linux-kernel - Re: [PATCH v2 9/9] module: Reorder functions

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Message-ID: <202006091331.A94BB0DA@keescook>
Date:   Tue, 9 Jun 2020 13:42:00 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Kristen Carlson Accardi <kristen@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Jessica Yu <jeyu@...nel.org>,
        arjan@...ux.intel.com, linux-kernel@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, rick.p.edgecombe@...el.com,
        Ard Biesheuvel <ardb@...nel.org>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH v2 9/9] module: Reorder functions

On Tue, Jun 09, 2020 at 01:14:04PM -0700, Kristen Carlson Accardi wrote:
> On Thu, 2020-05-21 at 14:33 -0700, Kees Cook wrote:
> > Oh! And I am reminded suddenly about CONFIG_FG_KASLR needing to
> > interact
> > correctly with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION in that we do NOT
> > want the sections to be collapsed at link time:
> 
> sorry - I'm a little confused and was wondering if you could clarify
> something. Does this mean you expect CONFIG_FG_KASLR=y and
> CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y to be a valid config? I am not

Yes, I don't see a reason they can't be used together.

> familiar with the option, but it seems like you are saying that it
> requires sections to be collapsed, in which case both of these options
> as yes would not be allowed? Should I actively prevent this in the
> Kconfig?

No, I'm saying that CONFIG_LD_DEAD_CODE_DATA_ELIMINATION does _not_
actually require that the sections be collapsed, but the Makefile
currently does this just to keep the resulting ELF "tidy". We want
that disabled (for the .text parts) in the case of CONFIG_FG_KASLR. The
dead code elimination step, is, IIUC, done at link time before the
output sections are written.

-- 
Kees Cook