Re: randomized placement of x86_64 vdso

["H. Peter Anvin" <hpa@zytor.com>]

On 04/21/2014 09:52 AM, Nathan Lynch wrote:
> Hi x86/vdso people,
> 
> I've been working on adding a vDSO to 32-bit ARM, and Kees suggested I
> look at x86_64's algorithm for placing the vDSO at a randomized offset
> above the stack VMA.  I found that when the stack top occupies the
> last slot in the PTE (is that the right term?), the vdso_addr routine
> returns an address below mm->start_stack, equivalent to
> (mm->start_stack & PAGE_MASK).  For instance if mm->start_stack is
> 0x7fff3ffffc96, vdso_addr returns 0x7fff3ffff000.
> 
> Since the address returned is always already occupied by the stack,
> get_unmapped_area detects the collision and falls back to
> vm_unmapped_area.  This results in the vdso being placed in the
> address space next to libraries etc.  While this is generally
> unnoticeable and doesn't break anything, it does mean that the vdso is
> placed below the stack when there is actually room above the stack.
> To me it also seems uncomfortably close to placing the vdso in the way
> of downward expansion of the stack.
> 
> I don't have a patch because I'm not sure what the algorithm should
> be, but thought I would bring it up as vdso_addr doesn't seem to be
> behaving as intended in all cases.
> 

If the stack occupies the last possible page, how can you say there is
"space above the stack"?

	-hpa