Loading GNU ld script with dlopen

Question

I have an C++14 code that should load an arbitrary shared object file with dlopen. Unfortunately on some systems (e.g. my archlinux, reportedly also applies to some .so on ubuntu and gentoo), these so-files can be "GNU ld scripts" instead of the actual binaries.

For reference, here is the content of my /usr/lib/libm.so:

/* GNU ld script
*/
OUTPUT_FORMAT(elf64-x86-64)
GROUP ( /usr/lib/libm.so.6  AS_NEEDED ( /usr/lib/libmvec.so.1 ) )

I have found a couple of code-pieces that deal with this issue in ghc or ruby. I would like to avoid resorting to manually parsing the text file based parsing the dlerror text and the file. I feel that is terribly evil and I won't be able to implement and maintain corner cases of this format.

Is there a clean way to implement handling this case? Frankly I am puzzled as to why dlopen does not actually handle these tranparaently.

Note: Considering the aforementioned patches I think this is not simply an issue with my system configuration / versions. If this should work out-of-the-box with dlopen (bug instead of missing feature), please let me know.

Answer 1

The linker scripts are intended to be used by the linker, not the run-time linker.

The GNU ld script comment should have been a giveaway: this is for ld, not for ld.so. ;-)

See for instance: http://www.math.utah.edu/docs/info/ld_3.html

So I guess using this with dlopen() would mean mimicking/importing part of ld's magic for this, which would confirm your fears about resorting to manually parsing the text and maintaining terribly evil code.

EDIT: There seems to be one thing that can help you though:

https://www.sourceware.org/ml/libc-alpha/2011-07/msg00152.html

<gnu/lib-names.h> should contain a define LIBM_SO which should point you to the correct file that you can actually dlopen().

That means that normally no evil code would be necessary.