How to make FS with only reading functions with libfuse on C

Question

I have a task to make FS with only reading functions (maintained on ext2 with libfuse on C). Basically I need to overwrite some functions from libfuse but I don't know how to overwrite read function without information storage function (how can I take data if I don't know how it stored)

I tried this but it's not what I want (here I replace file constant strings)

#define FUSE_USE_VERSION 30

#include <fuse.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <time.h>
#include <string.h>
#include <stdlib.h>

static int do_getattr( const char *path, struct stat *st )
{
    printf( "[getattr] Called\n" );
    printf( "\tAttributes of %s requested\n", path );

    st->st_uid = getuid();
    st->st_gid = getgid();
    st->st_atime = time( NULL );
    st->st_mtime = time( NULL );

    if ( strcmp( path, "/" ) == 0 )
    {
        st->st_mode = S_IFDIR | 0755;
        st->st_nlink = 2;
    }
    else
    {
        st->st_mode = S_IFREG | 0644;
        st->st_nlink = 1;
        st->st_size = 1024;
    }

    return 0;
}

static int do_readdir( const char *path, void *buffer, fuse_fill_dir_t filler, off_t offset, struct fuse_file_info *fi )
{
    printf( "--> Getting The List of Files of %s\n", path );

    filler( buffer, ".", NULL, 0 ); // Current Directory
    filler( buffer, "..", NULL, 0 ); // Parent Directory

    if ( strcmp( path, "/" ) == 0 ) // If the user is trying to show the files/directories of the root directory show the following
    {
        filler( buffer, "file54", NULL, 0 );
        filler( buffer, "file349", NULL, 0 );
    }

    return 0;
}

static int do_read( const char *path, char *buffer, size_t size, off_t offset, struct fuse_file_info *fi )
{
    char file54Text[] = "Hello World From File54!";
    char file349Text[] = "Hello World From File349!";
    char *selectedText = NULL;

    if ( strcmp( path, "/file54" ) == 0 )
        selectedText = file54Text;
    else if ( strcmp( path, "/file349" ) == 0 )
        selectedText = file349Text;
    else
        return -1;

    memcpy( buffer, selectedText + offset, size );

    return strlen( selectedText ) - offset;
}

static struct fuse_operations operations = {
        .getattr    = do_getattr,
        .readdir    = do_readdir,
        .read   = do_read,
};

int main( int argc, char *argv[] )
{
    return fuse_main( argc, argv, &operations, NULL );
}

I want to read files(not a constant strings) which stored in mounted dir. So I need to overwrite do_read but idk how(

Here is example of full FS

And I need to have FS with only reading functions