Re: [PATCH v1 1/1] x86/fred: Fix init_task thread stack pointer initialization

[Xin Li]

On 3/2/2024 5:20 AM, Brian Gerst wrote:
> On Fri, Mar 1, 2024 at 11:18 PM Xin Li <xin@xxxxxxxxx> wrote:
> > On 3/1/2024 5:15 AM, Brian Gerst wrote:
> > > On Fri, Mar 1, 2024 at 3:41 AM Xin Li (Intel) <xin@xxxxxxxxx> wrote:
> > > There is another spot in head_64.S that also needs this offset:
> >
> > I checked all references to __end_init_task before sending out this
> > patch, and I doubt we need to make more similar changes.
> >
> > First of all, "movq    TASK_threadsp(%rcx), %rsp" you added in
> > 3adee777ad0d ("x86/smpboot: Remove initial_stack on 64-bit") is exactly
> > what we need to set up %rsp for the init task.
> >
> > > /* Set up the stack for verify_cpu() */
> > > leaq (__end_init_task - PTREGS_SIZE)(%rip), %rsp
> >
> > As the comment says, it's a _temporary_ stack for calling verify_cpu()
> > (but only for BSP, as APs use a different bring up stack), at which
> > stage the concept of "task" has not formed. I'm thinking maybe it's
> > better to do:
> >
> > /* Set up the stack for verify_cpu() */
> > leaq __end_init_task(%rip), %rsp
> >
> > Previously it was "leaq    (__end_init_task - FRAME_SIZE)(%rip), %rsp",
> > but the kernel unwinder goes up only to secondary_startup_64_no_verify()
> > after the new way you introduced to set up %rsp for the init task, and
> > it seems to me that there is no point to subtract FRAME_SIZE or
> > PTREGS_SIZE.
> >
> > On the other hand, TOP_OF_KERNEL_STACK_PADDING is required for x86_32,
> > but probably not for x86_64 (defined as 0 before FRED). The most
> > important usage of TOP_OF_KERNEL_STACK_PADDING is to get the pt_regs
> > pointer for a task, i.e., task_pt_regs(task), which assumes a fixed
> > offset from the top of a task stack, but also limits the space that
> > could be used by future hardware above the pt_regs structure. Thus I
> > prefer to limit the usage of TOP_OF_KERNEL_STACK_PADDING on x86_64.
>
> The point is to keep consistency with other kernel threads, which have
> the pt_regs area cleared (see copy_thread()).  In particular, the CS
> field can't have junk in it or else user_mode(regs) could return the
> wrong result.  So the stack needs to start below pt_regs, or we need
> to explicitly zero pt_regs later.

Okay, I will add TOP_OF_KERNEL_STACK_PADDING to the spot in
arch/x86/kernel/head_64.S, plus another spot in arch/x86/xen/xen-head.S.

However, I still think it would be better to not have
TOP_OF_KERNEL_STACK_PADDING in these spots, instead we should explicitly
zero pt_regs later;  any *implicit* protocol is NOT welcome. I will find
a timer later to check with the x86 maintainers :)


> Ideally, the load from thread->sp should just shift RSP by phys_base,
> pointing to the same memory location in the virtual mapping.

I prefer to do this explicitly as what's done now; it may be easier to
understand for future kernel developers.

Thanks!
    Xin