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@xxxxxxxxx>
---
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
