| Message ID | 20171115213428.22559-7-samitolvanen@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed, Nov 15, 2017 at 01:34:16PM -0800, Sami Tolvanen wrote: > Use -fno-jump-tables to make sure clang doesn't generate branches > to EL1 virtual addresses. Can you elaborate a bit more on exactly what you saw failing here, please? Whilst it's obviously broken to jump to EL1 from EL2 w/o VHE, the commit message is a bit cryptic. Will
On Thu, Nov 16, 2017 at 11:46:17AM +0000, Will Deacon wrote: > Can you elaborate a bit more on exactly what you saw failing here, > please? Mark noticed that clang built kernels fail to boot when the kernel starts at EL2: http://lkml.iu.edu/hypermail/linux/kernel/1711.0/02817.html Turns out starting with LLVM r308050, the compiler decides to use a jump table in __init_stage2_translation, and generates code that branches into an EL1 virtual address. > Whilst it's obviously broken to jump to EL1 from EL2 w/o VHE, the > commit message is a bit cryptic. I agree, I'll add a more descriptive commit message for v3. Sami
On Wed, Nov 15, 2017 at 01:34:16PM -0800, Sami Tolvanen wrote: > Use -fno-jump-tables to make sure clang doesn't generate branches > to EL1 virtual addresses. Are there any other reasons that clang might generate absolute references/relocations? It would be nice if there was the option to disable that more generally, rather than disabling individual optimizations. Is there any PIC/PIE option that we could use? We might need something simnilar for GCC, even if we're not seeing problems today. Thanks, Mark. > Suggested-by: AKASHI Takahiro <takahiro.akashi@linaro.org> > Signed-off-by: Sami Tolvanen <samitolvanen@google.com> > --- > arch/arm64/kvm/hyp/Makefile | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/arm64/kvm/hyp/Makefile b/arch/arm64/kvm/hyp/Makefile > index f04400d494b7..19fa1c6b6b69 100644 > --- a/arch/arm64/kvm/hyp/Makefile > +++ b/arch/arm64/kvm/hyp/Makefile > @@ -5,6 +5,10 @@ > > ccflags-y += -fno-stack-protector -DDISABLE_BRANCH_PROFILING > > +ifeq ($(cc-name),clang) > +ccflags-y += -fno-jump-tables > +endif > + > KVM=../../../../virt/kvm > > obj-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/hyp/vgic-v2-sr.o > -- > 2.15.0.448.gf294e3d99a-goog >
On Mon, Nov 20, 2017 at 02:41:47PM +0000, Mark Rutland wrote: > On Wed, Nov 15, 2017 at 01:34:16PM -0800, Sami Tolvanen wrote: > > Use -fno-jump-tables to make sure clang doesn't generate branches > > to EL1 virtual addresses. > > Are there any other reasons that clang might generate absolute > references/relocations? > > It would be nice if there was the option to disable that more generally, > rather than disabling individual optimizations. Is there any PIC/PIE > option that we could use? > > We might need something simnilar for GCC, even if we're not seeing > problems today. ... and likewise for the EFI stub, which needs to be position-independent much like the hyp code. Thanks, Mark.
On 20 November 2017 at 14:43, Mark Rutland <mark.rutland@arm.com> wrote: > On Mon, Nov 20, 2017 at 02:41:47PM +0000, Mark Rutland wrote: >> On Wed, Nov 15, 2017 at 01:34:16PM -0800, Sami Tolvanen wrote: >> > Use -fno-jump-tables to make sure clang doesn't generate branches >> > to EL1 virtual addresses. >> >> Are there any other reasons that clang might generate absolute >> references/relocations? >> >> It would be nice if there was the option to disable that more generally, >> rather than disabling individual optimizations. Is there any PIC/PIE >> option that we could use? >> >> We might need something simnilar for GCC, even if we're not seeing >> problems today. > > ... and likewise for the EFI stub, which needs to be > position-independent much like the hyp code. > Actually, we are already using -fpie for the EFI stub, but it is a bit fiddly because it requires visibility overrides, given that PIC symbol references are usually indirected via GOT entries. At the time, it didn't occur to me that the HYP code has similar issues. Of course, in the EFI stub we already check for ABS64 relocations to make sure that the resulting code is correct (and /me makes mental note to add R_AARCH64_GOT_xxx relocations to that as well)
diff --git a/arch/arm64/kvm/hyp/Makefile b/arch/arm64/kvm/hyp/Makefile index f04400d494b7..19fa1c6b6b69 100644 --- a/arch/arm64/kvm/hyp/Makefile +++ b/arch/arm64/kvm/hyp/Makefile @@ -5,6 +5,10 @@ ccflags-y += -fno-stack-protector -DDISABLE_BRANCH_PROFILING +ifeq ($(cc-name),clang) +ccflags-y += -fno-jump-tables +endif + KVM=../../../../virt/kvm obj-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/hyp/vgic-v2-sr.o
Use -fno-jump-tables to make sure clang doesn't generate branches to EL1 virtual addresses. Suggested-by: AKASHI Takahiro <takahiro.akashi@linaro.org> Signed-off-by: Sami Tolvanen <samitolvanen@google.com> --- arch/arm64/kvm/hyp/Makefile | 4 ++++ 1 file changed, 4 insertions(+)