• Sylvain HENRY's avatar
    Systools: read ELF section without calling readelf · 109d7ce8
    Sylvain HENRY authored and Ben Gamari's avatar Ben Gamari committed
    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.
    2) 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: #10974, #11022