[llvm-dev] accessing stack frame after returning from the function

[Tim Northover via llvm-dev]

Hi Priyanka,

On Sat, 21 Dec 2019 at 14:09, Priyanka Panigrahi via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> 1. Whether the memory contents assigned for a function are accessible after we return from that function? If yes, how can we access it?

There's no well defined way to access that memory, but whatever was
stored there before doesn't get actively cleared to 0 by LLVM so some
kinds of buffer overrun can see what's there, as can future function
calls that reuse the space. Both would be undefined behaviour though.

> 2. Does llvm delete the stackframe assigned for a specific function, after we return from that function?

I'd say yes, but it kind of depends on what you mean by "delete". It
certainly deallocates the frame.

> 3. If not, how can we delete the stackframe or clear the memory content after we return from the function? Where do we need to change, in the assembly or llvm source?

If you want to prevent all access for security reasons, you'd probably
have to modify lib/Target/XYZ/XYZFrameLowering.cpp for each target to
insert some equivalent of memset(sp, 0, frame_size) at every function
return. I don't think there's a robust way to achieve it in IR alone.

Cheers.

Tim.