.org/show_bug.cgi?id=47162
> Link: https://github.com/ClangBuiltLinux/linux/issues/1126
> Link: https://man7.org/linux/man-pages/man3/stpcpy.3.html
> Link: https://pubs.opengroup.org/onlinepubs/9699919799/functions/stpcpy.html
> Link: https://reviews.llvm.org/D85963
> Reported-by: Sami Tolvanen <samitolvanen@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
>  include/linux/string.h |  3 +++
>  lib/string.c           | 23 +++++++++++++++++++++++
>  2 files changed, 26 insertions(+)

Thank you for the patch! I can confirm that this fixes the build for
me with ToT Clang.

Tested-by: Sami Tolvanen <samitolvanen@google.com>

Sami