Marvee Amasi Posts - Answer Overflow

Marvee Amasi

•Created by Marvee Amasi on 7/23/2024 in #middleware-and-os

LD_PRELOAD Not Working with Custom getpid Function

I'm trying to use the LD_PRELOAD to inject my custom function in a program. I've compiled a shared library getpid.so having a modified getpid function that prints a message:

#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

pid_t getpid(void)
{
    printf("Initializing..\n");
    return syscall(SYS_getpid);
}

#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

pid_t getpid(void)
{
    printf("Initializing..\n");
    return syscall(SYS_getpid);
}

I've also compiled a program testpid.c that calls the regular getpid function:

#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
int main()
{
    printf( "pid = %d!\n", getpid() );

    return 0;
}

#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
int main()
{
    printf( "pid = %d!\n", getpid() );

    return 0;
}

When I run the command:

LD_PRELOAD=./getpid.so ./testpid

LD_PRELOAD=./getpid.so ./testpid

I get the following error:

ERROR: ld.so: object 'getpid.so' from LD_PRELOAD cannot be preloaded: ignored

ERROR: ld.so: object 'getpid.so' from LD_PRELOAD cannot be preloaded: ignored

I've compiled both files with gcc -Wall -fPIC -shared -o getpid.so getpid.c and gcc testpid -o testpid.c Can anyone help me understand why LD_PRELOAD is not working and how to fix the issue?

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