Messages in this thread | | | From | Masahiro Yamada <> | Date | Sun, 7 Apr 2019 11:20:01 +0900 | Subject | Re: [PATCH v3] Makefile: lld: tell clang to use lld |
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Hi Kees, Nick,
On Sat, Apr 6, 2019 at 1:52 AM Nick Desaulniers <ndesaulniers@google.com> wrote: > > On Fri, Apr 5, 2019 at 9:11 AM Kees Cook <keescook@chromium.org> wrote: > > > > On Fri, Apr 5, 2019 at 3:17 AM Masahiro Yamada > > <yamada.masahiro@socionext.com> wrote: > > > I want to propose alternative solution. > > > Please check the attached patches. > > ``` > My plan is to rewrite all VDSO Makefiles to use $(LD), then delete cc-ldoption. > ``` > > I agree with that as a possible solution as well; I think it's best to > just invoke the linker directly rather than use the compiler as the > driver. Just small nits with the 2 attached patches, but I think they > will also work for us. > > Looks like there's 15 call sites across 12 files, plus Documentation/ > and scripts/Kbuild.include; removing it seems feasible and I'd be > happy to help if you'd welcome the patches?
Yeah, your help is appreciated.
I just worked on two Makefiles because I wanted to hear opinions before investing more efforts.
Basically, you are positive to this approach, so let's go this way.
> Let's start with that series of changes, but I suspect we will > ultimately need to revisit -fuse-ld=. I was just thinking that even > my proposed patch doesn't do anything for KBUILD_HOSTCFLAGS, for > example.
Host tools are not so important, but I agree.
BTW, why does clang invoke bfd linker instead of lld by default?
If we want to make llvm self-contained, I believe clang should use ld.lld by default.
> I worry that bfd may still be invoked by Clang at some point > in the build. I will try to test in an environment with no GNU > binutils. > > > In the 0001 patch of arch/arm/vdso/Makefile: > > > ... > > > -VDSO_LDFLAGS += $(call cc-ldoption, -fuse-ld=bfd) > > > +ldflags-y = -Bsymbolic --no-undefined -soname=linux-vdso.so.1 \ > > > + -z max-page-size=4096 -z common-page-size=4096 \ > > > + -nostdlib -shared \ > > > + $(call ld-option, --hash-style=sysv) \ > > I think the vdso's should all REQUIRE --hash-style=sysv; the kselftest > for vdso's will fail otherwise. I could always follow up with patches > for that if we didn't want to conflate changes. > > > > + $(call ld-option, --build-id) \ > > > + -T > > Was it intentional to add -T standalone? The man page for `ld` says > it needs an additional filename to follow -T.
This is because I wanted to re-use cmd_ld defined in scripts/Makefile.lib
$(real-prereqs) contains the linker scripts and objects.
> Or rather is optional > if a -L flag is specified? > > > > > I see that "-fuse-ld=bfd" is explicitly dropped here, which could > > reintroduce the problem fixed in d2b30cd4b722 ("ARM: 8384/1: VDSO: > > force use of BFD linker") (and 3473f26592c1c). Does ld.gold still > > exhibit this problem? If so, we'll need to detect gold and force bfd > > still maybe?
I intentionally dropped -fuse-ld=bfd.
With my patch applied, users have full control of the linker by passing LD=.
Since we cannot link vmlinux with gold, users should definitely pass a bfd linker.
> The arm32 vdso Makefile is an oddity in this regard. I think what > Kees described is best. Rather than always set the linker to bfd > (which harms linkers like lld), detect if gold is being used and if so > set the linker back to bfd. > > For arm64, lld has this issue with -n, > https://github.com/ClangBuiltLinux/linux/issues/340, but I think we > can fix that in a follow up patch. Same thoughts on --hash-style and > -T.
If we need to add something depending on the linker type,
I do not mind doing like this.
ldflags-$(CONFIG_LD_IS_GOLD) += ... ldflags-$(CONFIG_LD_IS_LLD) += ...
But, it is a different issue.
-- Best Regards Masahiro Yamada
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