mount: add string representation of mount flags

When reading mount errors, it is quite hard to make sense of mount flags
in their hex form. As this is the error path, the minor performance
impact of constructing a string is probably not worth hyper-optimising.

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

libcontainer/mount_linux.go

@@ -6,6 +6,7 @@ import (
 	"io/fs"
 	"os"
 	"strconv"
+	"strings"
 
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
@@ -45,6 +46,64 @@ type mountError struct {
 	err     error
 }
 
+// int32plus is a collection of int types with >=32 bits.
+type int32plus interface {
+	int | uint | int32 | uint32 | int64 | uint64 | uintptr
+}
+
+// stringifyMountFlags converts mount(2) flags to a string that you can use in
+// error messages.
+func stringifyMountFlags[Int int32plus](flags Int) string {
+	flagNames := []struct {
+		name string
+		bits Int
+	}{
+		{"MS_RDONLY", unix.MS_RDONLY},
+		{"MS_NOSUID", unix.MS_NOSUID},
+		{"MS_NODEV", unix.MS_NODEV},
+		{"MS_NOEXEC", unix.MS_NOEXEC},
+		{"MS_SYNCHRONOUS", unix.MS_SYNCHRONOUS},
+		{"MS_REMOUNT", unix.MS_REMOUNT},
+		{"MS_MANDLOCK", unix.MS_MANDLOCK},
+		{"MS_DIRSYNC", unix.MS_DIRSYNC},
+		{"MS_NOSYMFOLLOW", unix.MS_NOSYMFOLLOW},
+		// No (1 << 9) flag.
+		{"MS_NOATIME", unix.MS_NOATIME},
+		{"MS_NODIRATIME", unix.MS_NODIRATIME},
+		{"MS_BIND", unix.MS_BIND},
+		{"MS_MOVE", unix.MS_MOVE},
+		{"MS_REC", unix.MS_REC},
+		// MS_VERBOSE was deprecated and swapped to MS_SILENT.
+		{"MS_SILENT", unix.MS_SILENT},
+		{"MS_POSIXACL", unix.MS_POSIXACL},
+		{"MS_UNBINDABLE", unix.MS_UNBINDABLE},
+		{"MS_PRIVATE", unix.MS_PRIVATE},
+		{"MS_SLAVE", unix.MS_SLAVE},
+		{"MS_SHARED", unix.MS_SHARED},
+		{"MS_RELATIME", unix.MS_RELATIME},
+		// MS_KERNMOUNT (1 << 22) is internal to the kernel.
+		{"MS_I_VERSION", unix.MS_I_VERSION},
+		{"MS_STRICTATIME", unix.MS_STRICTATIME},
+		{"MS_LAZYTIME", unix.MS_LAZYTIME},
+	}
+	var (
+		flagSet  []string
+		seenBits Int
+	)
+	for _, flag := range flagNames {
+		if flags&flag.bits == flag.bits {
+			seenBits |= flag.bits
+			flagSet = append(flagSet, flag.name)
+		}
+	}
+	// If there were any remaining flags specified we don't know the name of,
+	// just add them in an 0x... format.
+	if remaining := flags &^ seenBits; remaining != 0 {
+		flagSet = append(flagSet, "0x"+strconv.FormatUint(uint64(remaining), 16))
+	}
+	return strings.Join(flagSet, "|")
+}
+
 // Error provides a string error representation.
 func (e *mountError) Error() string {
 	out := e.op + " "
@@ -62,7 +121,7 @@ func (e *mountError) Error() string {
 	}
 
 	if e.flags != uintptr(0) {
-		out += ", flags=0x" + strconv.FormatUint(uint64(e.flags), 16)
+		out += ", flags=" + stringifyMountFlags(e.flags)
 	}
 	if e.data != "" {
 		out += ", data=" + e.data

libcontainer/mount_linux_test.go

@@ -0,0 +1,54 @@
+package libcontainer
+
+import (
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestStringifyMountFlags(t *testing.T) {
+	for _, test := range []struct {
+		name     string
+		flags    uintptr
+		expected string
+	}{
+		{"Empty", 0, ""},
+		// Single valid flags.
+		{"Single-MS_RDONLY", unix.MS_RDONLY, "MS_RDONLY"},
+		{"Single-MS_NOSUID", unix.MS_NOSUID, "MS_NOSUID"},
+		{"Single-MS_NODEV", unix.MS_NODEV, "MS_NODEV"},
+		{"Single-MS_NOEXEC", unix.MS_NOEXEC, "MS_NOEXEC"},
+		{"Single-MS_SYNCHRONOUS", unix.MS_SYNCHRONOUS, "MS_SYNCHRONOUS"},
+		{"Single-MS_REMOUNT", unix.MS_REMOUNT, "MS_REMOUNT"},
+		{"Single-MS_MANDLOCK", unix.MS_MANDLOCK, "MS_MANDLOCK"},
+		{"Single-MS_DIRSYNC", unix.MS_DIRSYNC, "MS_DIRSYNC"},
+		{"Single-MS_NOSYMFOLLOW", unix.MS_NOSYMFOLLOW, "MS_NOSYMFOLLOW"},
+		{"Single-MS_NOATIME", unix.MS_NOATIME, "MS_NOATIME"},
+		{"Single-MS_NODIRATIME", unix.MS_NODIRATIME, "MS_NODIRATIME"},
+		{"Single-MS_BIND", unix.MS_BIND, "MS_BIND"},
+		{"Single-MS_MOVE", unix.MS_MOVE, "MS_MOVE"},
+		{"Single-MS_REC", unix.MS_REC, "MS_REC"},
+		{"Single-MS_SILENT", unix.MS_SILENT, "MS_SILENT"},
+		{"Single-MS_POSIXACL", unix.MS_POSIXACL, "MS_POSIXACL"},
+		{"Single-MS_UNBINDABLE", unix.MS_UNBINDABLE, "MS_UNBINDABLE"},
+		{"Single-MS_PRIVATE", unix.MS_PRIVATE, "MS_PRIVATE"},
+		{"Single-MS_SLAVE", unix.MS_SLAVE, "MS_SLAVE"},
+		{"Single-MS_SHARED", unix.MS_SHARED, "MS_SHARED"},
+		{"Single-MS_RELATIME", unix.MS_RELATIME, "MS_RELATIME"},
+		{"Single-MS_KERNMOUNT", unix.MS_KERNMOUNT, "0x400000"},
+		{"Single-MS_I_VERSION", unix.MS_I_VERSION, "MS_I_VERSION"},
+		{"Single-MS_STRICTATIME", unix.MS_STRICTATIME, "MS_STRICTATIME"},
+		{"Single-MS_LAZYTIME", unix.MS_LAZYTIME, "MS_LAZYTIME"},
+		// Invalid flag value.
+		{"Unknown-512", 1 << 9, "0x200"},
+		// Multiple flag values at the same time.
+		{"Multiple-Valid1", unix.MS_RDONLY | unix.MS_REC | unix.MS_BIND, "MS_RDONLY|MS_BIND|MS_REC"},
+		{"Multiple-Valid2", unix.MS_NOSUID | unix.MS_NODEV | unix.MS_NOEXEC | unix.MS_REC | unix.MS_NODIRATIME | unix.MS_I_VERSION, "MS_NOSUID|MS_NODEV|MS_NOEXEC|MS_NODIRATIME|MS_REC|MS_I_VERSION"},
+		{"Multiple-Mixed", unix.MS_REC | unix.MS_BIND | (1 << 9) | (1 << 31), "MS_BIND|MS_REC|0x80000200"},
+	} {
+		got := stringifyMountFlags(test.flags)
+		if got != test.expected {
+			t.Errorf("%s: stringifyMountFlags(0x%x) = %q, expected %q", test.name, test.flags, got, test.expected)
+		}
+	}
+}