Question

I am executing a program say A from another by first fork-ing followed by execve(). Now the problem is I would want A to use my library that I would generaly do by using LD_PRELOAD. How do I do it within execve().

Thanks

Answer 1

you can pass the LD_PRELOAD in envp execve’s argument:

the program that gets execved, named “run”:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char **argv) 
{

    printf("%s\n",getenv("LD_PRELOAD"));
}

the program that does the execve, named “ex”:

#include <stdio.h>
#include <unistd.h>


int main(int argc, char **argv)
{
    char *const args[] = {"./run",NULL};
    char *const envs[] = {"LD_PRELOAD=caca",NULL};
    execve("./run",args,envs);
}

running it:

root@pinkpony:~# ./ex
ERROR: ld.so: object 'caca' from LD_PRELOAD cannot be preloaded: ignored.
caca

EDIT:
the error shown gets thrown because “caca” lib can’t be preloaded for run, so it works. (I skipped the fork() part for clarity but the usage is the same)

EDIT:
doing something like:

LD_PRELOAD=caca ./ex

will not automagically preload caca lib when execve()-ing run if you’re not passing it via envp execve()’s argument