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

[Andy Lutomirski]

On Fri, Apr 29, 2016 at 1:50 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@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
>> > @@ -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

> --
> Josh



-- 
Andy Lutomirski
AMA Capital Management, LLC