Patching a binary in Go

patch.go

package main

import (
	"bytes"
	"os"
)

func main() {
	// find the file "program" and read it all into memory as a byte slice
	bs, err := os.ReadFile("program")
	if err != nil {
		panic(err)
	}

	// find the index of the first occurence of "Hello, world!"
	i := bytes.Index(bs, []byte("Hello, world!"))
	if i == -1 {
		// an index of -1 means that the string was not found
		println("string not found in binary")
		os.Exit(1)
	}

	// overwrite the bytes starting at index i with new values
	copy(bs[i:], []byte("Hello, there!"))

	// write the updated byte slice out to a file called "patched"
	if err := os.WriteFile("patched", bs, 0o755); err != nil {
		panic(err)
	}
}