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

[Nathan Chancellor <natechancellor@gmail.com>]

On Mon, Oct 19, 2020 at 10:59:47AM +0100, Vincenzo Frascino wrote:
> 
> Hi Nathan,
> 
> On 10/13/20 4:39 AM, Nathan Chancellor 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>
> 
> Just a small nit on my side (see below) and as you already stated it requires
> rebasing if we want to merge it in mainline. Otherwise:
> 
> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

Thank you for the review! I do not have any personal preference on how
fast this goes in. If we want it fast tracked, it can probably wait
until after -rc1 anyways so I am going to keep it rebased on -next since
presumably the kbuild tree will be merged into mainline by that point.
If anyone else feels differently, let me know.

> > ---
> > 
> > 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)
> 
> Nit: Here can we check 'ifneq ($(LLVM),)' for consistency with what the main
> Makefile does?

Sure, I will fix that up.

> > +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
> > 
> 
> -- 
> Regards,
> Vincenzo