Re: [PATCH v4 4/5] MIPS: VDSO: Use $(LD) instead of $(CC) to link VDSO

Fangrui Song
2020-04-28T22:54:07Z

On 2020-04-28, 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 clang is built in a default configuration, it first attempts to use
>the target triple's default linker then the system's default linker,
>unless told otherwise through -fuse-ld=... We do not use -fuse-ld=
>because it can be brittle and we have support for invoking $(LD)
>directly. See commit fe00e50b2db8c ("ARM: 8858/1: vdso: use $(LD)
>instead of $(CC) to link VDSO") and commit 691efbedc60d2 ("arm64: vdso:
>use $(LD) instead of $(CC) to link VDSO") for examples of doing this in
>the VDSO.
>
>Do the same thing here. Replace the custom linking logic with $(cmd_ld)
>and ldflags-y so that $(LD) is respected. We need to explicitly add two
>flags to the linker that were implicitly passed by the compiler:
>-G 0 (which comes from ccflags-vdso) and --eh-frame-hdr.
>
>Before this patch (generated by adding '-v' to VDSO_LDFLAGS):
>
><gcc_prefix>/libexec/gcc/mips64-linux/9.3.0/collect2 \
>-plugin <gcc_prefix>/libexec/gcc/mips64-linux/9.3.0/liblto_plugin.so \
>-plugin-opt=<gcc_prefix>/libexec/gcc/mips64-linux/9.3.0/lto-wrapper \
>-plugin-opt=-fresolution=/tmp/ccGEi5Ka.res \
>--eh-frame-hdr \
>-G 0 \
>-EB \
>-mips64r2 \
>-shared \
>-melf64btsmip \
>-o arch/mips/vdso/vdso.so.dbg.raw \
>-L<gcc_prefix>/lib/gcc/mips64-linux/9.3.0/64 \
>-L<gcc_prefix>/lib/gcc/mips64-linux/9.3.0 \
>-L<gcc_prefix>/lib/gcc/mips64-linux/9.3.0/../../../../mips64-linux/lib \
>-Bsymbolic \
>--no-undefined \
>-soname=linux-vdso.so.1 \
>-EB \
>--hash-style=sysv \
>--build-id \
>-T arch/mips/vdso/vdso.lds \
>arch/mips/vdso/elf.o \
>arch/mips/vdso/vgettimeofday.o \
>arch/mips/vdso/sigreturn.o
>
>After this patch:
>
><gcc_prefix>/bin/mips64-linux-ld \
>-m elf64btsmip \
>-Bsymbolic \
>--no-undefined \
>-soname=linux-vdso.so.1 \
>-EB \
>-nostdlib \
>-shared \
>-G 0 \
>--eh-frame-hdr \
>--hash-style=sysv \
>--build-id \
>-T arch/mips/vdso/vdso.lds \
>arch/mips/vdso/elf.o \
>arch/mips/vdso/vgettimeofday.o
>arch/mips/vdso/sigreturn.o \
>-o arch/mips/vdso/vdso.so.dbg.raw
>
>Note that we leave behind -mips64r2. Turns out that ld ignores it (see
>get_emulation in ld/ldmain.c). This is true of current trunk and 2.23,
>which is the minimum supported version for the kernel:
>
>https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/ldmain.c;hb=aa4209e7b679afd74a3860ce25659e71cc4847d5#l593
>https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/ldmain.c;hb=a55e30b51bc6227d8d41f707654d0a5620978dcf#l641
>
>Before this patch, LD=ld.lld did nothing:
>
>$ llvm-readelf -p.comment arch/mips/vdso/vdso.so.dbg | sed 's/(.*//'
>String dump of section '.comment':
>[ 0] ClangBuiltLinux clang version 11.0.0
>
>After this patch, it does:
>
>$ llvm-readelf -p.comment arch/mips/vdso/vdso.so.dbg | sed 's/(.*//'
>String dump of section '.comment':
>[ 0] Linker: LLD 11.0.0
>[ 62] ClangBuiltLinux clang version 11.0.0
>
>Link: https://github.com/ClangBuiltLinux/linux/issues/785
>Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
>---
>
>v3 -> v4:
>
>* Improve commit message to show that ld command is effectively the
> same as the one generated by GCC.
>
>* Add '-G 0' and '--eh-frame-hdr' because they were added by GCC.

My understanding is that we start to use more -fasynchronous-unwind-tables to eliminate .eh_frame in object files.
Without .eh_frame, LD --eh-frame-hdr is really not useful.

Sigh... -G 0. This is an option ignored by LLD. GCC devs probably should
have used the long option --gpsize rather than take the short option -G.
Even better, -z gpsize= or similar if this option is specific to ELF.

>v2 -> v3:
>
>* New patch.
>
> arch/mips/vdso/Makefile | 13 ++++---------
> 1 file changed, 4 insertions(+), 9 deletions(-)
>
>diff --git a/arch/mips/vdso/Makefile b/arch/mips/vdso/Makefile
>index 92b53d1df42c3..2e64c7600eead 100644
>--- a/arch/mips/vdso/Makefile
>+++ b/arch/mips/vdso/Makefile
>@@ -60,10 +60,9 @@ ifdef CONFIG_MIPS_DISABLE_VDSO
> endif
>
> # VDSO linker flags.
>-VDSO_LDFLAGS := \
>- -Wl,-Bsymbolic -Wl,--no-undefined -Wl,-soname=linux-vdso.so.1 \
>- $(addprefix -Wl$(comma),$(filter -E%,$(KBUILD_CFLAGS))) \
>- -nostdlib -shared -Wl,--hash-style=sysv -Wl,--build-id
>+ldflags-y := -Bsymbolic --no-undefined -soname=linux-vdso.so.1 \
>+ $(filter -E%,$(KBUILD_CFLAGS)) -nostdlib -shared \
>+ -G 0 --eh-frame-hdr --hash-style=sysv --build-id -T
>
> CFLAGS_REMOVE_vdso.o = -pg
>
>@@ -82,11 +81,7 @@ quiet_cmd_vdso_mips_check = VDSOCHK $@
> #
>
> quiet_cmd_vdsold_and_vdso_check = LD $@
>- cmd_vdsold_and_vdso_check = $(cmd_vdsold); $(cmd_vdso_check); $(cmd_vdso_mips_check)
>-
>-quiet_cmd_vdsold = VDSO $@
>- cmd_vdsold = $(CC) $(c_flags) $(VDSO_LDFLAGS) \
>- -Wl,-T $(filter %.lds,$^) $(filter %.o,$^) -o $@
>+ cmd_vdsold_and_vdso_check = $(cmd_ld); $(cmd_vdso_check); $(cmd_vdso_mips_check)
>
> quiet_cmd_vdsoas_o_S = AS $@
> cmd_vdsoas_o_S = $(CC) $(a_flags) -c -o $@ $<
>--
>2.26.2
>