linux-kernel - Re: [RFC PATCH 06/11] x86: make sure

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Message-ID: <20200210163627.GA1829035@rani.riverdale.lan>
Date:   Mon, 10 Feb 2020 11:36:29 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arjan van de Ven <arjan@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        rick.p.edgecombe@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH 06/11] x86: make sure _etext includes function
 sections

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.