linux-kernel - Re: [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK

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Date:	Tue, 10 Mar 2015 20:22:01 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>, x86@...nel.org,
	linux-kernel@...r.kernel.org
CC:	Borislav Petkov <bp@...en8.de>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK_PADDING
 macro

On 03/10/2015 07:05 PM, Andy Lutomirski wrote:
> x86_32, unlike x86_64, pads the top of the kernel stack.  Document
> this padding and give it a name.
> 
> This should make no change whatsoever to the compiled kernel image.
> It also doesn't fix any of the current bugs in this area.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
>  arch/x86/include/asm/processor.h   |  3 ++-
>  arch/x86/include/asm/thread_info.h | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 48a61c1c626e..88d9aa745898 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -849,7 +849,8 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
>  #define task_pt_regs(task)                                             \
>  ({                                                                     \
>         struct pt_regs *__regs__;                                       \
> -       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task))-8); \
> +       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task)) - \
> +				     TOP_OF_KERNEL_STACK_PADDING);     \
>         __regs__ - 1;                                                   \
>  })

The "magic 8" also hides here:

        /*
         * Push current_thread_info()->sysenter_return to the stack.
         * A tiny bit of offset fixup is necessary - 4*4 means the 4 words
         * pushed above; +8 corresponds to copy_thread's esp0 setting.
         */
        pushl_cfi ((TI_sysenter_return)-THREAD_SIZE+8+4*4)(%esp)
                                                   ^^^
        CFI_REL_OFFSET eip, 0


It may make sense to mention TOP_OF_KERNEL_STACK_PADDING here
(there are no useful comments in copy_thread() anymore):

        movl PT_EFLAGS(%esp), %eax      # mix EFLAGS, SS and CS
        # Warning: PT_OLDSS(%esp) contains the wrong/random values if we
        # are returning to the kernel.
        # See comments in process.c:copy_thread() for details.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
maybe s/comments in process.c:copy_thread()/TOP_OF_KERNEL_STACK_PADDING/ ?
        movb PT_OLDSS(%esp), %ah
        movb PT_CS(%esp), %al
        andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
        cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
        CFI_REMEMBER_STATE

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