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Message-ID: <20171020082646.bkxrps35sb3gq2nr@gmail.com>
Date: Fri, 20 Oct 2017 10:26:46 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Herbert Xu <herbert@...dor.apana.org.au>,
"David S . Miller" <davem@...emloft.net>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
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Subject: Re: [PATCH v1 06/27] x86/entry/64: Adapt assembly for PIE support
* Thomas Garnier <thgarnie@...gle.com> 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@...gle.com>
> ---
> 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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