Message-ID: <192270bd88d.f4dae43a112354.5469673441409436657@midstall.com>
Date: Tue, 24 Sep 2024 19:39:00 -0700
From: Tristan ross <tristan.ross@...stall.com>
To: "Fangrui Song" <maskray@...gle.com>
Cc: "Yuxuan Shui" <yshuiv7@...il.com>,
	"Masahiro Yamada" <masahiroy@...nel.org>,
	"llvm" <llvm@...ts.linux.dev>, "nathan" <nathan@...nel.org>,
	"nicolas" <nicolas@...sle.eu>,
	"linux-kbuild" <linux-kbuild@...r.kernel.org>,
	"tglx" <tglx@...utronix.de>, "mingo" <mingo@...hat.com>,
	"bp" <bp@...en8.de>, "dave.hansen" <dave.hansen@...ux.intel.com>,
	"x86" <x86@...nel.org>, "hpa" <hpa@...or.com>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kbuild: Fix building with LLVM on NixOS

LLVM maintainer for Nix here, I do agree that our wrapper is causing issues here. However, we're looking to fix this by mimicking GNU ld's behavior for checking --dynamic-linker. We're also going to look into getting LLVM lld to support the same behavior in case others have this issue. We might not be the only ones who could be affected so adding this behavior into LLVM definitely will be useful. I tested aarch64 building of the kernel and it works after we fixed our handling of hardening options. However, aside from aarch64 and x86_64, I am not sure what other things could be failing. I hope this clarifies things.

Thank you,
Tristan Ross

(Sorry if I sent two emails, I forgot to reply all last time. First time using the LKML)


 ---- On Mon, 12 Feb 2024 13:59:13 -0800  Fangrui Song  wrote --- 
 > 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
 > >
 > 
 > 
 > -- 
 > 宋方睿
 > 
 > 
 > 
Tristan ross
CEO & Founder of Midstall Software

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