Re: [Xen-devel] [RFC 06/22] kvm: Adapt assembly for PIE support

[Brian Gerst <brgerst@xxxxxxxxx>]

On Tue, Jul 18, 2017 at 6:33 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> Change the assembly code to use only relative references of symbols for the
> kernel to be PIE compatible. The new __ASM_GET_PTR_PRE macro is used to
> get the address of a symbol on both 32 and 64-bit with PIE support.
>
> Position Independent Executable (PIE) support will allow to extended the
> KASLR randomization range below the -2G memory limit.
>
> Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h | 6 ++++--
>  arch/x86/kernel/kvm.c           | 6 ++++--
>  arch/x86/kvm/svm.c              | 4 ++--
>  3 files changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 87ac4fba6d8e..3041201a3aeb 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1352,9 +1352,11 @@ asmlinkage void kvm_spurious_fault(void);
>         ".pushsection .fixup, \"ax\" \n" \
>         "667: \n\t" \
>         cleanup_insn "\n\t"                   \
> -       "cmpb $0, kvm_rebooting \n\t"         \
> +       "cmpb $0, kvm_rebooting" __ASM_SEL(,(%%rip)) " \n\t" \
>         "jne 668b \n\t"                       \
> -       __ASM_SIZE(push) " $666b \n\t"        \
> +       __ASM_SIZE(push) "%%" _ASM_AX " \n\t"           \
> +       __ASM_GET_PTR_PRE(666b) "%%" _ASM_AX "\n\t"     \
> +       "xchg %%" _ASM_AX ", (%%" _ASM_SP ") \n\t"      \
>         "call kvm_spurious_fault \n\t"        \
>         ".popsection \n\t" \
>         _ASM_EXTABLE(666b, 667b)
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 71c17a5be983..53b8ad162589 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -618,8 +618,10 @@ asm(
>  ".global __raw_callee_save___kvm_vcpu_is_preempted;"
>  ".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
>  "__raw_callee_save___kvm_vcpu_is_preempted:"
> -"movq  __per_cpu_offset(,%rdi,8), %rax;"
> -"cmpb  $0, " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax);"
> +"leaq  __per_cpu_offset(%rip), %rax;"
> +"movq  (%rax,%rdi,8), %rax;"
> +"addq  " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rip), %rax;"

This doesn't look right.  It's accessing a per-cpu variable.  The
per-cpu section is an absolute, zero-based section and not subject to
relocation.

> +"cmpb  $0, (%rax);
>  "setne %al;"
>  "ret;"
>  ".popsection");

--
Brian Gerst

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