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SubjectRe: [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@amacapital.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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