Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks

From: Brian Gerst
Date: Fri Apr 29 2016 - 14:46:15 EST


On Thu, Apr 28, 2016 at 4:44 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> Thanks to all the recent x86 entry code refactoring, most tasks' kernel
> stacks start at the same offset right above their saved pt_regs,
> regardless of which syscall was used to enter the kernel. That creates
> a nice convention which makes it straightforward to identify the
> "bottom" of the stack, which can be useful for stack walking code which
> needs to verify the stack is sane.
>
> However there are still a few types of tasks which don't yet follow that
> convention:
>
> 1) CPU idle tasks, aka the "swapper" tasks
>
> 2) freshly forked TIF_FORK tasks which don't have a stack at all
>
> Make the idle tasks conform to the new stack bottom convention by
> starting their stack at a sizeof(pt_regs) offset from the end of the
> stack page.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
> arch/x86/kernel/head_64.S | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 6dbd2c0..0b12311 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -296,8 +296,9 @@ ENTRY(start_cpu)
> * REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
> * address given in m16:64.
> */
> - movq initial_code(%rip),%rax
> - pushq $0 # fake return address to stop unwinder
> + call 1f # put return address on stack for unwinder
> +1: xorq %rbp, %rbp # clear frame pointer
> + movq initial_code(%rip), %rax
> pushq $__KERNEL_CS # set correct cs
> pushq %rax # target address in negative space
> lretq

This chunk looks like it should be a separate patch.

--
Brian Gerst