Systools: read ELF section without calling readelf

Authored by hsyl20.

Description

Systools: read ELF section without calling readelf

This patch tackles two issues:

  1. GHC stores a "link info" string into a ELF section. Initially a

section with type "note" was used but GHC didn't follow the ELF
specification which specifies a record-based format for these sections.
With D1375 we switched to a "progbits" section type for which there
isn't any format constraint. This is an issue for D1242 which use GCC's
--gc-sections which collects "unused" sections, such as our section
containing link info... In this patch, we fall back to a section with
type "note" but we respect the specified format.

  1. Reading back the ELF section was done by parsing the result of a

call to "readelf". Calling readelf is problematic because the program
may not be available or it may be renamed on some platforms (see
D1326). Moreover we have no garanty that its output layout will stay
the same in future releases of readelf. Finally we would need to fix
the parsing to support "note" sections because of 1. Instead, this
patch proposes to use Data.Binary.Get to directly read the "link info"
note into its section. ELF has a specification, hence it should work on
every conforming platform.

This patch "reverts" D1375, hence it supersedes D1432. It makes D1326
not necessary anymore.

Test Plan:

  • recomp011 should pass (test that relinking is avoided when both "link

info" match)

  • we should add a test for ELF objects with more than 0xff00 sections

> added test "recomp015"

  • we should check that GAS generates 32-bit words with .int on every

supported platform using ELF (or find a place where this is
documented). harbomaster and I (@hsyl20) only tested on x86-64. On
platforms where it is not true, it should make recomp011 fail. =>
tested to work on Linux/amd64, Solaris/i386 and OpenBSD/amd64

Reviewers: olsner, ony, thomie, kgardas, austin, bgamari

Reviewed By: thomie, bgamari

Subscribers: kgardas, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D1381

GHC Trac Issues: Trac #10974, Trac #11022