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

[Andrew Vagin]

On Fri, Sep 12, 2014 at 06:59:57PM +0400, Pavel Emelyanov wrote:
> On 09/12/2014 06:13 PM, Andrew Vagin wrote:
> > 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);
> 
> This is not equivalent to what I produce -- you give to child a single
> screwed up (zeroed) page as a stack.

I wanted to show that the parent is segfaulted, because it modifies this
stack.

> 
> > 	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
> >>
> >> _______________________________________________
> >> CRIU mailing list
> >> CRIU at openvz.org
> >> https://lists.openvz.org/mailman/listinfo/criu
> > .
> > 
>