Linker and Libraries Guide

Locating Associated Dependencies

Typically, an unbundled product is designed to be installed in a unique location. This product is composed of binaries, shared object dependencies, and associated configuration files. For example, the unbundled product ABC might have the layout shown in the following figure.

Figure C–1 Unbundled Dependencies

Assume that the product is designed for installation under /opt. Normally, you would augment the PATH with /opt/ABC/bin to locate the product's binaries. Each binary locates their dependencies using a hard-coded runpath within the binary. For the application abc, this runpath would be as follows.

$ cc -o abc abc.c -R/opt/ABC/lib -L/opt/ABC/lib -lA
$ elfdump -d abc | egrep 'NEEDED|RUNPATH'
       [0]  NEEDED            0x1b5               libA.so.1
       ....
       [4]  RUNPATH           0x1bf               /opt/ABC/lib

Similarly, for the dependency libA.so.1 the runpath would be as follows.

$ cc -o libA.so.1 -G -Kpic A.c -R/opt/ABC/lib -L/opt/ABC/lib -lB
$ elfdump -d libA.so.1 | egrep 'NEEDED|RUNPATH'
       [0]  NEEDED            0x96                libB.so.1
       [4]  RUNPATH           0xa0                /opt/ABC/lib

This dependency representation works until the product is installed in some directory other than the recommended default.

The dynamic token $ORIGIN expands to the directory in which an object originated. This token is available for filters, runpath, or dependency definitions. Use this technology to redefine the unbundled application to locate its dependencies in terms of $ORIGIN.

$ cc -o abc abc.c '-R$ORIGIN/../lib' -L/opt/ABC/lib -lA
$ elfdump -d abc | egrep 'NEEDED|RUNPATH'
       [0]  NEEDED            0x1b5               libA.so.1
       ....
       [4]  RUNPATH           0x1bf               $ORIGIN/../lib

The dependency libA.so.1 can also be defined in terms of $ORIGIN.

$ cc -o libA.so.1 -G -Kpic A.c '-R$ORIGIN' -L/opt/ABC/lib -lB
$ elfdump -d lib/libA.so.1 | egrep 'NEEDED|RUNPATH'
       [0]  NEEDED            0x96                libB.so.1
       [4]  RUNPATH           0xa0                $ORIGIN

If this product is now installed under /usr/local/ABC and the user's PATH is augmented with /usr/local/ABC/bin, invocation of the application abc result in a path name lookup for its dependencies as follows.

$ ldd -s abc
....
  find object=libA.so.1; required by abc
    search path=$ORIGIN/../lib  (RUNPATH/RPATH from file abc) 
      trying path=/usr/local/ABC/lib/libA.so.1
        libA.so.1 =>     /usr/local/ABC/lib/libA.so.1
 
  find object=libB.so.1; required by /usr/local/ABC/lib/libA.so.1
    search path=$ORIGIN  (RUNPATH/RPATH from file /usr/local/ABC/lib/libA.so.1)
      trying path=/usr/local/ABC/lib/libB.so.1
        libB.so.1 =>     /usr/local/ABC/lib/libB.so.1

Dependencies Between Unbundled Products

Another issue related to dependency location is how to establish a model whereby unbundled products express dependencies between themselves.

For example, the unbundled product XYZ might have dependencies on the product ABC. This dependency can be established by a host package installation script. This script generates a symbolic link to the installation point of the ABC product, as shown in the following figure.

Figure C–2 Unbundled Co-Dependencies

The binaries and shared objects of the XYZ product can represent their dependencies on the ABC product using the symbolic link. This link is now a stable reference point. For the application xyz, this runpath would be as follows.

$ cc -o xyz xyz.c '-R$ORIGIN/../lib:$ORIGIN/../ABC/lib' \
-L/opt/ABC/lib -lX -lA
$ elfdump -d xyz | egrep 'NEEDED|RUNPATH'
       [0]  NEEDED            0x1b5               libX.so.1
       [1]  NEEDED            0x1bf               libA.so.1
       ....
       [2]  NEEDED            0x18f               libc.so.1
       [5]  RUNPATH           0x1c9               $ORIGIN/../lib:$ORIGIN/../ABC/lib

and similarly for the dependency libX.so.1 this runpath would be as follows.

$ cc -o libX.so.1 -G -Kpic X.c '-R$ORIGIN:$ORIGIN/../ABC/lib' \
-L/opt/ABC/lib -lY -lC
$ elfdump -d libX.so.1 | egrep 'NEEDED|RUNPATH'
       [0]  NEEDED            0x96                libY.so.1
       [1]  NEEDED            0xa0                libC.so.1
       [5]  RUNPATH           0xaa                $ORIGIN:$ORIGIN/../ABC/lib

If this product is now installed under /usr/local/XYZ, its post-install script would be required to establish a symbolic link of.

$ ln -s ../ABC /usr/local/XYZ/ABC

If the user's PATH is augmented with /usr/local/XYZ/bin, then invocation of the application xyz result in a path name lookup for its dependencies as follows.

$ ldd -s xyz
....
  find object=libX.so.1; required by xyz
    search path=$ORIGIN/../lib:$ORIGIN/../ABC/lib  (RUNPATH/RPATH from file xyz)
      trying path=/usr/local/XYZ/lib/libX.so.1
        libX.so.1 =>     /usr/local/XYZ/lib/libX.so.1
 
  find object=libA.so.1; required by xyz
    search path=$ORIGIN/../lib:$ORIGIN/../ABC/lib  (RUNPATH/RPATH from file xyz)
      trying path=/usr/local/XYZ/lib/libA.so.1
      trying path=/usr/local/ABC/lib/libA.so.1
        libA.so.1 =>     /usr/local/ABC/lib/libA.so.1
 
  find object=libY.so.1; required by /usr/local/XYZ/lib/libX.so.1
     search path=$ORIGIN:$ORIGIN/../ABC/lib \
                (RUNPATH/RPATH from file /usr/local/XYZ/lib/libX.so.1)
      trying path=/usr/local/XYZ/lib/libY.so.1
        libY.so.1 =>     /usr/local/XYZ/lib/libY.so.1
 
  find object=libC.so.1; required by /usr/local/XYZ/lib/libX.so.1
     search path=$ORIGIN:$ORIGIN/../ABC/lib \
                (RUNPATH/RPATH from file /usr/local/XYZ/lib/libX.so.1)
      trying path=/usr/local/XYZ/lib/libC.so.1
      trying path=/usr/local/ABC/lib/libC.so.1
        libC.so.1 =>     /usr/local/ABC/lib/libC.so.1
 
  find object=libB.so.1; required by /usr/local/ABC/lib/libA.so.1
     search path=$ORIGIN  (RUNPATH/RPATH from file /usr/local/ABC/lib/libA.so.1)
       trying path=/usr/local/ABC/lib/libB.so.1
        libB.so.1 =>     /usr/local/ABC/lib/libB.so.1

Security

In a secure process, the expansion of the $ORIGIN string is allowed only if it expands to a trusted directory. The occurrence of other relative path names, poses a security risk.

A path like $ORIGIN/../lib apparently points to a fixed location, fixed by the location of the executable. However, the location is not actually fixed. A writable directory in the same file system could exploit a secure program that uses $ORIGIN.

The following example shows this possible security breach if $ORIGIN was arbitrarily expanded within a secure process.

$ cd /worldwritable/dir/in/same/fs
$ mkdir bin lib
$ ln $ORIGIN/bin/program bin/program
$ cp ~/crooked-libc.so.1 lib/libc.so.1
$ bin/program
.... using crooked-libc.so.1

You can use the utility crle(1) to specify trusted directories that enable secure applications to use $ORIGIN. Administrators who use this technique should ensure that the target directories are suitably protected from malicious intrusion.