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From:   Nathan Chancellor <natechancellor@gmail.com>
To:     Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com,
        Nathan Chancellor <natechancellor@gmail.com>
Subject: [PATCH v2] s390: vdso: Use $(LD) instead of $(CC) to link vDSO
Date:   Tue,  2 Jun 2020 12:25:24 -0700
Message-Id: <20200602192523.32758-1-natechancellor@gmail.com>
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Currently, the VDSO is being linked through $(CC). This does not match
how the rest of the kernel links objects, which is through the $(LD)
variable.

When clang is built in a default configuration, it first attempts to use
the target triple's default linker, which is just ld. However, the user
can override this through the CLANG_DEFAULT_LINKER cmake define so that
clang uses another linker by default, such as LLVM's own linker, ld.lld.
This can be useful to get more optimized links across various different
projects.

However, this is problematic for the s390 vDSO because ld.lld does not
have any s390 emulatiom support:

https://github.com/llvm/llvm-project/blob/llvmorg-10.0.1-rc1/lld/ELF/Driver.cpp#L132-L150

Thus, if a user is using a toolchain with ld.lld as the default, they
will see an error, even if they have specified ld.bfd through the LD
make variable:

$ make -j"$(nproc)" -s ARCH=s390 CROSS_COMPILE=s390x-linux-gnu- LLVM=1 \
                       LD=s390x-linux-gnu-ld \
                       defconfig arch/s390/kernel/vdso64/
ld.lld: error: unknown emulation: elf64_s390
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

Normally, '-fuse-ld=bfd' could be used to get around this; however, this
can be fragile, depending on paths and variable naming. The cleaner
solution for the kernel is to take advantage of the fact that $(LD) can
be invoked directly, which bypasses the heuristics of $(CC) and respects
the user's choice. Similar changes have been done for ARM, ARM64, and
MIPS.

Link: https://github.com/ClangBuiltLinux/linux/issues/1041
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
---

v1 -> v2:

* Add -fPIC per GCC's documentation, as pointed out by Nick.

 arch/s390/kernel/vdso64/Makefile | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/s390/kernel/vdso64/Makefile b/arch/s390/kernel/vdso64/Makefile
index bec19e7e6e1c..67c14732c304 100644
--- a/arch/s390/kernel/vdso64/Makefile
+++ b/arch/s390/kernel/vdso64/Makefile
@@ -18,8 +18,8 @@ KBUILD_AFLAGS_64 += -m64 -s
 
 KBUILD_CFLAGS_64 := $(filter-out -m64,$(KBUILD_CFLAGS))
 KBUILD_CFLAGS_64 += -m64 -fPIC -shared -fno-common -fno-builtin
-KBUILD_CFLAGS_64 += -nostdlib -Wl,-soname=linux-vdso64.so.1 \
-		    -Wl,--hash-style=both
+ldflags-y := -fPIC -shared -nostdlib -soname=linux-vdso64.so.1 \
+	     --hash-style=both -T
 
 $(targets:%=$(obj)/%.dbg): KBUILD_CFLAGS = $(KBUILD_CFLAGS_64)
 $(targets:%=$(obj)/%.dbg): KBUILD_AFLAGS = $(KBUILD_AFLAGS_64)
@@ -37,8 +37,8 @@ KASAN_SANITIZE := n
 $(obj)/vdso64_wrapper.o : $(obj)/vdso64.so
 
 # link rule for the .so file, .lds has to be first
-$(obj)/vdso64.so.dbg: $(src)/vdso64.lds $(obj-vdso64) FORCE
-	$(call if_changed,vdso64ld)
+$(obj)/vdso64.so.dbg: $(obj)/vdso64.lds $(obj-vdso64) FORCE
+	$(call if_changed,ld)
 
 # strip rule for the .so file
 $(obj)/%.so: OBJCOPYFLAGS := -S
@@ -50,8 +50,6 @@ $(obj-vdso64): %.o: %.S FORCE
 	$(call if_changed_dep,vdso64as)
 
 # actual build commands
-quiet_cmd_vdso64ld = VDSO64L $@
-      cmd_vdso64ld = $(CC) $(c_flags) -Wl,-T $(filter %.lds %.o,$^) -o $@
 quiet_cmd_vdso64as = VDSO64A $@
       cmd_vdso64as = $(CC) $(a_flags) -c -o $@ $<
 

base-commit: e1750a3d9abbea2ece29cac8dc5a6f5bc19c1492
-- 
2.27.0.rc2