Question

I read this article recently, and I tried overriding the libc printf function with a doing the following for fun :-

• Create an executable that uses printf to print this is a test(printer.c)
• Create an c file with a custom puts to print muhahaha, this is a test(custom.c)
• Create an object file gcc -fPIC -g -c -Wall custom.c
• Create an so file gcc -shared -Wl,-soname,libmystuff.so.1 -o libmystuff.so.1.0.1 custom.o
• I add the directory which contains the so file into the LD_PRELOAD environment variable. export LD_PRELOAD=$(pwd)
• Try running printer

I’d imagine that muhahaha, this is a test would be printed out but it seems like im doing something wrong. Have I got some concept wrong? Or am I just doing something wrong?

[EDIT]

The code snippets involved are :-

// printer.c
int main() {
printf("this is a test");
return 0;
}

// custom.c
void printf(char *t) {
puts("muhahaha, this is a test");
}

Answer 1

You’re supposed to name the library in the LD_PRELOAD environment variable, not the directory.

export LD_PRELOAD=/path/to/libmystuff.so.1.0.1