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

[Andrew Vagin]

Look at this code from glibc:
ENTRY (__clone)
        /* Sanity check arguments.  */
        movq    $-EINVAL,%rax
        testq   %rdi,%rdi               /* no NULL function pointers */
        jz      SYSCALL_ERROR_LABEL
        testq   %rsi,%rsi               /* no NULL stack pointers */
        jz      SYSCALL_ERROR_LABEL

        /* Insert the argument onto the new stack.  */
        subq    $16,%rsi
        movq    %rcx,8(%rsi)

On Fri, Sep 12, 2014 at 06:13:29PM +0400, 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);
> 	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
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