Re: [PATCH] mm: larger stack guard gap, between vmas

[Willy Tarreau]

On Tue, Jul 04, 2017 at 12:36:11PM +0100, Ben Hutchings wrote:
> @@ -2323,11 +2330,17 @@ int expand_downwards(struct vm_area_struct *vma,
>       if (error)
>               return error;
>  
> -     /* Enforce stack_guard_gap */
> +     /*
> +      * Enforce stack_guard_gap, but allow VM_NONE mappings in the gap
> +      * as some applications try to make their own stack guards
> +      */
>       gap_addr = address - stack_guard_gap;
>       if (gap_addr > address)
>               return -ENOMEM;
> -     prev = vma->vm_prev;
> +     for (prev = vma->vm_prev;
> +          prev && !(prev->vm_flags & (VM_READ | VM_WRITE | VM_EXEC));
> +          prev = prev->vm_prev)
> +             ;
>       if (prev && prev->vm_end > gap_addr) {
>               if (!(prev->vm_flags & VM_GROWSDOWN))
>                       return -ENOMEM;

Hmmm shouldn't we also stop looping when we're out of the gap ? Something
like this :

        for (prev = vma->vm_prev;
             prev && !(prev->vm_flags & (VM_READ | VM_WRITE | VM_EXEC)) &&
                address - prev->vm_end < stack_guard_gap;
             prev = prev->vm_prev)
                ;

This would limit the risk of runaway loops if someone is having fun
allocating a lot of memory in small chunks (eg: 4 GB in 1 million
independant mmap() calls).

Willy