Re: [PATCH 5/6] x86/shstk: don't create the shadow stack for PF_USER_WORKERs

[Oleg Nesterov <oleg@redhat.com>]

On 08/15, Mark Brown wrote:
>
> On Fri, Aug 15, 2025 at 05:43:11PM +0200, Oleg Nesterov wrote:
> > On 08/15, Mark Brown wrote:
>
> > > Sure, but OTOH at least for arm64 there's no cost to leaving the feature
> > > enabled unless you actually execute userspace code so if we never return
> > > to userspace writing the code to disable isn't really buying us anything.
>
> > The fact that a kernel thread can have the pointless ARCH_SHSTK_SHSTK is
> > the only reason I know why x86_task_fpu(PF_USER_WORKER) has to work.
>
> > I'd like to make this logic consistent with PF_KTHREAD, and in the longer
> > term change the x86 FPU code so that the kernel threads can run without
> > without "struct fpu" attached to task_struct.
>
> OK, that's entirely x86 specific - there's no reason we'd want to do
> that for arm64.

Since I know nothing about arm64. Any reason we do want to have the unnecessary
ARCH_SHSTK_SHSTK/shstk on arm64?

And... do you agree that shstk_alloc_thread_stack() without update_fpu_shstk()
in copy_thread() path doesn't look right? Even if nothing really bad can happen.

Oleg.