Re: [PATCH] lib/string.c: implement stpcpy

[Sami Tolvanen <samitolvanen@google.com>]

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