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

[Andy Lutomirski]

On Thu, Apr 28, 2016 at 1:44 PM, Josh Poimboeuf <jpoimboe at redhat.com> 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 at redhat.com>
> ---
>  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
> @@ -325,7 +326,7 @@ ENDPROC(start_cpu0)
>         GLOBAL(initial_gs)
>         .quad   INIT_PER_CPU_VAR(irq_stack_union)
>         GLOBAL(initial_stack)
> -       .quad  init_thread_union+THREAD_SIZE-8
> +       .quad  init_thread_union + THREAD_SIZE - SIZEOF_PTREGS

As long as you're doing this, could you also set orig_ax to -1?  I
remember running into some oddities resulting from orig_ax containing
garbage at some point.

--Andy