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

User · Accepted Answer

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

User · Answer

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

User · Answer

Here is a detailed blog post about preloading   https   blog cryptomilk org 2014 07 21 what-is-preloading

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

it s easy to export mylib so to env     export LD PRELOAD  path mylib so     mybin   to disable      export LD PRELOAD

User · Answer

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    gt  ldd  bin ls             libselinux so 1   gt   lib x86 64-linux-gnu libselinux so 1  0x00007f3927b1d000      libacl so 1   gt   lib x86 64-linux-gnu libacl so 1  0x00007f3927914000      libc so 6   gt   lib x86 64-linux-gnu libc so 6  0x00007f392754f000      libpcre so 3   gt   lib x86 64-linux-gnu libpcre so 3  0x00007f3927311000      libdl so 2   gt   lib x86 64-linux-gnu libdl so 2  0x00007f392710c000       lib64 ld-linux-x86-64 so 2  0x00007f3927d65000      libattr so 1   gt   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    gt ldd  bin ls             libselinux so 1   gt       home patric libselinux so 1  0x00007fb9245d8000

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Here is a detailed blog post about preloading   https   blog cryptomilk org 2014 07 21 what-is-preloading

User · Answer

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

User · Answer

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    gt  ldd  bin ls             libselinux so 1   gt   lib x86 64-linux-gnu libselinux so 1  0x00007f3927b1d000      libacl so 1   gt   lib x86 64-linux-gnu libacl so 1  0x00007f3927914000      libc so 6   gt   lib x86 64-linux-gnu libc so 6  0x00007f392754f000      libpcre so 3   gt   lib x86 64-linux-gnu libpcre so 3  0x00007f3927311000      libdl so 2   gt   lib x86 64-linux-gnu libdl so 2  0x00007f392710c000       lib64 ld-linux-x86-64 so 2  0x00007f3927d65000      libattr so 1   gt   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    gt ldd  bin ls             libselinux so 1   gt       home patric libselinux so 1  0x00007fb9245d8000

User · Answer

when LD PRELOAD is used that file will be loaded before any other   export LD PRELOAD  path lib lib to be pre loaded  even this can be used in programs too

User · Answer

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

User · Answer

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

User · Answer

when LD PRELOAD is used that file will be loaded before any other   export LD PRELOAD  path lib lib to be pre loaded  even this can be used in programs too

User · Answer

it s easy to export mylib so to env     export LD PRELOAD  path mylib so     mybin   to disable      export LD PRELOAD

User · Answer

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

[c] What is the LD_PRELOAD trick?

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