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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Fri, 21 Feb 2020 18:05:09 -0500
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Kristen Carlson Accardi <kristen@linux.intel.com>,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        rick.p.edgecombe@intel.com, x86@kernel.org,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com
Subject: Re: [RFC PATCH 06/11] x86: make sure _etext includes function
 sections
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On Fri, Feb 21, 2020 at 01:50:39PM -0600, Josh Poimboeuf wrote:
> On Mon, Feb 10, 2020 at 11:36:29AM -0500, Arvind Sankar wrote:
> > On Mon, Feb 10, 2020 at 07:54:58AM -0800, Arjan van de Ven wrote:
> > > > 
> > > > I'll leave it to others to figure out the exact details. But afaict it
> > > > should be possible to have fine-grained-randomization and preserve the
> > > > workaround in the end.
> > > > 
> > > 
> > > the most obvious "solution" is to compile with an alignment of 4 bytes (so tight packing)
> > > and then in the randomizer preserve the offset within 32 bytes, no matter what it is
> > > 
> > > that would get you an average padding of 16 bytes which is a bit more than now but not too insane
> > > (queue Kees' argument that tiny bits of padding are actually good)
> > > 
> > 
> > With the patchset for adding the mbranches-within-32B-boundaries option,
> > the section alignment gets forced to 32. With function-sections that
> > means function alignment has to be 32 too.
> 
> We should be careful about enabling -mbranches-within-32B-boundaries.
> It will hurt AMD, and presumably future Intel CPUs which don't need it.
> 
> -- 
> Josh
> 

And past Intel CPUs too :) As I understand it only appears from Skylake
onwards.