What is preloading?

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by Jakub Hrozek and Andreas Schneider

The LD_PRELOAD trick!

Preloading is a feature of the dynamic linker (ld). It is a available on most Unix system and allows to load a user specified, shared library before all other shared libraries which are linked to an executable.

Library pre-loading is most commonly used when you need a custom version of a library function to be called. You might want to implement your own malloc(3) and free(3) functions that would perform a rudimentary leak checking or memory access control for example, or you might want to extend the I/O calls to dump data when reverse engineering a binary blob. In this case, the library to be preloaded would implement the functions you want to override with prelinking. Only functions of dynamically loaded libraries can be overridden. You’re not able to override a function the application implements by itself or links statically with.

The library to preload is defined by the environment variable LD_PRELOAD, such as LD_PRELOAD=libwurst.so. The symbols of the preloaded library are bound first, before other linked shared libraries.
Lets look into symbol binding in more details. If your application calls a function, then the linker looks if it is available in the application itself first. If the symbol is not found, the linker checks all preloaded libraries and only then all the libraries which have been linked to your application. The shared libraries are searched in the order which has been given during compilation and linking. You can find out the linking order by calling 'ldd /path/to/my/applicaton'. If you’re interested how the linker is searching for the symbols it needs or if you want do debug if the symbol of your preloaded library is used correctly, you can do that by enabling tracing in the linker.

A simple example would be 'LD_DEBUG=symbols ls'. You can find more details about debugging with the linker in the manpage: 'man ld.so'.

Example:

Your application uses the function open(2).

  • Your application doesn’t implement it.
  • LD_PRELOAD=libcwrap.so provides open(2).
  • The linked libc.so provides open(2).

=> The open(2) symbol from libcwrap.so gets bound!

The wrappers used for creating complex testing environments of the cwrap project use preloading to supply their own variants of several system or library calls suitable for unit testing of networked software or privilege separation. For example, one wrapper includes its version of most of the standard API used to communicate over sockets that routes the communication over local sockets.

Samba AD DC in Fedora and RHEL

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Several people asked me about the status about the Active Directory Domain Controller support of Samba in Fedora. As Fedora and RHEL are using MIT Kerberos implementation as its Kerberos infrastructure of choice, the Samba Active Directory Domain Controller implementation is not available with MIT Kereberos at the moment. But we are working on it!

Günther Deschner and I gave a talk at the SambaXP conference about our development efforts in this direction:

The road to MIT KRB5 support

I hope this helps to understand that this is a huge task.

New features in socket_wrapper 1.1.0

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Maybe you already heard of the cwrap project. A set of tools to create a fully isolated network environment to test client/server components on a single host. socket_wrapper is a part of cwrap and I released version 1.1.0 today. In this release I worked together with Michael Adam and we implemented some nice new features like support for IP_PKTINFO for binding on UDP sockets, bindresvport() and more socket options via getsockopt(). This was mostly needed to be able to create a test environment for MIT Kerberos.

The upcoming features for the next version are support for passing file description between processes using a unix domain socket and sendmsg()/recvmsg() (SCM_RIGHTS). We would also like to make socket_wrapper thread-safe.

Group support for cmocka

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Last Friday I’ve released cmocka 0.4.0. It has several bugfixes and at least two new features. One is support for groups. This means you can define a setup and teardown function for a group of unit tests. I think some people have been waiting for this.

You can find an example here. It is simple and easy to use.
The other small feature is a new macro: assert_return_code(). It is designed for standard C function return values which return 0 for success and less than 0 to indicate an error with errno set. It will produce a nice error message! The rest are bugfixes and improvements for error message.

Thanks to all contributor and bug reporter!

If you think cmocka is a great piece of software, please vote it up on stackoverflow, thanks.

cwrap 1.0.0 – testing your full software stack …

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on one single machine!

FOSDEM/Brussels, February 2nd, I gave a talk about cwrap. I announced and released version 1.0.0 of cwrap, a set of tools to create a fully isolated network environment to test client/server components on a single host.
It provides synthetic account information, hostname resolution and privilege separation support. The heart of cwrap consists of three libraries you can preload to any executable.
The libc wrapper project does not require virtualization and can be used to build environments on different operating systems. The project
consists of a socket wrapper, NSS module wrapper (users, groups, hosts), and a (s)uid wrapper with support for GNU/Linux, BSD and Solaris.

The origin of these wrappers is the Samba project, where the wrappers are in use since many years to successfully test their protocol implementations. Now it is possible to use them outside of the Samba project. The wrappers have been improved and are loaded with new features.

Learn more at http://cwrap.org/

libssh 0.6.0 released

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After another development cycle, this time of 2,5 years, the libssh Team is proud to announce version 0.6.0 of libssh.

The most important functionality which has been added is a new callback-based server API. Also we added ECDSA support and a new algorithm called curve25519-sha256@libssh.org for key exchange to have something better than the NIST curves. OpenSSH also uses curve25519-sha256@libssh.org as the default for key exchange. For ECDSA there is a complete new API for public key management available. Also a big improvement is Kerberos support which has been tested by Red Hat engineers with FreeIPA and gssproxy.

Thanks to all contributors!

Powerline

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I spent the day to look at tmux and vim and found a lot of great plugins. What I really like for tmux and also vim is powerline. Powerline is a status-line and prompts utility to change the look and feel of your vim or tmux status lines. It looks like this:

vim

It consists of a special font, a python tool and plugins for applications. I’ve created package for Fedora and submitted a review request here.