Messages in this thread | | | Date | Wed, 23 Apr 2014 09:30:51 -0700 | From | "H. Peter Anvin" <> | Subject | Re: randomized placement of x86_64 vdso |
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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
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