#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: -------------------------------------+------------------------------------ Comment (by dagit): I just realized my previous reply probably leaves people wondering: Since libpthread also DOES NOT define environ why does it give the right address? The difference is that libpthread depends on ld-linux.so as a shared object dependency (grep the readelf -Wa output for `NEEDED`) and ld- linux.so defines a `LOCAL` symbol for environ. As I understand it, that means the dynamic linker thinks libpthread defines environ as `LOCAL` (so it goes to look for a different definition and ends up with the right one). For libgmp it looks in libc.so and finds a `WEAK` definition and uses it because there is no other `GLOBAL` definition. To help demonstrate this, consider this example. On my system, libz.so has dependencies similar to libgmp.so: {{{ $ readelf -Wa /usr/lib64/libgmp.so.10 | grep NEEDED 0x0000000000000001 (NEEDED) Shared library: [libc.so.6] $ readelf -Wa /usr/lib64/libz.so.1 | grep NEEDED 0x0000000000000001 (NEEDED) Shared library: [libc.so.6] }}} And look at this: {{{ $ ./check-environ /usr/lib64/libgmp.so /usr/lib64/libz.so.1 environ = 0x601058 dlsym(deflt, "environ") = 0x601058 dlsym("libgmp", "environ") = 0x31e45bd508 dlsym("libpthread", "environ") = 0x31e45bd508 }}} I'm still fuzzy on some of the details, but it seems that `WEAK` and `LOCAL` mean very different things to the dynamic linker. `WEAK` is the same as `GLOBAL` but the link priority is lower, while `LOCAL` means the symbol should not be exported. As far as I can tell, when it finds `LOCAL` it continues the search whereas with `WEAK` it stops earlier because technically it has found an exported symbol. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8935#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler