entry: convert hard link absolute paths to relative ones

When unpacking a docker image, some hard links may be pointing to, say,
/bin/gunzip. Those were failing the validate_inside_dst() check,
preventing to unpack the image.

Fix this by re-using the same logic as we are already using for actual
files, removing the leading '/' from the link target path.

We do this only if the absolute link is not an ancestor of the target
dir as we want to support this use case according to the existing
absolute_hardlink() test.

src/entry.rs

@@ -467,6 +467,9 @@ impl<'a> EntryFields<'a> {
                     // the destination of this hard link is both present and
                     // inside our own directory. This is needed because we want
                     // to make sure to not overwrite anything outside the root.
+                    // If the link points to an absolute path, assume it's relative
+                    // to the target dir by removing the leading '/', as we already
+                    // do for files, see unpack_in() comments.
                     //
                     // Note that this logic is only needed for hard links
                     // currently. With symlinks the `validate_inside_dst` which
@@ -475,6 +478,16 @@ impl<'a> EntryFields<'a> {
                     // links though they're canonicalized to their existing path
                     // so we need to validate at this time.
                     Some(ref p) => {
+                        let mut src = src.to_path_buf();
+                        if src.is_absolute() {
+                            let dest_canon = p.canonicalize()?;
+                            if !src.starts_with(&dest_canon) {
+                                // Skip root component, making the target relative to the target dir
+                                let components = src.components().skip(1);
+                                src = components.collect();
+                            }
+                        }
+
                         let link_src = p.join(src);
                         self.validate_inside_dst(p, &link_src)?;
                         link_src

tests/entry.rs

@@ -62,12 +62,22 @@ fn absolute_hardlink() {
     header.set_cksum();
     t!(ar.append(&header, &[][..]));
 
+    let mut header = tar::Header::new_gnu();
+    header.set_size(0);
+    header.set_entry_type(tar::EntryType::Link);
+    t!(header.set_path("baz"));
+    // This absolute path on root will be converted when unpacking
+    t!(header.set_link_name("/foo"));
+    header.set_cksum();
+    t!(ar.append(&header, &[][..]));
+
     let bytes = t!(ar.into_inner());
     let mut ar = tar::Archive::new(&bytes[..]);
 
     t!(ar.unpack(td.path()));
     t!(td.path().join("foo").metadata());
     t!(td.path().join("bar").metadata());
+    t!(td.path().join("baz").metadata());
 }
 
 #[test]