On Mon, 19 Oct 2020 13:41:21 PDT (-0700), maskray@google.com wrote:
On 2020-10-19, 'Nick Desaulniers' via Clang Built Linux wrote:
On Fri, Oct 16, 2020 at 5:44 PM 'Palmer Dabbelt' via Clang Built Linux clang-built-linux@googlegroups.com wrote:
We were relying on GNU ld's ability to re-link executable files in order to extract our VDSO symbols. This behavior was deemed a bug as of binutils-2.34 (specifically the binutils-gdb commit a87e1817a4 ("Have the linker fail if any attempt to link in an executable is made."), which IIUC landed in 2.34), which recently installed itself on my build setup.
I filed the issue https://sourceware.org/bugzilla/show_bug.cgi?id=26047 The commit is in 2.35 but not in the released 2.34 branch.
Thanks. Looks like it was backported to the Debian 2.34, I guess I was off by a release when looking. Regardless, I think the fix in binutils is reasonable and that we just shouldn't be doing this sort of thing.
The previous version of this was a bit of a mess: we were linking a static executable version of the VDSO, containing only a subset of the input symbols, which we then linked into the kernel. This worked, but certainly wasn't a supported path through the toolchain. Instead this new version parses the textual output of nm to produce a symbol table. Both rely on near-zero addresses being linkable, but as we rely on weak undefined symbols being linkable elsewhere I don't view this as a major issue.
Fixes: e2c0cdfba7f6 ("RISC-V: User-facing API") Cc: stable@vger.kernel.org Cc: clang-built-linux@googlegroups.com Signed-off-by: Palmer Dabbelt palmerdabbelt@google.com
Ah, I do see a build failure to link the vdso with: $ riscv64-linux-gnu-ld --version GNU ld (GNU Binutils for Debian) 2.34.90.20200706
riscv64-linux-gnu-ld: cannot use executable file 'arch/riscv/kernel/vdso/vdso-dummy.o' as input to a link
This patch fixes that for me, but there's a problem related to related to `nm` below.
After this, there's two other things we might want to fix up related to the build of the vdso:
- it looks like $(CC) is being used to link the vdso, rather than
$(LD). While it's generally fine to use the compiler as the driver for building a linked object file, it does not respect the set $(LD). `-fuse-ld=` needs to be passed to invoke the linker the user specified. See also: https://lore.kernel.org/linux-kbuild/20201013033947.2257501-1-natechancellor... (this has popped up in a few places when trying to do hermetic builds with LLD). 2. I observe the warning when building with clang: `argument unused during compilation: '-no-pie' [-Wunused-command-line-argument]`. IIRC, the top level Makefile sets `-Qunused-arguments` for builds with clang. `cmd_vdsold` may need that, but it's curious why it's unused and makes me wonder why/if `-no-pie` is necessary? That also might be fixed by fixing 1.
arch/riscv/kernel/vdso/.gitignore | 1 + arch/riscv/kernel/vdso/Makefile | 19 +++++++++---------- arch/riscv/kernel/vdso/so2s.sh | 7 +++++++ 3 files changed, 17 insertions(+), 10 deletions(-) create mode 100755 arch/riscv/kernel/vdso/so2s.sh
diff --git a/arch/riscv/kernel/vdso/.gitignore b/arch/riscv/kernel/vdso/.gitignore index 11ebee9e4c1d..3a19def868ec 100644 --- a/arch/riscv/kernel/vdso/.gitignore +++ b/arch/riscv/kernel/vdso/.gitignore @@ -1,3 +1,4 @@ # SPDX-License-Identifier: GPL-2.0-only vdso.lds *.tmp +vdso-syms.S diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile index 478e7338ddc1..2e02958f6224 100644 --- a/arch/riscv/kernel/vdso/Makefile +++ b/arch/riscv/kernel/vdso/Makefile @@ -43,19 +43,14 @@ $(obj)/vdso.o: $(obj)/vdso.so SYSCFLAGS_vdso.so.dbg = $(c_flags) $(obj)/vdso.so.dbg: $(src)/vdso.lds $(obj-vdso) FORCE $(call if_changed,vdsold) +SYSCFLAGS_vdso.so.dbg = -shared -s -Wl,-soname=linux-vdso.so.1 \
-Wl,--build-id -Wl,--hash-style=both# We also create a special relocatable object that should mirror the symbol # table and layout of the linked DSO. With ld --just-symbols we can then # refer to these symbols in the kernel code rather than hand-coded addresses.
-SYSCFLAGS_vdso.so.dbg = -shared -s -Wl,-soname=linux-vdso.so.1 \
-Wl,--build-id -Wl,--hash-style=both-$(obj)/vdso-dummy.o: $(src)/vdso.lds $(obj)/rt_sigreturn.o FORCE
$(call if_changed,vdsold)-LDFLAGS_vdso-syms.o := -r --just-symbols -$(obj)/vdso-syms.o: $(obj)/vdso-dummy.o FORCE
$(call if_changed,ld)+$(obj)/vdso-syms.S: $(obj)/vdso.so FORCE
$(call if_changed,so2s)# strip rule for the .so file $(obj)/%.so: OBJCOPYFLAGS := -S @@ -68,11 +63,15 @@ $(obj)/%.so: $(obj)/%.so.dbg FORCE # 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 $@.tmp && \
-Wl,-T,$(filter-out FORCE,$^) -o $@.tmp -Wl,-Map,$@.map && \Is -Wl,-Map,$@.map needed?
Nope, that's there be accident. I've fixed it for the v2.
Thanks!
$(CROSS_COMPILE)objcopy \ $(patsubst %, -G __vdso_%, $(vdso-syms)) $@.tmp $@ && \ rm $@.tmp+# Extracts +quiet_cmd_so2s = SO2S $@
cmd_so2s = $(CROSS_COMPILE)nm -D $< | $(src)/so2s.sh > $@This should use `$(NM)` rather than `$(CROSS_COMPILE)nm` which hardcodes the use of GNU nm from GNU binutils.
# install commands for the unstripped file quiet_cmd_vdso_install = INSTALL $@ cmd_vdso_install = cp $(obj)/$@.dbg $(MODLIB)/vdso/$@ diff --git a/arch/riscv/kernel/vdso/so2s.sh b/arch/riscv/kernel/vdso/so2s.sh new file mode 100755 index 000000000000..7862866b5ebb --- /dev/null +++ b/arch/riscv/kernel/vdso/so2s.sh @@ -0,0 +1,6 @@ +#!/bin/sh +# SPDX-License-Identifier: GPL-2.0+ +# Copyright 2020 Palmer Dabbelt palmerdabbelt@google.com
+sed 's!([0-9a-f]*) T ([a-z0-9_]*)@@LINUX_4.15!.global \2\n.set \2,0x\1!' \
+| grep '^.'
-- Thanks, ~Nick Desaulniers
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