linux-kernel - Re: [criu] 1M guard page ruined restore

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Date:   Wed, 21 Jun 2017 18:22:56 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrey Vagin <avagin@...nvz.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Andrew Morton <akpm@...uxfoundation.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [criu] 1M guard page ruined restore

On Tue, Jun 20, 2017 at 03:23:20AM -0700, Hugh Dickins wrote:
...
> 
> We do need to understand this fairly quickly, since those stable
> backports will pose more of a problem for you than the v4.12
> release itself.

The patches for criu are on the fly. Still one of the test case
start failing with the new kernels. Basically the test does
the following:

 - allocate growsdown memory area
 - touch first byte (which before the patch force the kernel
   to extend the stack allocating new page)
 - touch first-1 byte

---
int main(int argc, char **argv)
{
	char *start_addr, *start_addr1, *fake_grow_down, *test_addr, *grow_down;
	volatile char *p;

	start_addr = mmap(NULL, PAGE_SIZE * 10, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
	if (start_addr == MAP_FAILED) {
		printf("Can't mal a new region");
		return 1;
	}
	printf("start_addr %lx\n", start_addr);
	munmap(start_addr, PAGE_SIZE * 10);

	fake_grow_down = mmap(start_addr + PAGE_SIZE * 5, PAGE_SIZE,
			 PROT_READ | PROT_WRITE,
			 MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED | MAP_GROWSDOWN, -1, 0);
	if (fake_grow_down == MAP_FAILED) {
		printf("Can't mal a new region");
		return 1;
	}
	printf("start_addr %lx\n", fake_grow_down);

	p = fake_grow_down;
	*p-- = 'c';
	*p = 'b';
	...
}
---
This start failing because

 | static inline int check_stack_guard_page(struct vm_area_struct *vma, unsigned long address)

function get dropped off. Hugh, it is done on intent and
userspace programs have to extend stack manually?

	Cyrill