linux-kernel - Re: [RFC 06/14] fork: zero vmap stack using clear

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Date: Tue, 12 Mar 2024 12:53:16 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Nikolay Borisov <nik.borisov@...e.com>
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Subject: Re: [RFC 06/14] fork: zero vmap stack using clear_page() instead of memset()

On Tue, Mar 12, 2024 at 3:16 AM Nikolay Borisov <nik.borisov@...e.com> wrote:
>
>
>
> On 11.03.24 г. 18:46 ч., Pasha Tatashin wrote:
> > In preporation for dynamic kernel stacks do not zero the whole span of
> > the stack, but instead only the pages that are part of the vm_area.
> >
> > This is because with dynamic stacks we might have only partially
> > populated stacks.
> >
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> > ---
> >   kernel/fork.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 6a2f2c85e09f..41e0baee79d2 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -263,8 +263,8 @@ static int memcg_charge_kernel_stack(struct vm_struct *vm)
> >   static int alloc_thread_stack_node(struct task_struct *tsk, int node)
> >   {
> >       struct vm_struct *vm_area;
> > +     int i, j, nr_pages;
> >       void *stack;
> > -     int i;
> >
> >       for (i = 0; i < NR_CACHED_STACKS; i++) {
> >               vm_area = this_cpu_xchg(cached_stacks[i], NULL);
> > @@ -282,7 +282,9 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
> >               stack = kasan_reset_tag(vm_area->addr);
> >
> >               /* Clear stale pointers from reused stack. */
> > -             memset(stack, 0, THREAD_SIZE);
> > +             nr_pages = vm_area->nr_pages;
> > +             for (j = 0; j < nr_pages; j++)
> > +                     clear_page(page_address(vm_area->pages[j]));
>
> Can't this be memset(stack, 0, nr_pages*PAGE_SIZE) ?

No, we can't, because the pages can be physically discontiguous.

Pasha