linux-kernel - Re: [PATCH v7 4/5] x86/build: Warn on orphan section placement

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Message-ID: <20200908201734.GA87825@rani.riverdale.lan>
Date:   Tue, 8 Sep 2020 16:17:34 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        clang-built-linux@...glegroups.com, linux-arch@...r.kernel.org,
        linux-efi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 4/5] x86/build: Warn on orphan section placement

On Sat, Sep 05, 2020 at 06:48:35PM -0400, Arvind Sankar wrote:
> On Tue, Sep 01, 2020 at 07:53:46PM -0700, Kees Cook wrote:
> > We don't want to depend on the linker's orphan section placement
> > heuristics as these can vary between linkers, and may change between
> > versions. All sections need to be explicitly handled in the linker script.
> > 
> > Now that all sections are explicitly handled, enable orphan section
> > warnings.
> > 
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  arch/x86/Makefile | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index 4346ffb2e39f..154259f18b8b 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -209,6 +209,10 @@ ifdef CONFIG_X86_64
> >  LDFLAGS_vmlinux += -z max-page-size=0x200000
> >  endif
> >  
> > +# We never want expected sections to be placed heuristically by the
> > +# linker. All sections should be explicitly named in the linker script.
> > +LDFLAGS_vmlinux += $(call ld-option, --orphan-handling=warn)
> > +
> >  archscripts: scripts_basic
> >  	$(Q)$(MAKE) $(build)=arch/x86/tools relocs
> >  
> > -- 
> > 2.25.1
> > 
> 
> With LLVM=1 and GCOV_KERNEL/GCOV_PROFILE_ALL enabled, there are
> .eh_frame sections created. I see that KASAN and KCSAN currently discard
> them. Does GCOV actually need them or should it also discard?
> 
> Thanks.

Also, with LLD 10.0.1 which is going to be the minimum supported
version, the relocation sections etc still generate warnings.

ld.lld: warning:
arch/x86/video/built-in.a(fbdev.o):(.rela.orc_unwind_ip) is being placed
in '.rela.orc_unwind_ip'
ld.lld: warning: .tmp_vmlinux.kallsyms2.o:(.rela.rodata) is being placed
in '.rela.rodata'
ld.lld: warning: <internal>:(.bss.rel.ro) is being placed in
'.bss.rel.ro'
ld.lld: warning: <internal>:(.eh_frame) is being placed in '.eh_frame'
ld.lld: warning: <internal>:(.symtab_shndx) is being placed in
'.symtab_shndx'