Re: [PATCH 2/2] powerpc/vdso32: link vdso64 with linker

Christophe Leroy
2020-09-02T07:56:31Z

On 9/1/20 10:25 PM, Nick Desaulniers wrote:
> Rather than invoke the compiler as the driver, use the linker. That way
> we can check --orphan-handling=warn support correctly, as cc-ldoption
> was removed in
> commit 055efab3120b ("kbuild: drop support for cc-ldoption").
>
> Requires dropping the .got section. I couldn't find how it was used in
> the vdso32.
>
> Fixes: commit f2af201002a8 ("powerpc/build: vdso linker warning for orphan sections")
> Link: https://lore.kernel.org/lkml/CAKwvOdnn3wxYdJomvnveyD_njwRku3fABWT_bS92duihhywLJQ@mail.gmail.com/
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Not sure removing .got is a good idea or not. Otherwise I observe the
> following link error:
> powerpc-linux-gnu-ld: warning: orphan section `.got' from `arch/powerpc/kernel/vdso32/sigtramp.o' being placed in section `.got'
> powerpc-linux-gnu-ld: _GLOBAL_OFFSET_TABLE_ not defined in linker created .got
> powerpc-linux-gnu-ld: final link failed: bad value
>
> sigtramp.c doesn't mention anything from the GOT AFAICT, and doesn't
> look like it contains relocations that do, so I'm not sure where
> references to _GLOBAL_OFFSET_TABLE_ are coming from.

I'm getting the same but only when building for PPC64.
I don't get any reference to sigtramp.o though:

CALL scripts/checksyscalls.sh
CALL scripts/atomic/check-atomics.sh
VDSO32A arch/powerpc/kernel/vdso32/sigtramp.o
VDSO32A arch/powerpc/kernel/vdso32/gettimeofday.o
VDSO32A arch/powerpc/kernel/vdso32/datapage.o
VDSO32A arch/powerpc/kernel/vdso32/cacheflush.o
VDSO32A arch/powerpc/kernel/vdso32/note.o
VDSO32A arch/powerpc/kernel/vdso32/getcpu.o
LD arch/powerpc/kernel/vdso32/vdso32.so.dbg
powerpc64-linux-ld: _GLOBAL_OFFSET_TABLE_ not defined in linker created .got
powerpc64-linux-ld: final link failed: Bad value

(GCC 8.1, Binutils 2.30)

So it seems that the got section is being created by the linker. Don't
know why though.

With GCC 10.1, binutils 2.34 I get:

LDS arch/powerpc/kernel/vdso32/vdso32.lds
VDSO32A arch/powerpc/kernel/vdso32/sigtramp.o
VDSO32A arch/powerpc/kernel/vdso32/gettimeofday.o
VDSO32A arch/powerpc/kernel/vdso32/datapage.o
VDSO32A arch/powerpc/kernel/vdso32/cacheflush.o
VDSO32A arch/powerpc/kernel/vdso32/note.o
VDSO32A arch/powerpc/kernel/vdso32/getcpu.o
LD arch/powerpc/kernel/vdso32/vdso32.so.dbg
powerpc64-linux-ld: warning: orphan section `.branch_lt' from
`arch/powerpc/kernel/vdso32/sigtramp.o' being placed in section `.branch_lt'
powerpc64-linux-ld: _GLOBAL_OFFSET_TABLE_ not defined in linker created .got
powerpc64-linux-ld: final link failed: bad value

I can't see any .branch_lt section when objdumping sigtramp.o or any
other .o

When I move sigtramp.o at the end of the definition of obj-vdso32 in
Makefile, I then get:

powerpc64-linux-ld: warning: orphan section `.branch_lt' from
`arch/powerpc/kernel/vdso32/gettimeofday.o' being placed in section
`.branch_lt'
powerpc64-linux-ld: _GLOBAL_OFFSET_TABLE_ not defined in linker created .got
powerpc64-linux-ld: final link failed: bad value

gettimeofday.o now being the first object in obj-vdso32

Christophe

>
> arch/powerpc/kernel/vdso32/Makefile | 7 +++++--
> arch/powerpc/kernel/vdso32/vdso32.lds.S | 3 ++-
> 2 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/kernel/vdso32/Makefile b/arch/powerpc/kernel/vdso32/Makefile
> index 87ab1152d5ce..611a5951945a 100644
> --- a/arch/powerpc/kernel/vdso32/Makefile
> +++ b/arch/powerpc/kernel/vdso32/Makefile
> @@ -27,6 +27,9 @@ UBSAN_SANITIZE := n
> ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
> -Wl,-soname=linux-vdso32.so.1 -Wl,--hash-style=both
> asflags-y := -D__VDSO32__ -s
> +ldflags-y := -shared -soname linux-vdso32.so.1 \
> + $(call ld-option, --eh-frame-hdr) \
> + $(call ld-option, --orphan-handling=warn) -T
>
> obj-y += vdso32_wrapper.o
> extra-y += vdso32.lds
> @@ -49,8 +52,8 @@ $(obj-vdso32): %.o: %.S FORCE
> $(call if_changed_dep,vdso32as)
>
> # actual build commands
> -quiet_cmd_vdso32ld = VDSO32L $@
> - cmd_vdso32ld = $(VDSOCC) $(c_flags) $(CC32FLAGS) -o $@ $(call cc-ldoption, -Wl$(comma)--orphan-handling=warn) -Wl,-T$(filter %.lds,$^) $(filter %.o,$^)
> +quiet_cmd_vdso32ld = LD $@
> + cmd_vdso32ld = $(cmd_ld)
> quiet_cmd_vdso32as = VDSO32A $@
> cmd_vdso32as = $(VDSOCC) $(a_flags) $(CC32FLAGS) -c -o $@ $<
>
> diff --git a/arch/powerpc/kernel/vdso32/vdso32.lds.S b/arch/powerpc/kernel/vdso32/vdso32.lds.S
> index 4c985467a668..0ccdebad18b8 100644
> --- a/arch/powerpc/kernel/vdso32/vdso32.lds.S
> +++ b/arch/powerpc/kernel/vdso32/vdso32.lds.S
> @@ -61,7 +61,6 @@ SECTIONS
> .fixup : { *(.fixup) }
>
> .dynamic : { *(.dynamic) } :text :dynamic
> - .got : { *(.got) } :text
> .plt : { *(.plt) }
>
> _end = .;
> @@ -108,7 +107,9 @@ SECTIONS
> .debug_varnames 0 : { *(.debug_varnames) }
>
> /DISCARD/ : {
> + *(.got)
> *(.note.GNU-stack)
> + *(.branch_lt)
> *(.data .data.* .gnu.linkonce.d.* .sdata*)
> *(.bss .sbss .dynbss .dynsbss)
> *(.glink .iplt .plt .rela*)
>