Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 20 Apr 2022 08:17:14 -0700
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     joao@overdrivepizza.com, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org, andrew.cooper3@citrix.com,
        keescook@chromium.org, samitolvanen@google.com,
        mark.rutland@arm.com, hjl.tools@gmail.com,
        alyssa.milburn@linux.intel.com, ndesaulniers@google.com,
        gabriel.gomes@linux.intel.com, rick.p.edgecombe@intel.com
Subject: Re: [RFC PATCH 00/11] Kernel FineIBT Support
Message-ID: <20220420151714.fderdz4dzea75rvg@treble>
References: <20220420004241.2093-1-joao@overdrivepizza.com>
 <20220420074044.GC2731@worktop.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20220420074044.GC2731@worktop.programming.kicks-ass.net>
Precedence: bulk

On Wed, Apr 20, 2022 at 09:40:44AM +0200, Peter Zijlstra wrote:
> On Tue, Apr 19, 2022 at 05:42:30PM -0700, joao@overdrivepizza.com wrote:
> > @PeterZ @JoshP
> > 
> > I'm a bit unaware of the details on why the objtool approach to bypass ENDBRs
> > was removed from the IBT series. Is this approach now sensible considering that
> > it is a requirement for a new/enhanced feature? If not, how extending the Linker
> > to emit already fixed offsets sounds like?
> 
> Josh hates objtool modifying actualy code. He much prefers objtool only
> emits out of band data.
> 
> Now, I did sneak in that jump_label nop'ing, and necessity (broken
> compilers) had us do the KCOV nop'ing in noinstr, but if you look at the
> recent objtool series here:
> 
>   https://lkml.kernel.org/r/cover.1650300597.git.jpoimboe@redhat.com
> 
> you'll see his thoughs on that :-)
> 
> Now, I obviously don't mind, it's easy enough to figure out what objtool
> actually does with something like:
> 
>   $ OBJTOOL_ARGS="--backup" make O=ibt-build/ -j$lots vmlinux
>   $ objdiff.sh ibt-build/vmlinux.o
> 
> Where objdiff.sh is the below crummy script.
> 
> Now, one compromise that I did get out of Josh was that he objected less
> to rewriting relocations than to rewriting the immediates. From my
> testing the relocations got us the vast majority of direct call sites,
> very few are immediates.
> 
> Josh, any way you might reconsider all that? :-)

If I remember correctly, the goal of --ibt-fix-direct was to avoid
hitting unnecessary ENDBRs, which basically decode to NOPs, so the
ghastly hack wasn't worth it.

If FineIBT needs it, I could reconsider.  But I think there's a strong
case to be made that the linker should be doing that instead.

-- 
Josh