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

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Fri, Oct 20, 2017 at 1:26 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * 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?

It does add extra instructions when one is not possible, I preferred
that over ifdefing but I can change it.

>
> Thanks,
>
>         Ingo



-- 
Thomas

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