linux-kernel - [PATCH] arm64: vdso32: Allow ld.lld to properly link the VDSO

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Message-Id: <20201013033947.2257501-1-natechancellor@gmail.com>
Date:   Mon, 12 Oct 2020 20:39:48 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>
Cc:     Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH] arm64: vdso32: Allow ld.lld to properly link the VDSO

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@...gle.com>
Signed-off-by: Nathan Chancellor <natechancellor@...il.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