[PATCH] RISC-V: Fix the VDSO symbol generaton for binutils-2.34+

Mon Oct 19 19:38:12 EDT 2020

On Mon, 19 Oct 2020 13:41:21 PDT (-0700), maskray at 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 at 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 at vger.kernel.org
>>> Cc: clang-built-linux at googlegroups.com
>>> Signed-off-by: Palmer Dabbelt <palmerdabbelt at 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:
>>1. 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@gmail.com/T/#u
>>(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 at 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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