Linux Applications Debugging Techniques/The dynamic linker

Dependencies

• ldd -d -r /path/to/binary
• A script to visualize libraries and their dependencies.
• Binaries Analysis Tools
• tldd

Resolved symbols

To find out which dynamic library is a symbol coming from:

$ LD_DEBUG_OUTPUT=sym.log LD_DEBUG=bindings /bin/ls

$ cat sym.log.7688 | grep malloc
      7688:     binding file /lib/i686/cmov/libc.so.6 [0] to /lib/i686/cmov/libc.so.6 [0]: normal symbol `__malloc_initialize_hook' [GLIBC_2.0]
      7688:     binding file /lib/i686/cmov/libc.so.6 [0] to /bin/ls [0]: normal symbol `malloc' [GLIBC_2.0]
      7688:     binding file /lib/i686/cmov/libc.so.6 [0] to /lib/i686/cmov/libc.so.6 [0]: normal symbol `__malloc_hook' [GLIBC_2.0]
      7688:     binding file /lib/ld-linux.so.2 [0] to /lib/i686/cmov/libc.so.6 [0]: normal symbol `malloc' [GLIBC_2.0]
      7688:     binding file /lib/i686/cmov/libc.so.6 [0] to /lib/i686/cmov/libc.so.6 [0]: normal symbol `malloc' [GLIBC_2.0]
      7688:     binding file /bin/ls [0] to /lib/i686/cmov/libc.so.6 [0]: normal symbol `malloc' [GLIBC_2.0]

$ LD_DEBUG=help /bin/ls
Valid options for the LD_DEBUG environment variable are:

  libs        display library search paths
  reloc       display relocation processing
  files       display progress for input file
  symbols     display symbol table processing
  bindings    display information about symbol binding
  versions    display version dependencies
  all         all previous options combined
  statistics  display relocation statistics
  unused      determined unused DSOs
  help        display this help message and exit

To direct the debugging output into a file instead of standard output
a filename can be specified using the LD_DEBUG_OUTPUT environment variable.

References

• man 8 ld.so
• libc symbols visibility & linking

Notes

• It is run under whatever credentials would be accorded to the application program; thus, for example, if a set-user-ID-root program is being executed, the dynamic linker will run with an effective user ID of root.

Varia

• A look at dynamic linking