The issue is preventing pointers to the real stack on the real stack. I'm pretty sure you can't do that reliably at the LLVM IR level, since as I said such pointers can be introduced during code generation. In fact, I just looked at the source to the merged pass, and it doesn't even try - it only checks if stack pointers are passed to calls, but e.g.
int *p = cond ? &a : &b;
...later enough that this isn't trivially optimized into two stores...
*p = 1;
will probably not be flagged (it depends on what optimization passes have run before the SafeStack pass), but will put the pointer in the stack or a register that may be saved.
Comments
The issue is preventing pointers to the real stack on the real stack. I'm pretty sure you can't do that reliably at the LLVM IR level, since as I said such pointers can be introduced during code generation. In fact, I just looked at the source to the merged pass, and it doesn't even try - it only checks if stack pointers are passed to calls, but e.g.
will probably not be flagged (it depends on what optimization passes have run before the SafeStack pass), but will put the pointer in the stack or a register that may be saved.