Normally the Linux dynamic loader ld-linux (see ld-linux(8) man page) finds and loads the shared libraries needed by a program, prepare the program to run, and then run it. The shared libraries (shared objects) are loaded in whatever order the loader needs them in order to resolve symbols.
LD_PRELOAD is an optional environmental variable containing one or more paths to shared libraries, or shared objects, that the loader will load before any other shared library including the C runtime library (libc.so) This is called preloading a library.
Preloading a library means that its functions will be used before others of the same name in later libraries. This enables library functions to be intercepted and replaced (overwritten.) As a result program behavior can be non-invasively modified, i.e. a recompile is not necessary.
For example, you could write a library which implements alternative malloc and free functionality. By preloading the new library using LD_PRELOAD the new malloc and free functions will be used rather than the corresponding standard libc functions.
Shared library paths, if there is more than one, may be separated by either colons (preferred) or spaces. Entries in LD_PRELOAD containing ’/’are treated as pathnames whereas entries not containing ’/’are searched for as usual. Obviously this only affects dynamically linked – not statically linked – applications.
To avoid this mechanism being using as an attack vector for suid/sgid executable binaries, the loader ignores LD_PRELOAD if ruid != euid. For such binaries, only libraries in standard paths that are also suid/sgid will be preloaded.
Some users use LD_PRELOAD to specify libraries in nonstandard locations, but the LD_LIBRARY_PATH environmental variable is a better solution
Note that shared libraries specified in /etc/ld.so.preload are loaded before libraries specified by LD_PRELOAD. The libc library checks for the existing of /etc/ld.so.preload (see elf/rtld.c) and, if found, loads the listed shared libraries just as setting the environment variable LD_PRELOAD would do. The advantage of using /etc/ld.so.preload is that these shared libraries are implicitly trusted and hence the ruid != euid test does not apply. Thus the loader will load the shared objects listed in /etc/ld.so.preload even for suid/sgid executable binaries.