ln: don't refuse --backup when src/dst are different names for the same inode

src/uu/ln/src/ln.rs

@@ -402,6 +402,17 @@ fn relative_path<'a>(src: &'a Path, dst: &Path) -> Cow<'a, Path> {
     src.into()
 }
 
+/// Decide whether `src` and `dst` are actually the same directory entry.
+fn is_same_entry(src: &Path, dst: &Path) -> bool {
+    match (
+        canonicalize(src, MissingHandling::Missing, ResolveMode::Physical),
+        canonicalize(dst, MissingHandling::Missing, ResolveMode::Physical),
+    ) {
+        (Ok(src), Ok(dst)) => src == dst,
+        _ => true,
+    }
+}
+
 #[allow(clippy::cognitive_complexity)]
 fn link(src: &Path, dst: &Path, settings: &Settings) -> LnResult<()> {
     let mut backup_path = None;
@@ -415,7 +426,7 @@ fn link(src: &Path, dst: &Path, settings: &Settings) -> LnResult<()> {
         backup_path = backup_control::get_backup_path(settings.backup, dst, &settings.suffix);
         if settings.backup == BackupMode::Existing && !settings.symbolic {
             // when ln --backup f f, it should detect that it is the same file
-            if paths_refer_to_same_file(src, dst, true) {
+            if paths_refer_to_same_file(src, dst, true) && is_same_entry(src, dst) {
                 return Err(LnError::SameFile(src.to_owned(), dst.to_owned()));
             }
         }
@@ -438,18 +449,12 @@ fn link(src: &Path, dst: &Path, settings: &Settings) -> LnResult<()> {
                 // In case of error, don't do anything
             }
             OverwriteMode::Force => {
-                if !dst.is_symlink() && paths_refer_to_same_file(src, dst, true) {
+                if !dst.is_symlink()
+                    && paths_refer_to_same_file(src, dst, true)
+                    && is_same_entry(src, dst)
+                {
                     // 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()));
-                    }
+                    return Err(LnError::SameFile(src.to_owned(), dst.to_owned()));
                 }
                 let _ = fs::remove_file(dst);
                 // In case of error, don't do anything

tests/by-util/test_ln.rs

@@ -5,6 +5,8 @@
 #![allow(clippy::similar_names)]
 
 use std::path::PathBuf;
+#[cfg(all(unix, not(target_os = "android")))]
+use std::time::Duration;
 use uutests::at_and_ucmd;
 use uutests::new_ucmd;
 use uutests::util::TestScenario;
@@ -840,6 +842,38 @@ fn test_backup_same_file() {
         .stderr_contains("n: 'file1' and './file1' are the same file");
 }
 
+#[test]
+#[cfg(not(target_os = "android"))]
+fn test_backup_existing_hard_linked_under_different_name() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.touch("a");
+    at.hard_link("a", "b");
+
+    ucmd.args(&["--backup", "a", "b"]).succeeds().no_stderr();
+
+    assert!(at.file_exists("a"));
+    assert!(at.file_exists("b"));
+    assert!(at.file_exists("b~"));
+}
+
+#[test]
+#[cfg(all(unix, not(target_os = "android")))]
+fn test_backup_existing_hard_linked_target_is_fifo() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.touch("a");
+    at.hard_link("a", "b");
+    at.mkfifo("b~");
+
+    ucmd.args(&["--backup", "a", "b"])
+        .timeout(Duration::from_secs(10))
+        .succeeds()
+        .no_stderr();
+
+    assert!(at.file_exists("a"));
+    assert!(at.file_exists("b"));
+    assert!(!at.is_fifo("b~"));
+}
+
 #[test]
 fn test_backup_force() {
     let scene = TestScenario::new(util_name!());