linux-kernel - Re: [PATCH 4.14 09/69] x86: vdso: Use $LD instead of $CC to link

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Message-ID: <c9fc2241bb6687e6bfab50e329d81601ff6bc6bf.camel@nokia.com>
Date:   Fri, 26 Apr 2019 11:41:30 +0000
From:   "Rantala, Tommi T. (Nokia - FI/Espoo)" <tommi.t.rantala@...ia.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "sashal@...nel.org" <sashal@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "andi.kleen@...el.com" <andi.kleen@...el.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "natechancellor@...il.com" <natechancellor@...il.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "astrachan@...gle.com" <astrachan@...gle.com>,
        "kernel-team@...roid.com" <kernel-team@...roid.com>
Subject: Re: [PATCH 4.14 09/69] x86: vdso: Use $LD instead of $CC to link

On Mon, 2019-04-15 at 20:58 +0200, Greg Kroah-Hartman wrote:
> commit 379d98ddf41344273d9718556f761420f4dc80b3 upstream.
> 

Hi,

With this patch in 4.14.112 build-id is now missing in vdso32.so:

$ file arch/x86/entry/vdso/vdso*so*
arch/x86/entry/vdso/vdso32.so:     ELF 32-bit LSB pie executable, Intel
80386, version 1 (SYSV), dynamically linked, stripped
arch/x86/entry/vdso/vdso32.so.dbg: ELF 32-bit LSB pie executable, Intel
80386, version 1 (SYSV), dynamically linked, with debug_info, not
stripped
arch/x86/entry/vdso/vdso64.so:     ELF 64-bit LSB pie executable, x86-
64, version 1 (SYSV), dynamically linked,
BuildID[sha1]=d80730a5b561a3161e488a369d1c76c250b584b4, stripped
arch/x86/entry/vdso/vdso64.so.dbg: ELF 64-bit LSB pie executable, x86-
64, version 1 (SYSV), dynamically linked,
BuildID[sha1]=d80730a5b561a3161e488a369d1c76c250b584b4, with
debug_info, not stripped


Based on quick check, "$(call ld-option, --build-id)" fails due to some
32/64 bit mismatch, so the --build-id linker flag is not used when
linking vdso32.so

Perhaps scripts/Kbuild.include is missing some change in 4.14.y to make
this work properly.

-Tommi

> The vdso{32,64}.so can fail to link with CC=clang when clang tries to
> find
> a suitable GCC toolchain to link these libraries with.
> 
> /usr/bin/ld: arch/x86/entry/vdso/vclock_gettime.o:
>   access beyond end of merged section (782)
> 
> This happens because the host environment leaked into the cross
> compiler
> environment due to the way clang searches for suitable GCC
> toolchains.
> 
> Clang is a retargetable compiler, and each invocation of it must
> provide
> --target=<something> --gcc-toolchain=<something> to allow it to find
> the
> correct binutils for cross compilation. These flags had been added to
> KBUILD_CFLAGS, but the vdso code uses CC and not KBUILD_CFLAGS (for
> various
> reasons) which breaks clang's ability to find the correct linker when
> cross
> compiling.
> 
> Most of the time this goes unnoticed because the host linker is new
> enough
> to work anyway, or is incompatible and skipped, but this cannot be
> reliably
> assumed.
> 
> This change alters the vdso makefile to just use LD directly, which
> bypasses clang and thus the searching problem. The makefile will just
> use
> ${CROSS_COMPILE}ld instead, which is always what we want. This
> matches the
> method used to link vmlinux.
> 
> This drops references to DISABLE_LTO; this option doesn't seem to be
> set
> anywhere, and not knowing what its possible values are, it's not
> clear how
> to convert it from CC to LD flag.
> 
> Signed-off-by: Alistair Strachan <astrachan@...gle.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Acked-by: Andy Lutomirski <luto@...nel.org>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: kernel-team@...roid.com
> Cc: joel@...lfernandes.org
> Cc: Andi Kleen <andi.kleen@...el.com>
> Link: 
> https://lkml.kernel.org/r/20180803173931.117515-1-astrachan@google.com
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  arch/x86/entry/vdso/Makefile | 22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/entry/vdso/Makefile
> b/arch/x86/entry/vdso/Makefile
> index 0a550dc5c525..0defcc939ab4 100644
> --- a/arch/x86/entry/vdso/Makefile
> +++ b/arch/x86/entry/vdso/Makefile
> @@ -48,10 +48,8 @@ targets += $(vdso_img_sodbg)
>  
>  export CPPFLAGS_vdso.lds += -P -C
>  
> -VDSO_LDFLAGS_vdso.lds = -m64 -Wl,-soname=linux-vdso.so.1 \
> -			-Wl,--no-undefined \
> -			-Wl,-z,max-page-size=4096 -Wl,-z,common-page-
> size=4096 \
> -			$(DISABLE_LTO)
> +VDSO_LDFLAGS_vdso.lds = -m elf_x86_64 -soname linux-vdso.so.1 --no-
> undefined \
> +			-z max-page-size=4096 -z common-page-size=4096
>  
>  $(obj)/vdso64.so.dbg: $(src)/vdso.lds $(vobjs) FORCE
>  	$(call if_changed,vdso)
> @@ -103,10 +101,8 @@ CFLAGS_REMOVE_vvar.o = -pg
>  #
>  
>  CPPFLAGS_vdsox32.lds = $(CPPFLAGS_vdso.lds)
> -VDSO_LDFLAGS_vdsox32.lds = -Wl,-m,elf32_x86_64 \
> -			   -Wl,-soname=linux-vdso.so.1 \
> -			   -Wl,-z,max-page-size=4096 \
> -			   -Wl,-z,common-page-size=4096
> +VDSO_LDFLAGS_vdsox32.lds = -m elf32_x86_64 -soname linux-vdso.so.1 \
> +			   -z max-page-size=4096 -z common-page-
> size=4096
>  
>  # 64-bit objects to re-brand as x32
>  vobjs64-for-x32 := $(filter-out $(vobjs-nox32),$(vobjs-y))
> @@ -134,7 +130,7 @@ $(obj)/vdsox32.so.dbg: $(src)/vdsox32.lds
> $(vobjx32s) FORCE
>  	$(call if_changed,vdso)
>  
>  CPPFLAGS_vdso32.lds = $(CPPFLAGS_vdso.lds)
> -VDSO_LDFLAGS_vdso32.lds = -m32 -Wl,-m,elf_i386 -Wl,-soname=linux-
> gate.so.1
> +VDSO_LDFLAGS_vdso32.lds = -m elf_i386 -soname linux-gate.so.1
>  
>  # This makes sure the $(obj) subdirectory exists even though vdso32/
>  # is not a kbuild sub-make subdirectory.
> @@ -180,13 +176,13 @@ $(obj)/vdso32.so.dbg: FORCE \
>  # The DSO images are built using a special linker script.
>  #
>  quiet_cmd_vdso = VDSO    $@
> -      cmd_vdso = $(CC) -nostdlib -o $@ \
> +      cmd_vdso = $(LD) -nostdlib -o $@ \
>  		       $(VDSO_LDFLAGS) $(VDSO_LDFLAGS_$(filter
> %.lds,$(^F))) \
> -		       -Wl,-T,$(filter %.lds,$^) $(filter %.o,$^) && \
> +		       -T $(filter %.lds,$^) $(filter %.o,$^) && \
>  		 sh $(srctree)/$(src)/checkundef.sh '$(NM)' '$@'
>  
> -VDSO_LDFLAGS = -fPIC -shared $(call cc-ldoption, -Wl$(comma)--hash-
> style=both) \
> -	$(call cc-ldoption, -Wl$(comma)--build-id) -Wl,-Bsymbolic
> $(LTO_CFLAGS)
> +VDSO_LDFLAGS = -shared $(call ld-option, --hash-style=both) \
> +	$(call ld-option, --build-id) -Bsymbolic
>  GCOV_PROFILE := n
>  
>  #