[GHC] #8935: Obscure linker bug leads to crash in GHCi

[GHC]

#8935: Obscure linker bug leads to crash in GHCi
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              Reporter:  simonmar    |            Owner:
                  Type:  bug         |           Status:  patch
              Priority:  high        |        Milestone:  7.10.1
             Component:  Runtime     |          Version:  7.8.1-rc2
  System                             |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Rocket Science
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  GHCi crash  |  Related Tickets:  #9186, #9480
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D349   |
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Comment (by Austin Seipp <austin@…>):

 In [changeset:"383733b9191a36e2d3f757700842dbc3855911d9/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="383733b9191a36e2d3f757700842dbc3855911d9"
 Fix obscure problem with using the system linker (#8935)

 Summary:
 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
 matters.

 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
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8935#comment:64>
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