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    SubjectRe: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy
    On Tue, 18 Aug 2020 at 09:25, Greg KH <gregkh@linuxfoundation.org> wrote:
    >
    > On Tue, Aug 18, 2020 at 09:10:01AM +0200, Ard Biesheuvel wrote:
    > > On Tue, 18 Aug 2020 at 00:02, 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. This optimization was introduced into
    > > > clang-12. Because the kernel does not provide an implementation of
    > > > stpcpy, we observe linkage failures for almost all targets when building
    > > > with ToT clang.
    > > >
    > > > The interface is unsafe as it does not perform any bounds checking.
    > > > Disable this "libcall optimization" via `-fno-builtin-stpcpy`.
    > > >
    > > > Unlike
    > > > commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
    > > > which cited failures with `-fno-builtin-*` flags being retained in LLVM
    > > > LTO, that bug seems to have been fixed by
    > > > https://reviews.llvm.org/D71193, so the above sha can now be reverted in
    > > > favor of `-fno-builtin-bcmp`.
    > > >
    > > > Cc: stable@vger.kernel.org # 4.4
    > >
    > > Why does a fix for Clang-12 have to be backported all the way to v4.4?
    > > How does that meet the requirements for stable patches?
    >
    > Because people like to build older kernels with new compliler versions.
    >
    > And those "people" include me, who doesn't want to keep around old
    > compilers just because my distro moved to the latest one...
    >
    > We've been doing this for the past 4+ years, for new versions of gcc,
    > keeping 4.4.y building properly with the bleeding edge of that compiler,
    > why is clang any different here?
    >

    Fair enough. I am just struggling to match stable-kernel-rules.rst
    with the actual practices - perhaps it is time to update that
    document?

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