linux-kernel - Re: [PATCH] kbuild: Fix building with LLVM on NixOS

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Message-ID: <CAFP8O3Kqx-gdTBFn_hesWzd-6NCpGEz1=fMoJXuX+n4c7sp0Bw@mail.gmail.com>
Date: Mon, 12 Feb 2024 13:59:13 -0800
From: Fangrui Song <maskray@...gle.com>
To: Yuxuan Shui <yshuiv7@...il.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, llvm@...ts.linux.dev, nathan@...nel.org, 
	nicolas@...sle.eu, linux-kbuild@...r.kernel.org, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: Fix building with LLVM on NixOS

On Mon, Feb 12, 2024 at 1:30 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> +Cc: Fangrui Song <maskray@...gle.com>
>
>
>
> On Thu, Feb 8, 2024 at 10:22 AM Yuxuan Shui <yshuiv7@...il.com> wrote:
> >
> > NixOS is designed to have immutable packages, and explicit dependencies.
> > It allows multiple different versions of the same shared library to
> > co-exist in its file system.
> >
> > Each application built with Nix, the NixOS package manager, will have
> > paths to its dependency shared libraries hardcoded into its executable,
> > this includes the dynamic linker. To achieve this, Nix adds a
> > --dynamic-linker linker flag when building any application.
> >
> > This isn't a problem if the kernel is built with ld.bfd, because ld.bfd
> > ignores the --dynamic-linker flag when the resulting binary doesn't have
> > a DT_NEEDED entry. However, ld.lld respects --dynamic-linker
> > unconditionally, which breaks linking in several cases.
> >
> > This commit adds an explicit --no-dynamic-linker flag which overrides
> > the flag added by Nix.
>
>
>
> I expect some Acks from LLVM folks (especially, from Frangrui)
> if this is the right thing to do.

GNU ld seems to ignore --dynamic-linker for a position-dependent
executable (ET_EXEC) when there is no DT_NEEDED entry.
ld.lld respects --dynamic-linker in this case. Before this kernel
report, I do not know any user inconvenienced by this difference.
(mold respects --dynamic-linker as well.)
This could be helpful to test an executable with PT_INTERP but no DT_NEEDED.

I think this patch does not fix non-x86 builds.
It feels to me that NixOS should provide a linker wrapper that does
not force --dynamic-linker=.
While it's extremely uncommon (and generally not recommended), certain
programs invoke the linker directly (instead of using a compiler
driver).
Such programs would run into a problem when they make a
position-dependent executable with no dependency as well.
I don't feel that NixOS forcing --dynamic-linker= is enough
justification to change linkers.

> > Signed-off-by: Yuxuan Shui <yshuiv7@...il.com>
> > ---
> >  Makefile                      | 3 +++
> >  arch/x86/boot/Makefile        | 2 +-
> >  arch/x86/realmode/rm/Makefile | 2 +-
> >  3 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index a171eafce2a3b..10ed19caecb1b 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -531,6 +531,9 @@ RUSTFLAGS_KERNEL =
> >  AFLAGS_KERNEL  =
> >  LDFLAGS_vmlinux =
> >
> > +LDFLAGS_MODULE += --no-dynamic-linker
> > +LDFLAGS_vmlinux += --no-dynamic-linker
> > +
> >  # Use USERINCLUDE when you must reference the UAPI directories only.
> >  USERINCLUDE    := \
> >                 -I$(srctree)/arch/$(SRCARCH)/include/uapi \
> > diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> > index 3cece19b74732..390a4604166eb 100644
> > --- a/arch/x86/boot/Makefile
> > +++ b/arch/x86/boot/Makefile
> > @@ -102,7 +102,7 @@ $(obj)/zoffset.h: $(obj)/compressed/vmlinux FORCE
> >  AFLAGS_header.o += -I$(objtree)/$(obj)
> >  $(obj)/header.o: $(obj)/zoffset.h
> >
> > -LDFLAGS_setup.elf      := -m elf_i386 -z noexecstack -T
> > +LDFLAGS_setup.elf      := --no-dynamic-linker -m elf_i386 -z noexecstack -T
> >  $(obj)/setup.elf: $(src)/setup.ld $(SETUP_OBJS) FORCE
> >         $(call if_changed,ld)
> >
> > diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
> > index f614009d3e4e2..4b42006d9ce02 100644
> > --- a/arch/x86/realmode/rm/Makefile
> > +++ b/arch/x86/realmode/rm/Makefile
> > @@ -50,7 +50,7 @@ $(obj)/pasyms.h: $(REALMODE_OBJS) FORCE
> >  targets += realmode.lds
> >  $(obj)/realmode.lds: $(obj)/pasyms.h
> >
> > -LDFLAGS_realmode.elf := -m elf_i386 --emit-relocs -T
> > +LDFLAGS_realmode.elf := --no-dynamic-linker -m elf_i386 --emit-relocs -T
> >  CPPFLAGS_realmode.lds += -P -C -I$(objtree)/$(obj)
> >
> >  targets += realmode.elf
> > --
> > 2.43.0
> >
>
>
> --
> Best Regards
> Masahiro Yamada
>


-- 
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