Re: [Xen-devel] [PATCH v1 06/27] x86/entry/64: Adapt assembly for PIE support

[Ingo Molnar <mingo@xxxxxxxxxx>]

* Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:

> Change the assembly code to use only relative references of symbols for the
> kernel to be PIE compatible.
> 
> 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/entry/entry_64.S | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 49167258d587..15bd5530d2ae 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -194,12 +194,15 @@ entry_SYSCALL_64_fastpath:
>       ja      1f                              /* return -ENOSYS (already in 
> pt_regs->ax) */
>       movq    %r10, %rcx
>  
> +     /* Ensures the call is position independent */
> +     leaq    sys_call_table(%rip), %r11
> +
>       /*
>        * This call instruction is handled specially in stub_ptregs_64.
>        * It might end up jumping to the slow path.  If it jumps, RAX
>        * and all argument registers are clobbered.
>        */
> -     call    *sys_call_table(, %rax, 8)
> +     call    *(%r11, %rax, 8)
>  .Lentry_SYSCALL_64_after_fastpath_call:
>  
>       movq    %rax, RAX(%rsp)
> @@ -334,7 +337,8 @@ ENTRY(stub_ptregs_64)
>        * RAX stores a pointer to the C function implementing the syscall.
>        * IRQs are on.
>        */
> -     cmpq    $.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
> +     leaq    .Lentry_SYSCALL_64_after_fastpath_call(%rip), %r11
> +     cmpq    %r11, (%rsp)
>       jne     1f
>  
>       /*
> @@ -1172,7 +1176,8 @@ ENTRY(error_entry)
>       movl    %ecx, %eax                      /* zero extend */
>       cmpq    %rax, RIP+8(%rsp)
>       je      .Lbstep_iret
> -     cmpq    $.Lgs_change, RIP+8(%rsp)
> +     leaq    .Lgs_change(%rip), %rcx
> +     cmpq    %rcx, RIP+8(%rsp)
>       jne     .Lerror_entry_done
>  
>       /*
> @@ -1383,10 +1388,10 @@ ENTRY(nmi)
>        * resume the outer NMI.
>        */
>  
> -     movq    $repeat_nmi, %rdx
> +     leaq    repeat_nmi(%rip), %rdx
>       cmpq    8(%rsp), %rdx
>       ja      1f
> -     movq    $end_repeat_nmi, %rdx
> +     leaq    end_repeat_nmi(%rip), %rdx
>       cmpq    8(%rsp), %rdx
>       ja      nested_nmi_out
>  1:
> @@ -1440,7 +1445,8 @@ nested_nmi:
>       pushq   %rdx
>       pushfq
>       pushq   $__KERNEL_CS
> -     pushq   $repeat_nmi
> +     leaq    repeat_nmi(%rip), %rdx
> +     pushq   %rdx
>  
>       /* Put stack back */
>       addq    $(6*8), %rsp
> @@ -1479,7 +1485,9 @@ first_nmi:
>       addq    $8, (%rsp)      /* Fix up RSP */
>       pushfq                  /* RFLAGS */
>       pushq   $__KERNEL_CS    /* CS */
> -     pushq   $1f             /* RIP */
> +     pushq   %rax            /* Support Position Independent Code */
> +     leaq    1f(%rip), %rax  /* RIP */
> +     xchgq   %rax, (%rsp)    /* Restore RAX, put 1f */
>       INTERRUPT_RETURN        /* continues at repeat_nmi below */
>       UNWIND_HINT_IRET_REGS

This patch seems to add extra overhead to the syscall fast-path even when PIE 
is 
disabled, right?

Thanks,

        Ingo

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