Re: Alternative CET ABI

[Florian Weimer <fweimer@redhat.com>]

* Jann Horn:

> On Thu, Jul 30, 2020 at 6:02 PM Florian Weimer <fweimer@redhat.com> wrote:
>> Functions no longer start with the ENDBR64 prefix.  Instead, the link
>> editor produces a PLT entry with an ENDBR64 prefix if it detects any
>> address-significant relocation for it.  The PLT entry performs a NOTRACK
>> jump to the target address.  This assumes that the target address is
>> subject to RELRO, of course, so that redirection is not possible.
>> Without address-significant relocations, the link editor produces a PLT
>> entry without the ENDBR64 prefix (but still with the NOTRACK jump), or
>> perhaps no PLT entry at all.
>
> How would this interact with function pointer comparisons? As in, if
> library A exports a function func1 without referencing it, and
> libraries B and C both take references to func1, would they end up
> with different function pointers (pointing to their respective PLT
> entries)?

Same as today.  ELF already deals with this by picking one canonical
function address per process.

Some targets already need PLTs for inter-DSO calls, so the problem is
not new.  It happens even on x86 because the main program can refer to
its PLT stubs without run-time relocations, so those determine the
canonical address of those functions, and not the actual implementation
in a shared object.

> Would this mean that the behavior of a program that compares
> function pointers obtained through different shared libraries might
> change?

Hopefully not, because that would break things quite horribly (as it's
sometimes possible to observe if the RTLD_DEEPBIND flag is used).

Both the canonicalization and the fact in order to observe the function
pointer, you need to take its address should take care of this.

> I guess you could maybe canonicalize function pointers somehow, but
> that'd probably at least break dlclose(), right?

Ahh, dlclose.  I think in this case, my idea to generate a PLT stub
locally in the address-generating DSO will not work because the
canonical address must survive dlclose if it refers to another DSO.
There are two ways to deal with this: do not unload the PLT stub until
the target DSO is also unloaded (but make sure that the DSO can be
reloaded at a different address; probably not worth the complexity),
or use the dlsym hack I sketched for regular symbol binding as well.
Even more room for experiments, I guess.

Thanks,
Florian