Re: [PATCH] arm64: vdso32: Allow ld.lld to properly link the VDSO

[Nick Desaulniers <ndesaulniers@google.com>]

On Mon, Oct 12, 2020 at 8:41 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> As it stands now, the vdso32 Makefile hardcodes the linker to ld.bfd
> using -fuse-ld=bfd with $(CC). This was taken from the arm vDSO
> Makefile, as the comment notes, done in commit d2b30cd4b722 ("ARM:
> 8384/1: VDSO: force use of BFD linker").
>
> Commit fe00e50b2db8 ("ARM: 8858/1: vdso: use $(LD) instead of $(CC) to
> link VDSO") changed that Makefile to use $(LD) directly instead of
> through $(CC), which matches how the rest of the kernel operates. Since
> then, LD=ld.lld means that the arm vDSO will be linked with ld.lld,
> which has shown no problems so far.
>
> Allow ld.lld to link this vDSO as we do the regular arm vDSO. To do
> this, we need to do a few things:
>
> * Add a LD_COMPAT variable, which defaults to $(CROSS_COMPILE_COMPAT)ld
>   with gcc and $(LD) if LLVM is 1, which will be ld.lld, or
>   $(CROSS_COMPILE_COMPAT)ld if not, which matches the logic of the main
>   Makefile. It is overrideable for further customization and avoiding
>   breakage.
>
> * Eliminate cc32-ldoption, which matches commit 055efab3120b ("kbuild:
>   drop support for cc-ldoption").
>
> With those, we can use $(LD_COMPAT) in cmd_ldvdso and change the flags
> from compiler linker flags to linker flags directly. We eliminate
> -mfloat-abi=soft because it is not handled by the linker.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1033
> Reported-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

Thanks for the patch and summary of related changes! This is
immediately useful for Android.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>
> NOTE: This patch is currently based on the kbuild tree due to the
> --build-id -> --build-id=sha1 change that Bill did. If the arm64
> maintainers would prefer to take this patch, I can rebase it (althought
> presumably this won't hit mainline until at least 5.11 so it can
> probably just stay as is for now).
>
>  arch/arm64/kernel/vdso32/Makefile | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> index 7f96a1a9f68c..1cf00c58805d 100644
> --- a/arch/arm64/kernel/vdso32/Makefile
> +++ b/arch/arm64/kernel/vdso32/Makefile
> @@ -22,16 +22,21 @@ endif
>
>  CC_COMPAT ?= $(CC)
>  CC_COMPAT += $(CC_COMPAT_CLANG_FLAGS)
> +
> +ifeq ($(LLVM),1)
> +LD_COMPAT ?= $(LD)
> +else
> +LD_COMPAT ?= $(CROSS_COMPILE_COMPAT)ld
> +endif
>  else
>  CC_COMPAT ?= $(CROSS_COMPILE_COMPAT)gcc
> +LD_COMPAT ?= $(CROSS_COMPILE_COMPAT)ld
>  endif
>
>  cc32-option = $(call try-run,\
>          $(CC_COMPAT) $(1) -c -x c /dev/null -o "$$TMP",$(1),$(2))
>  cc32-disable-warning = $(call try-run,\
>         $(CC_COMPAT) -W$(strip $(1)) -c -x c /dev/null -o "$$TMP",-Wno-$(strip $(1)))
> -cc32-ldoption = $(call try-run,\
> -        $(CC_COMPAT) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
>  cc32-as-instr = $(call try-run,\
>         printf "%b\n" "$(1)" | $(CC_COMPAT) $(VDSO_AFLAGS) -c -x assembler -o "$$TMP" -,$(2),$(3))
>
> @@ -122,14 +127,10 @@ dmbinstr := $(call cc32-as-instr,dmb ishld,-DCONFIG_AS_DMB_ISHLD=1)
>  VDSO_CFLAGS += $(dmbinstr)
>  VDSO_AFLAGS += $(dmbinstr)
>
> -VDSO_LDFLAGS := $(VDSO_CPPFLAGS)
>  # From arm vDSO Makefile
> -VDSO_LDFLAGS += -Wl,-Bsymbolic -Wl,--no-undefined -Wl,-soname=linux-vdso.so.1
> -VDSO_LDFLAGS += -Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096
> -VDSO_LDFLAGS += -nostdlib -shared -mfloat-abi=soft
> -VDSO_LDFLAGS += -Wl,--hash-style=sysv
> -VDSO_LDFLAGS += -Wl,--build-id=sha1
> -VDSO_LDFLAGS += $(call cc32-ldoption,-fuse-ld=bfd)
> +VDSO_LDFLAGS += -Bsymbolic --no-undefined -soname=linux-vdso.so.1
> +VDSO_LDFLAGS += -z max-page-size=4096 -z common-page-size=4096
> +VDSO_LDFLAGS += -nostdlib -shared --hash-style=sysv --build-id=sha1
>
>
>  # Borrow vdsomunge.c from the arm vDSO
> @@ -189,8 +190,8 @@ quiet_cmd_vdsold_and_vdso_check = LD32    $@
>        cmd_vdsold_and_vdso_check = $(cmd_vdsold); $(cmd_vdso_check)
>
>  quiet_cmd_vdsold = LD32    $@
> -      cmd_vdsold = $(CC_COMPAT) -Wp,-MD,$(depfile) $(VDSO_LDFLAGS) \
> -                   -Wl,-T $(filter %.lds,$^) $(filter %.o,$^) -o $@
> +      cmd_vdsold = $(LD_COMPAT) $(VDSO_LDFLAGS) \
> +                   -T $(filter %.lds,$^) $(filter %.o,$^) -o $@
>  quiet_cmd_vdsocc = CC32    $@
>        cmd_vdsocc = $(CC_COMPAT) -Wp,-MD,$(depfile) $(VDSO_CFLAGS) -c -o $@ $<
>  quiet_cmd_vdsocc_gettimeofday = CC32    $@
>
> base-commit: 172aad81a882443eefe1bd860c4eddc81b14dd5b
> --
> 2.29.0.rc0
>


-- 
Thanks,
~Nick Desaulniers