archive: use Linkat on FreeBSD for hardlinks

Calling link(2) with a symlink as the target will cause FreeBSD to
attempt to create a new hard link pointing to the target of the symlink
rather than a hardlink to the symlink itself. By contrast, Linux creates
a hardlink to the symlink.

Use linkat(2) instead, which accepts a flag controlling this behavior.
If linkat(2) is called with AT_SYMLINK_FOLLOW, it will behave the same
as link(2).  If linkat(2) is called without AT_SYMLINK_FOLLOW, it will
behave the same as Linux's link(2) instead.

See FreeBSD's implementation of ln(1), which uses linkat(2) and controls
this behavior by way of the -P and -L flags:
3003117253/bin/ln/ln.c (L342-L343)

Signed-off-by: Samuel Karp <me@samuelkarp.com>

archive/link_default.go

@@ -0,0 +1,25 @@
+//go:build !freebsd
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package archive
+
+import "os"
+
+func link(oldname, newname string) error {
+	return os.Link(oldname, newname)
+}

archive/link_freebsd.go

@@ -0,0 +1,82 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package archive
+
+import (
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func link(oldname, newname string) error {
+	e := ignoringEINTR(func() error {
+		return unix.Linkat(unix.AT_FDCWD, oldname, unix.AT_FDCWD, newname, 0)
+	})
+	if e != nil {
+		return &os.LinkError{Op: "link", Old: oldname, New: newname, Err: e}
+	}
+	return nil
+}
+
+// The following contents were copied from Go 1.18.2.
+// Use of this source code is governed by the following
+// BSD-style license:
+//
+// Copyright (c) 2009 The Go Authors. All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//   * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//   * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//   * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// ignoringEINTR makes a function call and repeats it if it returns an
+// EINTR error. This appears to be required even though we install all
+// signal handlers with SA_RESTART: see #22838, #38033, #38836, #40846.
+// Also #20400 and #36644 are issues in which a signal handler is
+// installed without setting SA_RESTART. None of these are the common case,
+// but there are enough of them that it seems that we can't avoid
+// an EINTR loop.
+func ignoringEINTR(fn func() error) error {
+	for {
+		err := fn()
+		if err != syscall.EINTR {
+			return err
+		}
+	}
+}

archive/tar.go

@@ -361,7 +361,7 @@ func createTarFile(ctx context.Context, path, extractDir string, hdr *tar.Header
 			return err
 		}
 
-		if err := os.Link(targetPath, path); err != nil {
+		if err := link(targetPath, path); err != nil {
 			return err
 		}