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

Andy Lutomirski
2016-04-29T19:39:43Z

On Thu, Apr 28, 2016 at 1:44 PM, Josh Poimboeuf <jpoimboe@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@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