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

    > --
    > Josh



    --
    Andy Lutomirski
    AMA Capital Management, LLC

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