cp: Fix broken symlinks to parent-dir

According to the GNU cp official documentation[1], a symlink is only
created if all source paths are absolute, unless the destination files
are in the current directory.

This fix solves this GNU cp incompatibility by:
- Adding verifications when handling symlinks, to ensure that a symlink
  with relative path are only created if the destination is within the
  current directory.
- Adding unit tests to cover this modifications, and ensure the behavior
  is correctly align with GNU documentation.

[1]:
https://www.gnu.org/software/coreutils/manual/html_node/cp-invocation.html#index-_002ds-16

src/uu/cp/src/cp.rs

@@ -7,6 +7,7 @@
 use quick_error::quick_error;
 use std::cmp::Ordering;
 use std::collections::{HashMap, HashSet};
+use std::env;
 #[cfg(not(windows))]
 use std::ffi::CString;
 use std::fs::{self, File, Metadata, OpenOptions, Permissions};
@@ -1864,6 +1865,22 @@ fn handle_copy_mode(
             if dest.exists() && options.overwrite == OverwriteMode::Clobber(ClobberMode::Force) {
                 fs::remove_file(dest)?;
             }
+            if source.is_relative() {
+                let current_dir = env::current_dir()?;
+                let abs_dest =
+                    canonicalize(dest, MissingHandling::Missing, ResolveMode::Physical).unwrap();
+                if let Some(parent) = abs_dest.parent() {
+                    if parent != current_dir {
+                        {
+                            return Err(format!(
+                                "{}: can make relative symbolic links only in current directory",
+                                dest.maybe_quote()
+                            )
+                            .into());
+                        }
+                    }
+                }
+            }
             symlink_file(source, dest, symlinked_files)?;
         }
         CopyMode::Update => {

tests/by-util/test_cp.rs

@@ -5573,3 +5573,31 @@ fn test_preserve_attrs_overriding_2() {
         assert_ne!(dest_file1_metadata.ino(), dest_file2_metadata.ino());
     }
 }
+
+#[test]
+fn test_symlink_to_subdir() {
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    at.touch("file");
+    at.mkdir("dir");
+
+    ucmd.args(&["--symbolic", "file", "dir"])
+        .fails()
+        .no_stdout()
+        .stderr_is("cp: dir/file: can make relative symbolic links only in current directory\n");
+
+    assert!(at.dir_exists("dir"));
+    assert!(!at.file_exists("dir/file"));
+}
+
+#[test]
+fn test_symlink_from_subdir() {
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    at.mkdir("dir");
+    at.touch("dir/file");
+
+    ucmd.args(&["--symbolic", "dir/file", "."]).succeeds();
+
+    assert!(at.symlink_exists("file"));
+}