[Bug symtab/11717] New: common/.bss variables from shared libraries not displayed correctly

["lance604 at gmail dot com" <sourceware-bugzilla@sourceware.org>]

Observed with version 7.0-ubuntu under Kubuntu 9.10 with gcc version 4.4.1.

Also occurs for gdb 6.8 and gcc version 4.4.3 in x86_64 hosted MIPS
cross-compilation toolchain.

Apparently, when linking a shared library which contains a variable in .bss with
an executable referencing that variable, ld allocates space for that variable in
the executable's .bss section and creates a duplicate symbol for this variable.

The executable's copy of the variable will be used due to the dynamic linker's
search order (executable first).

However, gdb always used data from the shared library .bss section (always zero)
instead of the executable's .bss section.

Here's a minimal example:

Contents of shared.c:

     int var;

Build shared.c into libshared.so:
     gcc -g -o libshared.so -fPIC -shared shared.c

Contents of executable.c:
     #include <stdlib.h>
     extern int var;

     int main(int argc, char **argv)
     {
         var = 42;
         abort();
         return 0;
     
     }

Build executable:
     gcc -g -L. -Wl,-rpath,. -o executable -lshared executable.c

Display symbols (note that "var" is defined in both files):
     nm -aA libshared.so executable | grep var
     libshared.so:0000000000201020 B var
     executable:0000000000601020 B var

GDB session demonstrating problem:
     gdb executable
     GNU gdb (GDB) 7.0-ubuntu
     Copyright (C) 2009 Free Software Foundation, Inc.
     License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
     This is free software: you are free to change and redistribute it.
     There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
     and "show warranty" for details.
     This GDB was configured as "x86_64-linux-gnu".
     For bug reporting instructions, please see:
     <http://www.gnu.org/software/gdb/bugs/>...
     Reading symbols from /home/lrichardson/test/executable...done.
     (gdb) run
     Starting program: /home/lrichardson/test/executable

     Program received signal SIGABRT, Aborted.
     0x00007ffff78a04b5 in raise () from /lib/libc.so.6
     (gdb) p &var
     $1 = (int *) 0x7ffff7dde020
     (gdb) p var
     $2 = 0
     (gdb) p *(int *)0x601020
     $3 = 42
     (gdb) info shared
     From                To                  Syms Read   Shared Object Library
     0x00007ffff7ddfaf0  0x00007ffff7df7354  Yes (*)     /lib64/ld-linux-x86-64.so.2
     0x00007ffff7bdd4a0  0x00007ffff7bdd5a8  Yes         ./libshared.so
     0x00007ffff788b730  0x00007ffff798b7fc  Yes (*)     /lib/libc.so.6
     (*): Shared library is missing debugging information.

Note that &var and the value of var (zero) come from the shared library's
version of the variable.  Using the address of var from the executable shows the
expected value (42) as modified by the program.

-- 
           Summary: common/.bss variables from shared libraries not
                    displayed correctly
           Product: gdb
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: symtab
        AssignedTo: unassigned at sourceware dot org
        ReportedBy: lance604 at gmail dot com
                CC: gdb-prs at sourceware dot org
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://sourceware.org/bugzilla/show_bug.cgi?id=11717

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