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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Apr 29, 2016 at 02:38:02PM -0700, Andy Lutomirski wrote:
> On Fri, Apr 29, 2016 at 1:50 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > On Fri, Apr 29, 2016 at 12:39:16PM -0700, Andy Lutomirski wrote:
> >> 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.
> >
> > I assume you mean to initialize the orig_rax value in the pt_regs at the
> > bottom of the stack of the idle task?
> >
> > How could that cause a problem?  Since the idle task never returns from
> > a system call, I'd assume that memory never gets accessed?
> >
> 
> Look at collect_syscall in lib/syscall.c

I don't see how collect_syscall() can be called for the per-cpu idle
"swapper" tasks (which is what the above code affects).  They don't have
pids or /proc entries so you can't do /proc/<pid>/syscall on them.

-- 
Josh