Fix ln -f handling when source and destination are the same entry (uutils#8838)

* fix(ln): enhance same-file detection with canonical paths

Improved the `link` function in `ln.rs` to use canonical path resolution for accurate same-file detection when forcing overwrites, preventing incorrect errors for equivalent paths. Added tests to verify behavior for self-linking and hard link relinking scenarios.

Co-authored-by: Sylvestre Ledru <[email protected]>

Author: mattsu2020

src/uu/ln/src/ln.rs

@@ -410,7 +410,17 @@ fn link(src: &Path, dst: &Path, settings: &Settings) -> UResult<()> {
             }
             OverwriteMode::Force => {
                 if !dst.is_symlink() && paths_refer_to_same_file(src, dst, true) {
-                    return Err(LnError::SameFile(src.to_owned(), dst.to_owned()).into());
+                    // Even in force overwrite mode, verify we are not targeting the same entry and return a SameFile error if so
+                    let same_entry = match (
+                        canonicalize(src, MissingHandling::Missing, ResolveMode::Physical),
+                        canonicalize(dst, MissingHandling::Missing, ResolveMode::Physical),
+                    ) {
+                        (Ok(src), Ok(dst)) => src == dst,
+                        _ => true,
+                    };
+                    if same_entry {
+                        return Err(LnError::SameFile(src.to_owned(), dst.to_owned()).into());
+                    }
                 }
                 if fs::remove_file(dst).is_ok() {}
                 // In case of error, don't do anything

tests/by-util/test_ln.rs

@@ -793,6 +793,52 @@ fn test_symlink_remove_existing_same_src_and_dest() {
     assert_eq!(at.read("a"), "sample");
 }
 
+#[test]
+fn test_force_same_file_detected_after_canonicalization() {
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    at.write("file", "hello");
+
+    ucmd.args(&["-f", "file", "./file"])
+        .fails_with_code(1)
+        .stderr_contains("are the same file");
+
+    assert!(at.file_exists("file"));
+    assert_eq!(at.read("file"), "hello");
+}
+
+#[test]
+#[cfg(not(target_os = "android"))]
+fn test_force_ln_existing_hard_link_entry() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    at.write("file", "hardlink\n");
+    at.mkdir("dir");
+
+    scene.ucmd().args(&["file", "dir"]).succeeds().no_stderr();
+    assert!(at.file_exists("dir/file"));
+
+    scene
+        .ucmd()
+        .args(&["-f", "file", "dir"])
+        .succeeds()
+        .no_stderr();
+
+    assert!(at.file_exists("file"));
+    assert!(at.file_exists("dir/file"));
+    assert_eq!(at.read("file"), "hardlink\n");
+    assert_eq!(at.read("dir/file"), "hardlink\n");
+
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::MetadataExt;
+        let source_inode = at.metadata("file").ino();
+        let target_inode = at.metadata("dir/file").ino();
+        assert_eq!(source_inode, target_inode);
+    }
+}
+
 #[test]
 #[cfg(not(target_os = "android"))]
 fn test_ln_seen_file() {