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

[Fangrui Song <maskray@google.com>]

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.

>> 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:
>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?

>>                     $(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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