Kernel-hardening archive on
 help / color / Atom feed
From: "H.J. Lu" <>
To: Florian Weimer <>
Cc: Jann Horn <>,,
	 x86-64-abi <>,
	 Kernel Hardening <>,
	Szabolcs Nagy <>
Subject: Re: Alternative CET ABI
Date: Thu, 30 Jul 2020 10:14:10 -0700
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, Jul 30, 2020 at 9:54 AM Florian Weimer <> wrote:
> * Jann Horn:
> > On Thu, Jul 30, 2020 at 6:02 PM Florian Weimer <> 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

FWIW, we can introduce a different CET PLT as long as it is compatible
with the past, current and future binaries.


      reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-30 16:01 Florian Weimer
2020-07-30 16:41 ` Jann Horn
2020-07-30 16:47   ` Szabolcs Nagy
2020-07-30 16:54   ` Florian Weimer
2020-07-30 17:14     ` H.J. Lu [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Kernel-hardening archive on

Archives are clonable:
	git clone --mirror kernel-hardening/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 kernel-hardening kernel-hardening/ \
	public-inbox-index kernel-hardening

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone