Introduce and use internal/linux

This package is to provide unix.* wrappers to ensure that:
 - they retry on EINTR;
 - a "rich" error is returned on failure.

 A first such wrapper, Sendmsg, is introduced.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

internal/linux/eintr.go

@@ -0,0 +1,18 @@
+package linux
+
+import (
+	"errors"
+
+	"golang.org/x/sys/unix"
+)
+
+// retryOnEINTR takes a function that returns an error and calls it
+// until the error returned is not EINTR.
+func retryOnEINTR(fn func() error) error {
+	for {
+		err := fn()
+		if !errors.Is(err, unix.EINTR) {
+			return err
+		}
+	}
+}

internal/linux/linux.go

@@ -0,0 +1,15 @@
+package linux
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// Sendmsg wraps [unix.Sendmsg].
+func Sendmsg(fd int, p, oob []byte, to unix.Sockaddr, flags int) error {
+	err := retryOnEINTR(func() error {
+		return unix.Sendmsg(fd, p, oob, to, flags)
+	})
+	return os.NewSyscallError("sendmsg", err)
+}

libcontainer/utils/cmsg.go

@@ -21,6 +21,7 @@ import (
 	"os"
 	"runtime"
 
+	"github.com/opencontainers/runc/internal/linux"
 	"golang.org/x/sys/unix"
 )
 
@@ -126,10 +127,5 @@ func SendFile(socket *os.File, file *os.File) error {
 // SendRawFd sends a specific file descriptor over the given AF_UNIX socket.
 func SendRawFd(socket *os.File, msg string, fd uintptr) error {
 	oob := unix.UnixRights(int(fd))
-	for {
-		err := unix.Sendmsg(int(socket.Fd()), []byte(msg), oob, nil, 0)
-		if err != unix.EINTR {
-			return os.NewSyscallError("sendmsg", err)
-		}
-	}
+	return linux.Sendmsg(int(socket.Fd()), []byte(msg), oob, nil, 0)
 }

notify_socket.go

@@ -11,6 +11,7 @@ import (
 	"strconv"
 	"time"
 
+	"github.com/opencontainers/runc/internal/linux"
 	"github.com/opencontainers/runc/libcontainer"
 	"github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/sirupsen/logrus"
@@ -181,7 +182,7 @@ func sdNotifyBarrier(client *net.UnixConn) error {
 		return err
 	}
 
-	// Get the FD for the unix socket file to be able to do perform syscall.Sendmsg.
+	// Get the FD for the unix socket file to be able to use sendmsg.
 	clientFd, err := client.File()
 	if err != nil {
 		return err
@@ -189,9 +190,9 @@ func sdNotifyBarrier(client *net.UnixConn) error {
 
 	// Send the write end of the pipe along with a BARRIER=1 message.
 	fdRights := unix.UnixRights(int(pipeW.Fd()))
-	err = unix.Sendmsg(int(clientFd.Fd()), []byte("BARRIER=1"), fdRights, nil, 0)
+	err = linux.Sendmsg(int(clientFd.Fd()), []byte("BARRIER=1"), fdRights, nil, 0)
 	if err != nil {
-		return &os.SyscallError{Syscall: "sendmsg", Err: err}
+		return err
 	}
 
 	// Close our copy of pipeW.