[PATCH/user_cr] pass a valid stack address to clone_with_pids

[Oren Laadan]

Pulled, thanks.

Nathan Lynch wrote:
> Off-by-one error: the stack address passed to clone/clone_with_pids
> must be within the region allocated.  (Also, arithmetic on void * is a
> gcc extension; change the relevant variables to char *).
> 
> Signed-off-by: Nathan Lynch <ntl at pobox.com>
> ---
>  mktree.c |   15 ++++++++-------
>  1 files changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/mktree.c b/mktree.c
> index 63be82d..2d8d796 100644
> --- a/mktree.c
> +++ b/mktree.c
> @@ -1367,18 +1367,19 @@ int ckpt_fork_stub(void *data)
>  static pid_t ckpt_fork_child(struct ckpt_ctx *ctx, struct task *child)
>  {
>  	struct target_pid_set pid_set;
> -	void *stack = NULL;
> +	char *stack_region;
> +	char *stack_start;
>  	unsigned long flags = SIGCHLD;
>  	pid_t pid = 0;
>  
>  	ckpt_dbg("forking child vpid %d flags %#x\n", child->pid, child->flags);
>  
> -	stack = malloc(PTHREAD_STACK_MIN);
> -	if (!stack) {
> +	stack_region = malloc(PTHREAD_STACK_MIN);
> +	if (!stack_region) {
>  		perror("stack malloc");
>  		return -1;
>  	}
> -	stack += PTHREAD_STACK_MIN;
> +	stack_start = stack_region + PTHREAD_STACK_MIN - 1;
>  
>  	pid_set.target_pids = &pid;
>  	pid_set.num_pids = 1;
> @@ -1406,15 +1407,15 @@ static pid_t ckpt_fork_child(struct ckpt_ctx *ctx, struct task *child)
>  	else
>  		child->real_parent = _getpid();
>  
> -	pid = clone_with_pids(ckpt_fork_stub, stack, flags, &pid_set, child);
> +	pid = clone_with_pids(ckpt_fork_stub, stack_start, flags, &pid_set, child);
>  	if (pid < 0) {
>  		perror("clone");
> -		free(stack - PTHREAD_STACK_MIN);
> +		free(stack_region);
>  		return -1;
>  	}
>  
>  	if (!(child->flags & TASK_THREAD))
> -		free(stack - PTHREAD_STACK_MIN);
> +		free(stack_region);
>  
>  	ckpt_dbg("forked child vpid %d (asked %d)\n", pid, child->pid);
>  	return pid;