[PATCH] arm64: vdso: work-around broken ELF toolchains in Makefile

[PATCH] arm64: vdso: work-around broken ELF toolchains in Makefile

[Will Deacon]

When building the kernel with a bare-metal (ELF) toolchain, the -shared
option may not be passed down to collect2, resulting in silent corruption
of the vDSO image (in particular, the DYNAMIC section is omitted).

The effect of this corruption is that the dynamic linker fails to find
the vDSO symbols and libc is instead used for the syscalls that we
intended to optimise (e.g. gettimeofday). Functionally, there is no
issue as the sigreturn trampoline is still intact and located by the
kernel.

This patch fixes the problem by explicitly passing -shared to the linker
when building the vDSO.

Cc: <stable@vger.kernel.org>
Reported-by: Szabolcs Nagy <Szabolcs.Nagy@arm.com>
Reported-by: James Greenlaigh <james.greenhalgh@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 arch/arm64/kernel/vdso/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index ff3bddea482d..f6fe17d88da5 100644
--- a/arch/arm64/kernel/vdso/Makefile
+++ b/arch/arm64/kernel/vdso/Makefile
@@ -15,6 +15,10 @@ ccflags-y := -shared -fno-common -fno-builtin
 ccflags-y += -nostdlib -Wl,-soname=linux-vdso.so.1 \
 		$(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
 
+# Workaround for bare-metal (ELF) toolchains that neglect to pass -shared
+# down to collect2, resulting in silent corruption of the vDSO image.
+ccflags-y += -Wl,-shared
+
 obj-y += vdso.o
 extra-y += vdso.lds vdso-offsets.h
 CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
-- 
2.1.4

[PATCH] arm64: vdso: work-around broken ELF toolchains in Makefile

[Catalin Marinas]

On Fri, Jun 19, 2015 at 01:56:33PM +0100, Will Deacon wrote:
> When building the kernel with a bare-metal (ELF) toolchain, the -shared
> option may not be passed down to collect2, resulting in silent corruption
> of the vDSO image (in particular, the DYNAMIC section is omitted).
> 
> The effect of this corruption is that the dynamic linker fails to find
> the vDSO symbols and libc is instead used for the syscalls that we
> intended to optimise (e.g. gettimeofday). Functionally, there is no
> issue as the sigreturn trampoline is still intact and located by the
> kernel.
> 
> This patch fixes the problem by explicitly passing -shared to the linker
> when building the vDSO.
> 
> Cc: <stable@vger.kernel.org>
> Reported-by: Szabolcs Nagy <Szabolcs.Nagy@arm.com>
> Reported-by: James Greenlaigh <james.greenhalgh@arm.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>

Queued for 4.2. Thanks.

-- 
Catalin

[PATCH] arm64: vdso: work-around broken ELF toolchains in Makefile

[Nathan Lynch]

On 06/19/2015 07:56 AM, Will Deacon wrote:
> When building the kernel with a bare-metal (ELF) toolchain, the -shared
> option may not be passed down to collect2, resulting in silent corruption
> of the vDSO image (in particular, the DYNAMIC section is omitted).

I checked arch/arm with

arm-none-eabi-gcc (Fedora 4.9.2-3.fc21) 4.9.2

and it doesn't seem to be affected.  Do you know whether the toolchain
issue is aarch64-specific?

[PATCH] arm64: vdso: work-around broken ELF toolchains in Makefile

[Will Deacon]

On Fri, Jun 19, 2015 at 03:47:46PM +0100, Nathan Lynch wrote:
> On 06/19/2015 07:56 AM, Will Deacon wrote:
> > When building the kernel with a bare-metal (ELF) toolchain, the -shared
> > option may not be passed down to collect2, resulting in silent corruption
> > of the vDSO image (in particular, the DYNAMIC section is omitted).
> 
> I checked arch/arm with
> 
> arm-none-eabi-gcc (Fedora 4.9.2-3.fc21) 4.9.2
> 
> and it doesn't seem to be affected.  Do you know whether the toolchain
> issue is aarch64-specific?

I don't know for sure, but it seems to affect all AArch64 toolchains so
from your observations, arch/arm/ should be ok.

Will