Messages in this thread | | | Date | Tue, 10 Mar 2015 20:22:01 +0100 | From | Denys Vlasenko <> | Subject | Re: [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK_PADDING macro |
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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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