[LLVMdev] Position-independent stacks

[Mark Seaborn]

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:

https://docs.google.com/presentation/d/1RD3bxsBfTZOIfrlq7HzGMsygPHgb61A1eTdelIYOurs/edit

Cheers,
Mark
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