[CRIU] [PATCH 1/3] rst: Don't allocate page for child stack

[Andrew Vagin]

On Fri, Sep 12, 2014 at 04:42:43PM +0400, Pavel Emelyanov wrote:
> When clone-ing kids we can set their stack on current, as
> it will anyway be COW-ed later.

glibc() will modify a new stack. Look at this:

[avagin at localhost ~]$ cat test.c 
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/mman.h>
#include <signal.h>
#include <stdlib.h>

int child(void *args)
{
	exit(0);
}

int main()
{
	void *stack;

	stack = mmap(NULL, 4096, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
	clone(child, stack, SIGCHLD, 0);

	return 0;
}
[avagin at localhost ~]$ strace ./a.out 
execve("./a.out", ["./a.out"], [/* 60 vars */]) = 0
...
mmap(NULL, 4096, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa331ab5000
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0x7fa331ab4ff8} ---
+++ killed by SIGSEGV (core dumped) +++
Segmentation fault (core dumped)


> 
> Signed-off-by: Pavel Emelyanov <xemul at parallels.com>
> ---
>  cr-restore.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/cr-restore.c b/cr-restore.c
> index 4d5ccd5..ee15b6c 100644
> --- a/cr-restore.c
> +++ b/cr-restore.c
> @@ -945,7 +945,6 @@ static int restore_one_task(int pid, CoreEntry *core)
>  
>  /* All arguments should be above stack, because it grows down */
>  struct cr_clone_arg {
> -	char stack[PAGE_SIZE] __attribute__((aligned (8)));
>  	char stack_ptr[0];
>  	struct pstree_item *item;
>  	unsigned long clone_flags;
> @@ -993,8 +992,8 @@ static void maybe_clone_parent(struct pstree_item *item,
>  
>  static inline int fork_with_pid(struct pstree_item *item)
>  {
> -	int ret = -1, fd;
>  	struct cr_clone_arg ca;
> +	int ret = -1, fd;
>  	pid_t pid = item->pid.virt;
>  
>  	if (item->state != TASK_HELPER) {
> @@ -1037,6 +1036,8 @@ static inline int fork_with_pid(struct pstree_item *item)
>  	ca.item = item;
>  	ca.clone_flags = item->rst->clone_flags;
>  
> +	BUG_ON(ca.clone_flags & CLONE_VM);
> +
>  	pr_info("Forking task with %d pid (flags 0x%lx)\n", pid, ca.clone_flags);
>  
>  	if (!(ca.clone_flags & CLONE_NEWPID)) {
> -- 
> 1.8.4.2
> 
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