Messages in this thread | | | Date | Fri, 26 Mar 2021 19:02:16 -0700 | From | Fangrui Song <> | Subject | Re: [PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO |
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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 > >-- >You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. >To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com. >To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20210325215156.1986901-1-nathan%40kernel.org.
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