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Date: Thu, 30 Jul 2020 17:47:14 +0100
From: Szabolcs Nagy <szabolcs.nagy@....com>
To: Jann Horn <jannh@...gle.com>
Cc: Florian Weimer <fweimer@...hat.com>, oss-security@...ts.openwall.com,
x86-64-abi@...glegroups.com,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: Alternative CET ABI
The 07/30/2020 18:41, Jann Horn wrote:
> On Thu, Jul 30, 2020 at 6:02 PM Florian Weimer <fweimer@...hat.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)? Would this mean that the behavior of a program that compares
ld.so only needs to generate one plt entry
for a function in a process and that entry
can provided the canonical address that is
loaded from some got entry when the address
is used, so there is double indirection, but
it works.
> function pointers obtained through different shared libraries might
> change?
>
> I guess you could maybe canonicalize function pointers somehow, but
> that'd probably at least break dlclose(), right?
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