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

[Ben Hutchings <ben@decadent.org.uk>]

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On Tue, 2017-07-04 at 19:11 +0200, Willy Tarreau wrote:
> 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 ?

Yes, either that or only allow one such vma.

Ben.

> 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).

-- 
Ben Hutchings
Anthony's Law of Force: Don't force it, get a larger hammer.


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