Can't preload a function in C program - c

Question

I wrote some code in nasm and I'm trying to implement it in a C program as a replacement of strlen through a shared library, but it doesn't work.
nasm code:
section .text
global strlen:function
strlen:
mov rax, 42
ret
C code:
#include <stdio.h>
size_t strlen(const char *s);
int main()
{
printf("%zu\n", strlen("foobar"));
return (0);
}
I compile the C program just using gcc without any arguments, and I create the shared library with the following commands:
nasm -f elf64 strlen.asm
gcc -shared -fPIC -o libasm.so strlen.o
Finally, I include the shared library:
export LD_PRELOAD=`pwd`/libasm.so
But it displays '6' where I expect it to display '42'.
I don't think the problem comes from my library, because I get segmentation fault when I execute the ls command with LD_PRELOAD.
I'm working on Ubuntu 16.04.

Answer 1

This is not related to nasm at all. A C equivalent of your strlen() function does not work either.
$ cat strlen.c
#include <stddef.h>
size_t strlen(const char *s)
{
return 43;
}
$ cat s.c
#include <stdio.h>
size_t strlen(const char *s);
int main()
{
printf("%zu\n", strlen("foobar"));
return 0;
}
$ make s
cc s.c -o s
$ gcc -shared -fPIC -o strlen.so strlen.c
$ LD_PRELOAD=$PWD/strlen.so ./s
6
What is happening here is that gcc is using its own built-in version of strlen() that cannot be overridden. If the C program that calls strlen() is recompiled to not use this built-in version of strlen(), then your shared library can override it.
$ rm s
$ make s CFLAGS=-fno-builtin-strlen
cc -fno-builtin-strlen s.c -o s
$ LD_PRELOAD=$PWD/strlen.so ./s
43
$ LD_PRELOAD=$PWD/libasm.so ./s
42