LD_PRELOAD

• environment variable
• entries containin / are treated as full path, otherwise usual LD search applies.
• system wide basis: */etc/profile (overiddable) or /etc/ld.so.preload (non-overridable)
  • affects all programs - be careful!
• does not work for statically linked programs. Can be good and bad.
• effects all child processes (unless library takes care to remove itself).
• does not work for setuid/setgid unless libraries are in standard path and also setuid/setgid.
• can only override library functions, not system calls.
• only way to get parameters is through environment.

Existing hacks

Filesystem - a lot of these before FUSE, often a better solution.

• flcow - FileLink Copy On Write. Break hard links on file write.
• plasticfs - https://pasticffs.sourceforge.net/
• installwatch

Network

• libshape
• netjail - https://netjail.sourceforge.net/

Debugging and testing - force library functions to fail when they would

• failmalloc - intercepts malloc and causes it to fail.
• libeatmydata - disables fsync

Debugging and testing

• electric fence
• segv_handler - produces stack track that dumps back trace on segfault.

Graphical

• libglfps - adds framerate display to OpenGL programs.

Writing hacks

dlfcn - Dynamic Linker FunCtioNs - use these functions to call original library.

sysconfcplus

sysconf(_SC_NPROCESSORS_CONF)

We want a library that overrides the return value of this sysconf, and ensure closed source process doesn’t spawn to many threads on a shared system.

xlibtrace

strace/ltrace like program for X-Windows calls. Interpret and displays libx11 specific data structures. More portable.

xmultiwin

multiple head displays

https://www.kev.pulo.com.au/xmultiwin/

Currently: Redraws reflected on both windows.