On Sun, Nov 11, 2018 at 08:38:53PM +0000, Nadav Amit wrote:
> From: Peter Zijlstra
> > On Sat, Nov 10, 2018 at 03:17:27PM -0800, Nadav Amit wrote:
> >> +void __init poking_init(void)
> >> +{
> >> + spinlock_t *ptl;
> >> + pte_t *ptep;
> >> +
> >> + poking_mm = copy_init_mm();
> >> + if (!poking_mm) {
> >> + pr_err("x86/mm: error setting a separate poking address space");
> >> + return;
> >> + }
> >> +
> >> + /*
> >> + * Randomize the poking address, but make sure that the following page
> >> + * will be mapped at the same PMD. We need 2 pages, so find space for 3,
> >> + * and adjust the address if the PMD ends after the first one.
> >> + */
> >> + poking_addr = TASK_UNMAPPED_BASE +
> >> + (kaslr_get_random_long("Poking") & PAGE_MASK) %
> >> + (TASK_SIZE - TASK_UNMAPPED_BASE - 3 * PAGE_SIZE);
> >> +
> >> + if (((poking_addr + PAGE_SIZE) & ~PMD_MASK) == 0)
> >> + poking_addr += PAGE_SIZE;
> >> +
> >> + /*
> >> + * We need to trigger the allocation of the page-tables that will be
> >> + * needed for poking now. Later, poking may be performed in an atomic
> >> + * section, which might cause allocation to fail.
> >> + */
> >> + ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
> >> + if (!WARN_ON(!ptep))
> >> + pte_unmap_unlock(ptep, ptl);
> >> +}
> >
> > The difference in how we deal with -ENOMEM here is weird. I think we
> > have a _lot_ of code that simply hard assumes we don't fail memory alloc
> > on init.
> >
> > I for instance would not mind to simply remove both branches and let the
> > kernel crash and burn if we ever fail here.
>
> Actually, now that we removed the fallback of patching without poking_mm, a
> failure to allocate poking_mm should have had a BUG_ON().
>
> For the second case, I think we still need either WARN_ON() or BUG_ON(), at
> least as some sort of an in-code comment. I’ll change it to BUG_ON() if you
> prefer.
Sure, two BUG_ON()s works for me.
