Re: [PATCH v4 05/10] x86/alternative: initializing temporary mm for patching

[Peter Zijlstra]

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. Iâll change it to BUG_ON() if you
> prefer.

Sure, two BUG_ON()s works for me.