Re: [PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO

[Fangrui Song <maskray@google.com>]

On 2021-03-25, Nathan Chancellor wrote:
>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 linking with clang, there are a couple of warnings about flags that
>will not be used during the link:
>
>clang-12: warning: argument unused during compilation: '-no-pie' [-Wunused-command-line-argument]
>clang-12: warning: argument unused during compilation: '-pg' [-Wunused-command-line-argument]
>
>'-no-pie' was added in commit 85602bea297f ("RISC-V: build vdso-dummy.o
>with -no-pie") to override '-pie' getting added to the ld command from
>distribution versions of GCC that enable PIE by default. It is
>technically no longer needed after commit c2c81bb2f691 ("RISC-V: Fix the
>VDSO symbol generaton for binutils-2.35+"), which removed vdso-dummy.o
>in favor of generating vdso-syms.S from vdso.so with $(NM) but this also
>resolves the issue in case it ever comes back due to having full control
>over the $(LD) command. '-pg' is for function tracing, it is not used
>during linking as clang states.

Looks good.

-pg affects the link action: it changes crt1.o to gcrt1.o.
Since the Makefile uses -nostdlib, crt1.o is suppressed, so -pg
is entirely unneeded.
(-nostdlib implies -nostartfiles so the previous usage has a redundant
option.)

>These flags could be removed/filtered to fix the warnings but it is
>easier to just match the rest of the kernel and use $(LD) directly for
>linking. See commits
>
>  fe00e50b2db8 ("ARM: 8858/1: vdso: use $(LD) instead of $(CC) to link VDSO")
>  691efbedc60d ("arm64: vdso: use $(LD) instead of $(CC) to link VDSO")
>  2ff906994b6c ("MIPS: VDSO: Use $(LD) instead of $(CC) to link VDSO")
>  2b2a25845d53 ("s390/vdso: Use $(LD) instead of $(CC) to link vDSO")
>
>for more information.
>
>The flags are converted to linker flags and '--eh-frame-hdr' is added to
>match what is added by GCC implicitly, which can be seen by adding '-v'
>to GCC's invocation.

Another minor change which may be shipped together: --hash-style=both
can be --hash-style=gnu. We don't need sysv .hash . The glibc/musl
support for .gnu.hash has been there for years. .gnu.hash is often
smaller than .hash .

Reviewed-by: Fangrui Song <maskray@google.com>

>Additionally, since this area is being modified, use the $(OBJCOPY)
>variable instead of an open coded $(CROSS_COMPILE)objcopy so that the
>user's choice of objcopy binary is respected.
>
>Link: https://github.com/ClangBuiltLinux/linux/issues/803
>Link: https://github.com/ClangBuiltLinux/linux/issues/970
>Signed-off-by: Nathan Chancellor <nathan@kernel.org>
>---
> arch/riscv/kernel/vdso/Makefile | 12 ++++--------
> 1 file changed, 4 insertions(+), 8 deletions(-)
>
>diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
>index 71a315e73cbe..ca2b40dfd24b 100644
>--- a/arch/riscv/kernel/vdso/Makefile
>+++ b/arch/riscv/kernel/vdso/Makefile
>@@ -41,11 +41,10 @@ KASAN_SANITIZE := n
> $(obj)/vdso.o: $(obj)/vdso.so
>
> # link rule for the .so file, .lds has to be first
>-SYSCFLAGS_vdso.so.dbg = $(c_flags)
> $(obj)/vdso.so.dbg: $(src)/vdso.lds $(obj-vdso) FORCE
> 	$(call if_changed,vdsold)
>-SYSCFLAGS_vdso.so.dbg = -shared -s -Wl,-soname=linux-vdso.so.1 \
>-	-Wl,--build-id=sha1 -Wl,--hash-style=both
>+LDFLAGS_vdso.so.dbg = -shared -s -soname=linux-vdso.so.1 \
>+	--build-id=sha1 --hash-style=both --eh-frame-hdr
>
> # We also create a special relocatable object that should mirror the symbol
> # table and layout of the linked DSO. With ld --just-symbols we can then
>@@ -60,13 +59,10 @@ $(obj)/%.so: $(obj)/%.so.dbg FORCE
>
> # actual build commands
> # The DSO images are built using a special linker script
>-# Add -lgcc so rv32 gets static muldi3 and lshrdi3 definitions.
> # Make sure only to export the intended __vdso_xxx symbol offsets.
> quiet_cmd_vdsold = VDSOLD  $@
>-      cmd_vdsold = $(CC) $(KBUILD_CFLAGS) $(call cc-option, -no-pie) -nostdlib -nostartfiles $(SYSCFLAGS_$(@F)) \
>-                           -Wl,-T,$(filter-out FORCE,$^) -o $@.tmp && \
>-                   $(CROSS_COMPILE)objcopy \
>-                           $(patsubst %, -G __vdso_%, $(vdso-syms)) $@.tmp $@ && \
>+      cmd_vdsold = $(LD) $(ld_flags) -T $(filter-out FORCE,$^) -o $@.tmp && \
>+                   $(OBJCOPY) $(patsubst %, -G __vdso_%, $(vdso-syms)) $@.tmp $@ && \
>                    rm $@.tmp
>
> # Extracts symbol offsets from the VDSO, converting them into an assembly file
>-- 
>2.31.0
>
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