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

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Date:   Thu, 6 Feb 2020 09:39:43 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Kristen Carlson Accardi <kristen@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        arjan@...ux.intel.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 Thu, Feb 06, 2020 at 04:26:23AM -0800, Kees Cook wrote:
> I know x86_64 stack alignment is 16 bytes. I cannot find evidence for
> what function start alignment should be. It seems the linker is 16 byte
> aligning these functions, when I think no alignment is needed for
> function starts, so we're wasting some memory (average 8 bytes per
> function, at say 50,000 functions, so approaching 512KB) between
> functions. If we can specify a 1 byte alignment for these orphan
> sections, that would be nice, as mentioned in the cover letter: we lose
> a 4 bits of entropy to this alignment, since all randomized function
> addresses will have their low bits set to zero.
> 

The default function alignment is 16-bytes for x64 at least with gcc.
You can use -falign-functions to specify a different alignment.

There was some old discussion on reducing it [1] but it doesn't seem to
have been merged.

[1] https://lore.kernel.org/lkml/tip-4874fe1eeb40b403a8c9d0ddeb4d166cab3f37ba@git.kernel.org/