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

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Nick Desaulniers <ndesaulniers@google.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Joe Lawrence <joe.lawrence@redhat.com>,
	Christophe Leroy <christophe.leroy@c-s.fr>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com,
	Kees Cook <keescook@chromium.org>,
	Fangrui Song <maskray@google.com>
Subject: Re: [PATCH 2/2] powerpc/vdso32: link vdso64 with linker
Date: Wed, 2 Sep 2020 07:56:13 +0000	[thread overview]
Message-ID: <371fd179-7d25-1393-f878-10eeb6ad12cc@csgroup.eu> (raw)
In-Reply-To: <20200901222523.1941988-3-ndesaulniers@google.com>

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*)
> 