archive: disable looking up usernames and groupnames on the host

These strings were resolved using the user information on the host
filesystem and were decreasing reproducibility.

For moby/buildkit issue 3688

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

.golangci.yml

@@ -29,6 +29,9 @@ issues:
       text: "blank-imports:"
     - path: 'contrib[\\/]fuzz[\\/]'
       text: "exported: func name will be used as fuzz.Fuzz"
+    - path: 'archive[\\/]tarheader[\\/]'
+      # conversion is necessary on Linux, unnecessary on macOS
+      text: "unnecessary conversion"
 
 linters-settings:
   gosec:

archive/tar.go

@@ -29,6 +29,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/containerd/containerd/archive/tarheader"
 	"github.com/containerd/containerd/log"
 	"github.com/containerd/containerd/pkg/epoch"
 	"github.com/containerd/containerd/pkg/userns"
@@ -586,7 +587,8 @@ func (cw *ChangeWriter) HandleChange(k fs.ChangeKind, p string, f os.FileInfo, e
 			}
 		}
 
-		hdr, err := tar.FileInfoHeader(f, link)
+		// Use FileInfoHeaderNoLookups to avoid propagating user names and group names from the host
+		hdr, err := tarheader.FileInfoHeaderNoLookups(f, link)
 		if err != nil {
 			return err
 		}

archive/tarheader/tarheader.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.
+*/
+
+/*
+   Portions from https://github.com/moby/moby/blob/v23.0.1/pkg/archive/archive.go#L419-L464
+   Copyright (C) Docker/Moby authors.
+   Licensed under the Apache License, Version 2.0
+   NOTICE: https://github.com/moby/moby/blob/v23.0.1/NOTICE
+*/
+
+package tarheader
+
+import (
+	"archive/tar"
+	"os"
+)
+
+// nosysFileInfo hides the system-dependent info of the wrapped FileInfo to
+// prevent tar.FileInfoHeader from introspecting it and potentially calling into
+// glibc.
+//
+// From https://github.com/moby/moby/blob/v23.0.1/pkg/archive/archive.go#L419-L434 .
+type nosysFileInfo struct {
+	os.FileInfo
+}
+
+func (fi nosysFileInfo) Sys() interface{} {
+	// A Sys value of type *tar.Header is safe as it is system-independent.
+	// The tar.FileInfoHeader function copies the fields into the returned
+	// header without performing any OS lookups.
+	if sys, ok := fi.FileInfo.Sys().(*tar.Header); ok {
+		return sys
+	}
+	return nil
+}
+
+// sysStat, if non-nil, populates hdr from system-dependent fields of fi.
+//
+// From https://github.com/moby/moby/blob/v23.0.1/pkg/archive/archive.go#L436-L437 .
+var sysStat func(fi os.FileInfo, hdr *tar.Header) error
+
+// FileInfoHeaderNoLookups creates a partially-populated tar.Header from fi.
+//
+// Compared to the archive/tar.FileInfoHeader function, this function is safe to
+// call from a chrooted process as it does not populate fields which would
+// require operating system lookups. It behaves identically to
+// tar.FileInfoHeader when fi is a FileInfo value returned from
+// tar.Header.FileInfo().
+//
+// When fi is a FileInfo for a native file, such as returned from os.Stat() and
+// os.Lstat(), the returned Header value differs from one returned from
+// tar.FileInfoHeader in the following ways. The Uname and Gname fields are not
+// set as OS lookups would be required to populate them. The AccessTime and
+// ChangeTime fields are not currently set (not yet implemented) although that
+// is subject to change. Callers which require the AccessTime or ChangeTime
+// fields to be zeroed should explicitly zero them out in the returned Header
+// value to avoid any compatibility issues in the future.
+//
+// From https://github.com/moby/moby/blob/v23.0.1/pkg/archive/archive.go#L439-L464 .
+func FileInfoHeaderNoLookups(fi os.FileInfo, link string) (*tar.Header, error) {
+	hdr, err := tar.FileInfoHeader(nosysFileInfo{fi}, link)
+	if err != nil {
+		return nil, err
+	}
+	if sysStat != nil {
+		return hdr, sysStat(fi, hdr)
+	}
+	return hdr, nil
+}

archive/tarheader/tarheader_unix.go

@@ -0,0 +1,59 @@
+//go:build !windows
+
+/*
+   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.
+*/
+
+/*
+   Portions from https://github.com/moby/moby/blob/v23.0.1/pkg/archive/archive_unix.go#L52-L70
+   Copyright (C) Docker/Moby authors.
+   Licensed under the Apache License, Version 2.0
+   NOTICE: https://github.com/moby/moby/blob/v23.0.1/NOTICE
+*/
+
+package tarheader
+
+import (
+	"archive/tar"
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func init() {
+	sysStat = statUnix
+}
+
+// statUnix populates hdr from system-dependent fields of fi without performing
+// any OS lookups.
+// From https://github.com/moby/moby/blob/v23.0.1/pkg/archive/archive_unix.go#L52-L70
+func statUnix(fi os.FileInfo, hdr *tar.Header) error {
+	s, ok := fi.Sys().(*syscall.Stat_t)
+	if !ok {
+		return nil
+	}
+
+	hdr.Uid = int(s.Uid)
+	hdr.Gid = int(s.Gid)
+
+	if s.Mode&unix.S_IFBLK != 0 ||
+		s.Mode&unix.S_IFCHR != 0 {
+		hdr.Devmajor = int64(unix.Major(uint64(s.Rdev)))
+		hdr.Devminor = int64(unix.Minor(uint64(s.Rdev)))
+	}
+
+	return nil
+}