NetBSD ELF FAQ

ELF Issues


ELF Issues

What is ELF?

ELF is a binary format designed to support dynamic objects and shared libraries. On older COFF and ECOFF systems, dynamic support and shared libraries were not naturally supported by the underlying format, leading to dynamic implementations that were at times complex, quirky, and slow.

A dynamically loaded module at run-time couldn't find symbols from my program image.

You probably left off the --export-dynamic option when linking the application. This is required only if arbitrary symbols in the program might be needed by the dynamically loaded module, for example, if a program intends to make run-time decisions to dynamically load modules it was never linked with. Note, when running cc(1) instead of ld(1) this will be specified as -Wl,--export-dynamic.

No need for ldconfig or for ld.so.conf!

Ideally, there is no need for ldconfig or for /etc/ld.so.conf, since ELF provides good and predictable (read portable) mechanism to locate shared libraries. Unfortunately there are still a few corner cases (like wanting to run setuid binaries that you don't have the source for, that want shared libraries to be installed somewhere you don't like). For those corner cases, you'll find that creating an /etc/ld.so.conf file, will still work. Read on though about other ways of doing this and why it is not a good idea. The next section discusses the ELF mechanisms for locating shared libraries.

My program can't find its shared library

An ELF program needs to know the directory and the filename required to mmap(2) its shared libraries. Encoded within the file name is version information. There are one set of mechanisms for the directories and a different mechanism for the file names.

Directories

Although rarely used, the optional LD_LIBRARY_PATH environment variable specifies a colon-separated search path. This can be used in wrapper scripts as needed for misconfigured applications. It is ignored for setuid binaries.

There is a built-in search path in the run-time loader: ld.elf_so. On many systems this path consists only of /usr/lib; although on NetBSD versions prior to 1.4 it also searched /usr/local/lib.

The primary directory locating mechanism is the ``rpath'' search list contained within the executable image. This search list is set with the -R directive to ld(1). The POSIX syntax for passing ld(1) options through the compiler front end is:

    -Wl, option,option,...

For example: -Wl,-R/usr/something/lib. Multiple -R directives can be given to a single application to create a shared library search path.

This directive is also known as -rpath. Using -R has the advantage of working in older versions of NetBSD as well.

File Names and Versions

When shared libraries are created, the -soname directive is used to record the major version number of the library in the internal DT_SONAME field. The actual library is installed as, for example,

  • libgizmo.so.4.2   (the actual file)
  • libgizmo.so.4   (symbolic link)
  • libgizmo.so   (symbolic link)

The idea is that Makefiles will want to link against only the plain .so file. (Who would want to go around changing all the Makefiles just because a new library version was installed?) Once linked, however, the program does want to be aware of the major version but does not want to deal with the minor version.

Consequently, the library itself knows that it is libgizmo.so.4 because a -soname libgizmo.so.4 directive was used when it was created. The program knows it got major version 4 because the linker copied the libgizmo.so.4 DT_SONAME string out of the library and saved it in the executable.

You don't say -soname libgizmo.so, because then the program would use the latest major number and would break if that ever changed. (The major number only changes if the new library is incompatible.) You don't say -soname libgizmo.so.4.2, because then the installation of a compatible change that bumps the minor number would unnecessarily break the linked images.

Elf Shared Library Examples

To compile f.c and make an installable shared library out of it:

cc -O  -Werror  -c -fpic -DPIC f.c -o f.so
ar cq libf_pic.a `NM=nm lorder f.so | tsort -q`
ld -x -shared -R/my/directory/lib -soname libf.so.4 -o \
   libf.so.4.9  /usr/lib/crtbeginS.o  --whole-archive \
   libf_pic.a /usr/lib/crtendS.o

There is another way:

% cat Makefile
LIB=f
SRCS=f.c
.include <bsd.lib.mk>
% cat shlib_version
major=4
minor=9
% make

You can disable some of the Makefile targets with NOPROFILE=1 and NOSTATICLIB=1.

And there is another way:

libtool - The libtool package is a large shell script used to manage shared and static libraries in a platform-independent fashion. There is a NetBSD devel/libtool package and even a libtool home page.

But I Want ldconfig!    I Want ld.so.conf! I want it! I want it! I want it!

At first glance, it might seem reasonable, why shouldn't people be able to move things around to anywhere they want and correct the consequent lossage in a /etc file?

In fact, some developers of ELF systems have apparently added such a file, but with mixed results. The ELF mechanism was designed to correct some of the previous problems, and introducing the old mechanism would bring many of those old problems back.

Currently we are even supporting the /etc/ld.so.conf functionality in our ELF linker, but it is not at all clear that a hybrid mechanism is the right solution. For that reason we do not advertise its existence, advocate its use, or even provide a default installation template. It is there for those who think that they really need it, and cannot live without it.

Here are some of the problems.

  1. It's a shotgun approach that incorrectly assumes all images on the system will want to use the same search path. One advantage of the -R mechanism is that different applications can use different library search paths. We could add exceptions to this, but it's more configuration steps...see below...
  2. Another file in /etc is yet another configuration knob to turn. This works against making the system easy to install and use.
  3. The /etc file gets out of sync with the installed system when configurations are changed or packages are added. The resulting failure mode can be confusing to some users.
  4. The system should `just work'. We don't want to have to indoctrinate users into editing the config file for packages, X11, /usr/local.
  5. Would you think it is reasonable to provide a local configuration search path for random data configuration files? Would a single path really apply for all applications? What if two applications had the same configuration file? What if two packages each want a "libutil.so.1"?

ELF tools are standardized packages maintained by third parties; these tools are used consistently on different operating systems and platforms. In the long run, the standardization provided by ELF will increase the quality of both systems and applications.

How do I tell if my system is ELF?

If you are running an ELF system your compiler will define the constant __ELF__. You can use this in your C programs of course but you can also use the following shell script to determine it as well.

if echo __ELF__ | ${CC:-cc} -E - | grep -q __ELF__
then echo "Not ELF"
else echo "It is an ELF system"
fi

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