CODEWITHSUNDEEP
clinuxenvironment-variables
Hank Gay
Hank Gay

Reputation: 71959

What is the LD_PRELOAD trick?

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.

Upvotes: 488

Views: 546532

Answers (9)

JulienGenoud
JulienGenoud

Reputation: 630

To export mylib.so to env:

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

To disable it:

$ unset LD_PRELOAD

Upvotes: 10

Sumith Senarathne
Sumith Senarathne

Reputation: 41

When LD_PRELOAD is used, that file will be loaded before any other. Use $export LD_PRELOAD=/path/lib for lib to be pre-loaded. This can even be used in programs too.

Upvotes: 3

Patric
Patric

Reputation: 2131

As many people mentioned, using LD_PRELOAD to preload library. By the way, 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)
    ...

Upvotes: 64

dnahc araknayirp
dnahc araknayirp

Reputation: 131

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.

Upvotes: 3

Joshua
Joshua

Reputation: 43280

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.

Upvotes: 77

Rajesh
Rajesh

Reputation: 346

LD_PRELOAD lists shared libraries with functions that override the standard set, just as /etc/ld.so.preload does. These are implemented by the loader /lib/ld-linux.so. If you want to override just a few selected functions, you can do this by creating an overriding object file and setting LD_PRELOAD; the functions in this object file will override just those functions leaving others as they were.

For more information on shared libraries visit http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html

Upvotes: 11

asn
asn

Reputation: 838

Here is a detailed blog post about preloading:

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

Upvotes: 4

JesperE
JesperE

Reputation: 64414

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

Upvotes: 577

Ronny Brendel
Ronny Brendel

Reputation: 4845

With LD_PRELOAD you can give libraries precedence.

For example you can write a library which implement malloc and free. And by loading these with LD_PRELOAD your malloc and free will be executed rather than the standard ones.

Upvotes: 52

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