[LLVMdev] Position-independent stacks

Mark Seaborn mseaborn at chromium.org
Thu Jan 23 08:22:03 PST 2014

On 22 January 2014 22:10, Vadim <vadimcn at gmail.com> wrote:

> Hi,
> I am toying with an idea of having LLVM generate code that has
> position-independent stacks.   This would be a very useful property for
> implementing all sorts of micro-thread libraries (I am thinking something
> similar to Python greenlets <http://stackoverflow.com/a/17447308>),
> because you'd be able to easily save threadlet state from one OS thread and
> later restore it into another.
> On the surface, it seems entirely do-able - basically, one needs to get
> rid of all the things that point into the stack.  It should be sufficient
> to:
> 1. write a function pass that finds all local variables, whose address is
> ever taken, and hoists them into a heap-allocated secondary "stack frame",
> 2. either turn off frame base pointers, or make sure they are adjusted
> after the stack had been relocated,
> 3. ... can't think of anything else, actually.
> What do you guys think?  Any reasons this approach wouldn't fly?

I've implemented something similar, but with the motivation of implementing
SFI sandboxing rather than making the stack relocatable.

The code is here: https://codereview.chromium.org/29743003/  In particular,
see the ExpandAlloca pass.

That code implements sandboxing at the level of LLVM IR.  It restricts all
memory accesses to a range of address space by truncating the memory
address and adding a base pointer.  Here are some notes explaining further:


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