[Devel] [PATCH 2/2] parasite: Don't try to start executing syscall from guard page

[Pavel Emelyanov]

On 11/24/2016 06:48 PM, Cyrill Gorcunov wrote:
> When looking for place where we can do syscall we should care
> if there a guard page present. So check for it first.

Have we found a task with executable stack?

> https://jira.sw.ru/browse/PSBM-55989
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov at virtuozzo.com>
> ---
>  criu/parasite-syscall.c | 39 ++++++++++++++++++++++-----------------
>  1 file changed, 22 insertions(+), 17 deletions(-)
> 
> diff --git a/criu/parasite-syscall.c b/criu/parasite-syscall.c
> index 0fe7e83..07c4ef2 100644
> --- a/criu/parasite-syscall.c
> +++ b/criu/parasite-syscall.c
> @@ -49,30 +49,36 @@ static int can_run_syscall(unsigned long ip, unsigned long start,
>  	return ip >= start && ip < (end - BUILTIN_SYSCALL_SIZE - pad);
>  }
>  
> -static int syscall_fits_vma_area(struct vma_area *vma_area, unsigned long pad)
> +static int syscall_fits_vma_area(unsigned long start, unsigned long end, unsigned long pad)
>  {
> -	return can_run_syscall((unsigned long)vma_area->e->start,
> -			       (unsigned long)vma_area->e->start,
> -			       (unsigned long)vma_area->e->end,
> -			       pad);
> +	return can_run_syscall(start, start, end, pad);
>  }
>  
> -static struct vma_area *get_vma_by_ip(struct list_head *vma_area_list,
> -				      unsigned long ip,
> -				      unsigned long pad)
> +static unsigned long find_syscall_ip(struct list_head *vma_area_list,
> +				     unsigned long ip, unsigned long pad)
>  {
> -	struct vma_area *vma_area;
> +	struct vma_area *vma_area, *vma_prev = NULL;
>  
>  	list_for_each_entry(vma_area, vma_area_list, list) {
> +		unsigned long start, end;
> +
>  		if (vma_area->e->start >= kdat.task_size)
>  			continue;
>  		if (!(vma_area->e->prot & PROT_EXEC))
>  			continue;
> -		if (syscall_fits_vma_area(vma_area, pad))
> -			return vma_area;
> +
> +		start = vma_area->e->start;
> +		end = vma_area->e->end;
> +
> +		if (stack_guard_page_start(vma_area, vma_prev, start))
> +			start += PAGE_SIZE;
> +
> +		if (syscall_fits_vma_area(start, end, pad))
> +			return start;
> +		vma_prev = vma_area;
>  	}
>  
> -	return NULL;
> +	return 0;
>  }
>  
>  static inline int ptrace_get_regs(int pid, user_regs_struct_t *regs)
> @@ -1154,7 +1160,6 @@ err:
>  struct parasite_ctl *parasite_prep_ctl(pid_t pid, struct vm_area_list *vma_area_list)
>  {
>  	struct parasite_ctl *ctl = NULL;
> -	struct vma_area *vma_area;
>  
>  	if (!arch_can_dump_task(pid))
>  		goto err;
> @@ -1180,15 +1185,15 @@ struct parasite_ctl *parasite_prep_ctl(pid_t pid, struct vm_area_list *vma_area_
>  		return ctl;
>  
>  	/* Search a place for injecting syscall */
> -	vma_area = get_vma_by_ip(&vma_area_list->h, REG_IP(ctl->orig.regs),
> -				 MEMFD_FNAME_SZ);
> -	if (!vma_area) {
> +	ctl->syscall_ip	= find_syscall_ip(&vma_area_list->h,
> +					  REG_IP(ctl->orig.regs),
> +					  MEMFD_FNAME_SZ);
> +	if (!ctl->syscall_ip) {
>  		pr_err("No suitable VMA found to run parasite "
>  		       "bootstrap code (pid: %d)\n", pid);
>  		goto err;
>  	}
>  
> -	ctl->syscall_ip	= vma_area->e->start;
>  	pr_debug("Parasite syscall_ip at %p\n", (void *)ctl->syscall_ip);
>  
>  	return ctl;
>