What does the "no version information available" error from linux dynamic linker mean?

Question

In our product we ship some linux binaries that dynamically link to system libraries like "libpam". On some customer systems we get the following error on stderr when the program runs:

./authpam: /lib/libpam.so.0: no version information available (required by authpam)

The application runs fine and executes code from the dynamic library. So this is not a fatal error, it's really just a warning.

I figure that this is error comes from the dynamic linker when the system installed library is missing something our executable expects. I don't know much about the internals of the dynamic linking process ... and googling the topic doesn't help much. :(

Anyone know what causes this error? ... how I can diagnose the cause? ... and how we could change our executables to avoid this problem?

Update: The customer upgraded to the latest version of debian "testing" and the same error occurred. So it's not an out of date libpam library. I guess I'd like to understand what the linker is complaining about? How can I investigate the underlying cause, etc?

Obrian

Answer 1

The "no version information available" means that the library version number is lower on the shared object. For example, if your major.minor.patch number is 7.15.5 on the machine where you build the binary, and the major.minor.patch number is 7.12.1 on the installation machine, ld will print the warning.

You can fix this by compiling with a library (headers and shared objects) that matches the shared object version shipped with your target OS. E.g., if you are going to install to RedHat 3.4.6-9 you don't want to compile on Debian 4.1.1-21. This is one of the reasons that most distributions ship for specific linux distro numbers.

Otherwise, you can statically link. However, you don't want to do this with something like PAM, so you want to actually install a development environment that matches your client's production environment (or at least install and link against the correct library versions.)

Advice you get to rename the .so files (padding them with version numbers,) stems from a time when shared object libraries did not use versioned symbols. So don't expect that playing with the .so.n.n.n naming scheme is going to help (much - it might help if you system has been trashed.)

You last option will be compiling with a library with a different minor version number, using a custom linking script: http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/gnu-linker/scripts.html

To do this, you'll need to write a custom script, and you'll need a custom installer that runs ld against your client's shared objects, using the custom script. This requires that your client have gcc or ld on their production system.

Aletaaletha

Answer 2

What this message from the glibc dynamic linker actually means is that the library mentioned (/lib/libpam.so.0 in your case) doesn't have the VERDEF ELF section while the binary (authpam in your case) has some version definitions in VERNEED section for this library (presumably, libpam.so.0). You can easily see it with readelf, just look at .gnu.version_d and .gnu.version_r sections (or lack thereof).

So it's not a symbol version mismatch, because if the binary wanted to get some specific version via VERNEED and the library didn't provide it in its actual VERDEF, that would be a hard linker error and the binary wouldn't run at all (like this compared to this or that). It's that the binary wants some versions, but the library doesn't provide any information about its versions.

What does it mean in practice? Usually, exactly what is seen in this example — nothing, things just work ignoring versioning. Could things break? Of course, yes, so the other answers are correct in the fact that one should use the same libraries at runtime as the ones the binary was linked to at build time.

More information could be found in Ulrich Dreppers "ELF Symbol Versioning".

Aswan

I recommend running 'readelf -V <exePath>' to see the versioning section. notice capital V – Counterpoint

I had deduced this was the reason of the warning for (newer! versions of system) libraries I build myself and install in a parallel prefix. I always thought it was because I use the LLVM toolchain, but I just noticed that building with the system gcc doesn't put those version tags into the library automatically. Do I need to add an option via CFLAGS and/or LDFLAGS? – Hexateuch

Answer 3

Fwiw, I had this problem when running check_nrpe on a system that had the zenoss monitoring system installed. To add to the confusion, it worked fine as root user but not as zenoss user.

I found out that the zenoss user had an LD_LIBRARY_PATH that caused it to use zenoss libraries, which issue these warnings. Ie:

root@monitoring:$ echo $LD_LIBRARY_PATH

su - zenoss
zenoss@monitoring:/root$ echo $LD_LIBRARY_PATH
/usr/local/zenoss/python/lib:/usr/local/zenoss/mysql/lib:/usr/local/zenoss/zenoss/lib:/usr/local/zenoss/common/lib::
zenoss@monitoring:/root$ /usr/lib/nagios/plugins/check_nrpe -H 192.168.61.61 -p 6969 -c check_mq
/usr/lib/nagios/plugins/check_nrpe: /usr/local/zenoss/common/lib/libcrypto.so.0.9.8: no version information available (required by /usr/lib/libssl.so.0.9.8)
(...)
zenoss@monitoring:/root$ LD_LIBRARY_PATH= /usr/lib/nagios/plugins/check_nrpe -H 192.168.61.61 -p 6969 -c check_mq
(...)

So anyway, what I'm trying to say: check your variables like LD_LIBRARY_PATH, LD_PRELOAD etc as well.

Tenia

Answer 4

How are you compiling your app? What compiler flags?

In my experience, when targeting the vast realm of Linux systems out there, build your packages on the oldest version you are willing to support, and because more systems tend to be backwards compatible, your app will continue to work. Actually this is the whole reason for library versioning - ensuring backward compatibility.

Hershberger

Answer 5

Have you seen this already? The cause seems to be a very old libpam on one of the sides, probably on that customer.

Or the links for the version might be missing : http://www.linux.org/docs/ldp/howto/Program-Library-HOWTO/shared-libraries.html

Atmospherics

I did find that one, but it didn't really help understanding the cause. I don't think it's an old pam library, coz they updated to the latest debian testing. – Obrian

Then maybe you are compiling on an old machine? :) Have you tried compiling it on the client's machine? nondot.org/sabre/Mirrored/libtool-2.1a/libtool_toc.html#TOC36 – Atmospherics

Arrrgh broken link for the HOWTO... you can still retrieve it from the Wayback Machine: web.archive.org/web/20100611124512/http://www.linux.org/docs/… ... or directly from the author's own website: dwheeler.com/program-library/Program-Library-HOWTO/index.html ... or even directly from the Linux Documentation Project: tldp.org/HOWTO/Program-Library-HOWTO – Dionnedionysia