Finding .so files without setting LD_LIBRARY_PATH

[eryk sun]

On Wed, May 11, 2016 at 10:39 PM, Paul Smith <paul at mad-scientist.net> wrote:
> Hi all.  I have a locally-built version of Python (2.7.11) that I'm
> copying around to different systems, running all different versions of
> GNU/Linux.
...
> What I'd like to do is have a way of setting the library path that
> Python uses when it tries to load .so files (for example in the ssl
> module which loads lib-dynload/_ssl.so which links to libssl.so and
> libcrypto.so), WITHOUT setting LD_LIBRARY_PATH in the environment that
> Python passes to its children.

An ELF header can contain either an RPATH or a RUNPATH to extend the
library search path at load time. RPATH is searched before
LD_LIBRARY_PATH, while RUNPATH is searched after LD_LIBRARY_PATH. Note
that the Linux loader only uses an embedded RUNPATH when searching for
the immediate dependencies of an object. Use an RPATH instead if you
need to extend the path that dlopen searches at runtime (e.g. for
loading shared libraries via ctypes).

Paths can be defined relative to the object using ${ORIGIN}. For
example, the following configures a Python build to add a relative
RPATH:

    $ ./configure LDFLAGS='-Wl,-rpath,\$${ORIGIN}/lib'
    $ make

Or a RUNPATH:

    $ ./configure LDFLAGS='-Wl,-rpath,\$${ORIGIN}/lib,--enable-new-dtags'
    $ make

Here's the resulting RPATH in the ELF header:

    $ readelf -d python | grep RPATH
     0x000000000000000f (RPATH)              Library rpath: [${ORIGIN}/lib]