[PATCH v3] RISC-V: Fix the VDSO symbol generaton for binutils-2.35+

[Nick Desaulniers]

On Fri, Oct 23, 2020 at 10:03 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-2.35 (specifically the binutils-gdb commit a87e1817a4 ("Have
> the linker fail if any attempt to link in an executable is made."), but as that
> has been backported to at least Debian's binutils-2.34 in may manifest in other
> places.
>
> 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: clang-built-linux at googlegroups.com
> Cc: stable at vger.kernel.org
> Signed-off-by: Palmer Dabbelt <palmerdabbelt at google.com>

Any way to improve the error message if/when this fails?
https://travis-ci.com/github/ClangBuiltLinux/continuous-integration/jobs/407165683

>
> ---
>
> Changes since v2 <20201019235630.762886-1-palmerdabbelt at google.com>:
>
> * Uses $(srctree)/$(src) to allow for out-of-tree builds.
>
> Changes since v1 <20201017002500.503011-1-palmerdabbelt at google.com>:
>
> * Uses $(NM) instead of $(CROSS_COMPILE)nm.  We use the $(CROSS_COMPILE) form
>   elsewhere in this file, but we'll fix that later.
> * Removed the unnecesary .map file creation.
>
> ---
>  arch/riscv/kernel/vdso/.gitignore |  1 +
>  arch/riscv/kernel/vdso/Makefile   | 17 ++++++++---------
>  arch/riscv/kernel/vdso/so2s.sh    |  6 ++++++
>  3 files changed, 15 insertions(+), 9 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..a8ecf102e09b 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
> @@ -73,6 +68,10 @@ quiet_cmd_vdsold = VDSOLD  $@
>                             $(patsubst %, -G __vdso_%, $(vdso-syms)) $@.tmp $@ && \
>                     rm $@.tmp
>
> +# Extracts
> +quiet_cmd_so2s = SO2S    $@
> +      cmd_so2s = $(NM) -D $< | $(srctree)/$(src)/so2s.sh > $@
> +
>  # 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..3c5b43207658
> --- /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 '^\.'
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>
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-- 
Thanks,
~Nick Desaulniers