Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy

[Ard Biesheuvel]

On Tue, 18 Aug 2020 at 09:25, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx # 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?