What is the LD_PRELOAD trick?

Question

I came across a reference to it recently on proggit and (as of now) it is not explained.

I suspect this might be it, but I don't know for sure.

Answer 1

Using LD_PRELOAD path, you can force the application loader to load provided shared object, over the default provided.

Developers uses this to debug their applications by providing different versions of the shared objects.

We've used it to hack certain applications, by overriding existing functions using prepared shared objects.

Answer 2

As many people mentioned, using LD_PRELOAD to preload library. BTW, you can CHECK if the setting is available by ldd command.

Example: suppose you need to preload your own libselinux.so.1.

> ldd /bin/ls
    ...
    libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007f3927b1d000)
    libacl.so.1 => /lib/x86_64-linux-gnu/libacl.so.1 (0x00007f3927914000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f392754f000)
    libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f3927311000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f392710c000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f3927d65000)
    libattr.so.1 => /lib/x86_64-linux-gnu/libattr.so.1 (0x00007f3926f07000)

Thus, set your preload environment:

  export LD_PRELOAD=/home/patric/libselinux.so.1

Check your library again:

>ldd /bin/ls
    ...
    libselinux.so.1 =>
    /home/patric/libselinux.so.1 (0x00007fb9245d8000)
    ...

Answer 3

Here is a detailed blog post about preloading:

https://blog.cryptomilk.org/2014/07/21/what-is-preloading/

Answer 4

If you set LD_PRELOAD to the path of a shared object, that file will be loaded before any other library (including the C runtime, libc.so). So to run ls with your special malloc() implementation, do this:

$ LD_PRELOAD=/path/to/my/malloc.so /bin/ls

Answer 5

You can override symbols in the stock libraries by creating a library with the same symbols and specifying the library in LD_PRELOAD.

Some people use it to specify libraries in nonstandard locations, but LD_LIBRARY_PATH is better for that purpose.

Answer 6

it's easy to export mylib.so to env:

$ export LD_PRELOAD=/path/mylib.so
$ ./mybin

to disable :

$ export LD_PRELOAD=