g304: use a more convincing example

docs/rules/g304_file-path_provided_as_taint_input.md

@@ -6,7 +6,7 @@ title: G304: File path provided as taint input
 Trying to open a file provided as an input in a variable. The content of this variable might be controlled by an attacker who could change it to hold unauthorised file paths from the system.  In this way, it is possible to exfiltrate confidential information or such.
 
 ## Example problematic code:
-
+This code lets an attacker read a `/private/path`
 ```
 package main
 
@@ -17,7 +17,10 @@ import (
 )
 
 func main() {
-	repoFile := "path_of_file"
+	repoFile := "/safe/path/../../private/path"
+	if !strings.HasPrefix(repoFile, "/safe/path/") {
+		panic(fmt.Errorf("Unsafe input"))
+	}
 	byContext, err := ioutil.ReadFile(repoFile)
 	if err != nil {
 		panic(err)
@@ -34,7 +37,7 @@ func main() {
 ```
 
 ## The right way
-
+This code panics if `/safe/path` was removed by an attacker
 ```
 package main
 
@@ -46,15 +49,18 @@ import (
 )
 
 func main() {
-	repoFile := "path_of_file"
-	byContext, err := ioutil.ReadFile(filepath.Clean(repoFile))
+	repoFile := "/safe/path/../../private/path"
+	repoFile = filepath.Clean(repoFile)
+	if !strings.HasPrefix(repoFile, "/safe/path/") {
+		panic(fmt.Errorf("Unsafe input"))
+	}
+	byContext, err := ioutil.ReadFile(repoFile)
 	if err != nil {
 		panic(err)
 	}
-	fmt.Printf("%s", string(byContext))
-}
+	fmt.Printf("%s", string(byContext))}
 ```
 
 ## See also
 
-* https://pkg.go.dev/path/filepath?tab=doc#Clean
+* https://pkg.go.dev/path/filepath?tab=doc#Clean