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

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Date:   Thu, 6 Feb 2020 09:57:40 -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:
> On Wed, Feb 05, 2020 at 02:39:45PM -0800, Kristen Carlson Accardi wrote:
> > We will be using -ffunction-sections to place each function in
> > it's own text section so it can be randomized at load time. The
> > linker considers these .text.* sections "orphaned sections", and
> > will place them after the first similar section (.text). However,
> > we need to move _etext so that it is after both .text and .text.*
> > We also need to calculate text size to include .text AND .text.*
> 
> The dependency on the linker's orphan section handling is, I feel,
> rather fragile (during work on CFI and generally building kernels with
> Clang's LLD linker, we keep tripping over difference between how BFD and
> LLD handle orphans). However, this is currently no way to perform a
> section "pass through" where input sections retain their name as an
> output section. (If anyone knows a way to do this, I'm all ears).
> 
> Right now, you can only collect sections like this:
> 
>         .text :  AT(ADDR(.text) - LOAD_OFFSET) {
> 		*(.text.*)
> 	}
> 
> or let them be orphans, which then the linker attempts to find a
> "similar" (code, data, etc) section to put them near:
> https://sourceware.org/binutils/docs-2.33.1/ld/Orphan-Sections.html
> 
> So, basically, yes, this works, but I'd like to see BFD and LLD grow
> some kind of /PASSTHRU/ special section (like /DISCARD/), that would let
> a linker script specify _where_ these sections should roughly live.
> 

You could go through the objects that are being linked and find the
individual text sections, and generate the linker script using that?