• Peter Trommler's avatar
    Fix obscure problem with using the system linker (#8935) · 383733b9
    Peter Trommler authored
    In a statically linked GHCi symbol `environ` resolves to NULL when
    called from a Haskell script.
    When resolving symbols in a Haskell script we need to search the
    executable program and its dependent (DT_NEEDED) shared libraries
    first and then search the loaded libraries.
    We want to be able to override functions in loaded libraries later.
    Libraries must be opened with local scope (RTLD_LOCAL) and not global.
    The latter adds all symbols to the executable program's symbols where
    they are then searched in loading order. We want reverse loading order.
    When libraries are loaded with local scope the dynamic linker
    cannot use symbols in that library when resolving the dependencies
    in another shared library. This changes the way files compiled to
    object code must be linked into temporary shared libraries. We link
    with the last temporary shared library created so far if it exists.
    Since each temporary shared library is linked to the previous temporary
    shared library the dynamic linker finds the latest definition of a
    symbol by following the dependency chain.
    See also Note [RTLD_LOCAL] for a summary of the problem and solution.
    Cherry-picked commit 2f8b4c
    Changed linker argument ordering
    On some ELF systems GNU ld (and others?) default to
    --as-needed and the order of libraries in the link
    The last temporary shared library, must appear
    before all other libraries. Switching the position
    of extra_ld_inputs and lib_path_objs does that.
    Fixes #8935 and #9186
    Reviewers: austin, hvr, rwbarton, simonmar
    Reviewed By: simonmar
    Subscribers: thomie, carter, simonmar
    Differential Revision: https://phabricator.haskell.org/D349
    GHC Trac Issues: #8935, #9186, #9480
SysTools.lhs 62.5 KB