Re: [PATCH v3 09/51] x86/dumpstack: fix x86_32 kernel_stack_pointer() previous stack access

[Andy Lutomirski]

On Fri, Aug 12, 2016 at 7:28 AM, Josh Poimboeuf <jpoim...@redhat.com> wrote:
> On x86_32, when an interrupt happens from kernel space, SS and SP aren't
> pushed and the existing stack is used.  So pt_regs is effectively two
> words shorter, and the previous stack pointer is normally the memory
> after the shortened pt_regs, aka '&regs->sp'.
>
> But in the rare case where the interrupt hits right after the stack
> pointer has been changed to point to an empty stack, like for example
> when call_on_stack() is used, the address immediately after the
> shortened pt_regs is no longer on the stack.  In that case, instead of
> '&regs->sp', the previous stack pointer should be retrieved from the
> beginning of the current stack page.
>
> kernel_stack_pointer() wants to do that, but it forgets to dereference
> the pointer.  So instead of returning a pointer to the previous stack,
> it returns a pointer to the beginning of the current stack.
>
> Fixes: 0788aa6a23cb ("x86: Prepare removal of previous_esp from i386 
> thread_info structure")
> Signed-off-by: Josh Poimboeuf <jpoim...@redhat.com>

This seems like a valid fix, but I'm not sure I agree with the intent
of the code.  &regs->sp really is the previous stack pointer in the
sense that the stack pointer was &regs->sp when the entry happened.
>From an unwinder's perspective, how is:

movl [whatever], $esp
<-- interrupt

any different from:

movl [whatever], $esp
pushl [something]
<-- interrupt

Also, does x86_32 do this type of stack switching at all?  AFAICS
32-bit kernels don't use IRQ stacks in the first place.  Do they?  Am
I just missing the code that does it?

--Andy