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

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Message-ID: <20200206154547.GA3064177@rani.riverdale.lan>
Date:   Thu, 6 Feb 2020 10:45:47 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Kees Cook <keescook@...omium.org>,
        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 09:57:40AM -0500, Arvind Sankar wrote:
> 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?

Also, one thing to note about the orphan section handling -- by default
ld will combine multiple orphan sections with the same name into a
single output section. So if you have sections corresponding to static
functions with the same name but from different files, they will get
unnecessarily combined. You may want to add --unique to the ld options
to keep them separate. That will create multiple sections with the same
name instead of merging them.