Linker Version Scripts

Using linker version scripts to control symbol visibility

We can use a version script to explicitly control the symbols exported by a library:

bash$ cat vis.map
VER_1 {
    global:
        vis_f1;
        vis_f2;
    local:
        *;
};

VER_1 is a version tag (more later).
After global, we list symbols that should be exported.
After local, we list symbols that should not be exported.
* and ? can be used as wildcards (as with filename wildcards). * matches 0 or more characters, ? matches a single character.

Here, our local specification says that all symbols are not exported, other than those explicitly exported by exported by global).

We can then build our shared library using the version script as follows:

bash$ gcc -g -c -fPIC -Wall vis_comm.c vis_f1.c vis_f2.c
bash$ gcc -g -shared -o vis.so vis_comm.o vis_f1.o vis_f2.o \
             -Wl,--version-script,vis.map

And readelf shows that vis_comm() is no longer externally visible:

bash$ readelf --syms --use-dynamic vis.so | grep vis_
   25   0: 00000730    73    FUNC GLOBAL DEFAULT  11 vis_f2
   29  16: 000006f0    59    FUNC GLOBAL DEFAULT  11 vis_f1