dmz: use sendfile(2) when cloning /proc/self/exe

This results in a 5-20% speedup of dmz.CloneBinary(), depending on the
machine.

io.Copy:

  goos: linux
  goarch: amd64
  pkg: github.com/opencontainers/runc/libcontainer/dmz
  cpu: Intel(R) Core(TM) i5-10210U CPU @ 1.60GHz
  BenchmarkCloneBinary
  BenchmarkCloneBinary-8               139           8075074 ns/op
  PASS
  ok      github.com/opencontainers/runc/libcontainer/dmz 2.286s

unix.Sendfile:

  goos: linux
  goarch: amd64
  pkg: github.com/opencontainers/runc/libcontainer/dmz
  cpu: Intel(R) Core(TM) i5-10210U CPU @ 1.60GHz
  BenchmarkCloneBinary
  BenchmarkCloneBinary-8               192           6382121 ns/op
  PASS
  ok      github.com/opencontainers/runc/libcontainer/dmz 2.415s

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

libcontainer/system/linux.go

@@ -5,6 +5,7 @@ package system
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"strconv"
@@ -174,3 +175,42 @@ func ExecutableMemfd(comment string, flags int) (*os.File, error) {
 	}
 	return os.NewFile(uintptr(memfd), "/memfd:"+comment), nil
 }
+
+// Copy is like io.Copy except it uses sendfile(2) if the source and sink are
+// both (*os.File) as an optimisation to make copies faster.
+func Copy(dst io.Writer, src io.Reader) (copied int64, err error) {
+	dstFile, _ := dst.(*os.File)
+	srcFile, _ := src.(*os.File)
+
+	if dstFile != nil && srcFile != nil {
+		fi, err := srcFile.Stat()
+		if err != nil {
+			goto fallback
+		}
+		size := fi.Size()
+		for size > 0 {
+			n, err := unix.Sendfile(int(dstFile.Fd()), int(srcFile.Fd()), nil, int(size))
+			if n > 0 {
+				size -= int64(n)
+				copied += int64(n)
+			}
+			if err == unix.EINTR {
+				continue
+			}
+			if err != nil {
+				if copied == 0 {
+					// If we haven't copied anything so far, we can safely just
+					// fallback to io.Copy. We could always do the fallback but
+					// it's safer to error out in the case of a partial copy
+					// followed by an error (which should never happen).
+					goto fallback
+				}
+				return copied, fmt.Errorf("partial sendfile copy: %w", err)
+			}
+		}
+		return copied, nil
+	}
+
+fallback:
+	return io.Copy(dst, src)
+}