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

[GHC]

#8935: Obscure linker bug leads to crash in GHCi
-------------------------------------+------------------------------------
        Reporter:  simonmar          |            Owner:  simonmar
            Type:  bug               |           Status:  new
        Priority:  high              |        Milestone:  7.8.3
       Component:  Runtime System    |          Version:  7.8.1-rc2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  GHCi crash        |       Difficulty:  Rocket Science
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by dagit):

 I haven't tested if I have the crash on my system, but suspect you're
 seeing something to do with weak symbols. On my system, Fedora 19, I see
 that glibc defines environ as a weak symbol and gives it the same address
 that your libgmp check reports.

 I modified your C program slightly so that it takes the paths in `argv[1]`
 and `argv[2]`:
 {{{
 $ ./check-environ /usr/lib64/libgmp.so /usr/lib64/libpthread.so
 environ = 0x601058
 dlsym(deflt, "environ") = 0x601058
 dlsym("libgmp", "environ") = 0x31e45bd508
 dlsym("libpthread", "environ") = 0x601058
 }}}

 My libgmp doesn't define environ so it must find it in libc, and indeed:
 {{{
 $ readelf -Wa /usr/lib64/libc.so.6 | grep environ
 00000031e45b9dd8  0000011900000006 R_X86_64_GLOB_DAT      00000031e45bd508
 _environ + 0
 00000031e45b9ea0  0000050000000006 R_X86_64_GLOB_DAT      00000031e45bd508
 __environ + 0
    281: 00000031e45bd508     8 OBJECT  WEAK   DEFAULT   33
 _environ@@GLIBC_2.2.5
    956: 00000031e45bd508     8 OBJECT  WEAK   DEFAULT   33
 environ@@GLIBC_2.2.5
   1280: 00000031e45bd508     8 OBJECT  GLOBAL DEFAULT   33
 __environ@@GLIBC_2.2.5
    349: 00000031e45bbe70     8 OBJECT  LOCAL  DEFAULT   33 last_environ
   1813: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS environ.c
   4539: 00000031e4238a10  1122 FUNC    LOCAL  DEFAULT   12
 __add_to_environ
   5707: 00000031e45bd508     8 OBJECT  WEAK   DEFAULT   33 _environ
   6374: 00000031e45bd508     8 OBJECT  GLOBAL DEFAULT   33 __environ
   6483: 00000031e45bd508     8 OBJECT  WEAK   DEFAULT   33 environ
 }}}

 I suspect that the way you're searching for the symbol needs to look for a
 (the?) strong binding first. I hope that helps.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8935#comment:5>
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