Re: [RFC PATCH 06/11] x86: make sure _etext includes function sections

Kristen Carlson Accardi
2020-02-06T19:41:17Z

On Thu, 2020-02-06 at 04:26 -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.
>
> Related thoughts:
>
> 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.

So, when I was developing this patch set, I initially ignored the value
of sh_addralign and just packed the functions in one right after
another when I did the new layout. They were byte aligned :). I later
realized that I should probably pay attention to alignment and thus
started respecting the value that was in sh_addralign. There is
actually nothing stopping me from ignoring it again, other than I am
concerned that I will make runtime performance suffer even more than I
already have.

>
> And we can't adjust function section alignment, or there is some
> benefit to a larger alignment, I would like to have a way for the
> linker
> to specify the inter-section padding (or fill) bytes. Right now, the
> FILL(0xnn) (or =0xnn) can be used to specify the padding bytes
> _within_
> a section, but not between sections. Right now, BFD appears to 0-pad.
> I'd
> like that to be 0xCC so "guessing" addresses incorrectly will trigger
> a trap.

Padding the space between functions with int3 is easy to add during
boot time, and I've got it on my todo list.