[llvm-dev] Caller callee calling convention enforcement in C++ bin. code

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Thu Jul 6 08:53:20 PDT 2017

Is it enough to compute the set of all possible return addresses, or do you
need to limit the set to only C++ method calls? If you just need the full
set of return addresses for a given DSO, I'd recommend disassembling the
object after linking, scraping the output for "callq" instructions, and
taking the address of the next instruction. This will give you the return
address "VA" (I think, in ELF parlance), which is the address of the
instruction assuming the ELF binary is loaded at the address listed in its
program headers. You can compute the possible return addresses at runtime
by adding the difference between the on-disk p_vaddr values and the actual
addresses that the loader used at runtime. You can probably discover the
load addresses with dl_iterate_phdr.

If you need only some specific annotated list of return addresses, you will
probably have to make complicated changes to LLVM that insert labels after
certain CALL instructions and emit some object file section with
relocations against those labels. This is doable but complicated. You can
follow the EH label machinery to see how to insert labels into the
instruction stream and create relocations against them from read-only data

On Wed, Jul 5, 2017 at 9:22 AM, Paul Muntean via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi guys,
> maybe you can help with an issue which I have.
> I want to recuperate for a C++ program compiled with Clang/LLVM on an
> Ubuntu CPU x86_64 bit architecture all the addresses of the call
> instructions (C++ object dispatches) or directly the return address
> which are just the next address after a call instruction.
> I think that this information is not obtainable during link time since
> we have at that moment only IR code. Please corect me if I am wrong.
> So my assumption is that in the compiler back end after the IR code is
> lowered to machine code and the addresses for the call instructions
> and the addresses next to the call instructions are available.
> Has anybody a suggestion where are the possible places in the compiler
> where I should look for?
> Since I am new to this topic suggestions or solutions are highly welcome.
> -Paul
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