Obscure linker bug leads to crash in GHCi

I have a build of GHC (with DYNAMIC_GHC_PROGRAMS=NO) that exhibits the following crash:

  $ ghc -e 'System.Environment.getEnvironment'
  <segfault>

I tracked it down, eventually, to a bad reference to the symbol environ from __hscore_environ in libraries/base/includes/HsBase.h. Somehow, environ had got linked to the wrong address.

Lots more investigation lead me to discover this: internal_dlsym() in Linker.c tries to look up a symbol in all the different shared libraries we have loaded so far, one by one. (see be497c20). Unfortunately, this seems to break things in my case. Here's a simple test program that works on Ubuntu 12.04:

#include <dlfcn.h>
#include <stdio.h>

char *so = "/usr/lib/x86_64-linux-gnu/libgmp.so";
char *so2 = "/usr/lib/x86_64-linux-gnu/libpthread.so";

extern char**environ;

int main(int argc, char *argv[])
{
  void *deflt, *hdl;
  deflt = dlopen(NULL, RTLD_LAZY | RTLD_GLOBAL);
  printf("environ = %p\n", &environ);
  printf("dlsym(deflt, \"environ\") = %p\n", dlsym(deflt,"environ"));
  hdl = dlopen(so, RTLD_LAZY);
  printf("dlsym(\"libgmp\", \"environ\") = %p\n", dlsym(hdl,"environ"));
  hdl = dlopen(so2, RTLD_LAZY);
  printf("dlsym(\"libpthread\", \"environ\") = %p\n", dlsym(hdl,"environ"));
}

And the output:

environ = 0x601040
dlsym(deflt, "environ") = 0x601040
dlsym("libgmp", "environ") = 0x2aaaab290568
dlsym("libpthread", "environ") = 0x601040

Note that the value we get from looking up environ in libgmp is different to the others. The correct value is 0x601040. gdb thinks that 0x2aaaab290568 is also environ:

(gdb) p4 0x2aaaab290568
0x2aaaab290580 <buflen.9817>:   0x0
0x2aaaab290578: 0x0
0x2aaaab290570 <miss_F_GETOWN_EX>:      0x0
0x2aaaab290568 <environ>:       0x0

but note that it contains zero. The real one is:

(gdb) p4 0x601040
0x601058:       0x0
0x601050 <dtor_idx.6533>:       0x0
0x601048 <completed.6531>:      0x0
0x601040 <environ@@GLIBC_2.2.5>:        0x7fffffffe268

In GHC we're loading libgmp when we load the integer-gmp package, and this causes future references to environ to go wrong.

I've locally fixed this by changing internal_dlsym to dlsym, but since there was a reason to make this change in the first place I haven't pushed it to master. Suggestions welcome.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	igloo, simonmar
Operating system
Architecture