rootless: allow single mappings

we were playing safe and not allowed any container to have less than
65536 mappings.  There are a couple of reasons to change it:

- it blocked libpod to work in an environment where
  newuidmap/newgidmap are not available, or not configured.

- not allowed to use different partitions of subuids, where each user
  has less than 65536 ids available.

Hopefully this change in containers/storage:

https://github.com/containers/storage/pull/303

will make error clearers if there are not enough IDs for the image
that is being used.

Closes: https://github.com/containers/libpod/issues/1651

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/rootless/rootless_linux.go

@@ -11,7 +11,6 @@ import (
 	"os/user"
 	"runtime"
 	"strconv"
-	"strings"
 	"sync"
 	"syscall"
 	"unsafe"
@@ -192,22 +191,6 @@ func JoinNSPath(path string) (bool, int, error) {
 	return true, int(ret), nil
 }
 
-const defaultMinimumMappings = 65536
-
-func getMinimumIDs(p string) int {
-	content, err := ioutil.ReadFile(p)
-	if err != nil {
-		logrus.Debugf("error reading data from %q, use a default value of %d", p, defaultMinimumMappings)
-		return defaultMinimumMappings
-	}
-	ret, err := strconv.Atoi(strings.TrimSuffix(string(content), "\n"))
-	if err != nil {
-		logrus.Debugf("error reading data from %q, use a default value of %d", p, defaultMinimumMappings)
-		return defaultMinimumMappings
-	}
-	return ret + 1
-}
-
 // BecomeRootInUserNS re-exec podman in a new userNS.  It returns whether podman was re-executed
 // into a new user namespace and the return code from the re-executed podman process.
 // If podman was re-executed the caller needs to propagate the error code returned by the child
@@ -237,47 +220,18 @@ func BecomeRootInUserNS() (bool, int, error) {
 		return false, -1, errors.Errorf("cannot re-exec process")
 	}
 
-	allowSingleIDMapping := os.Getenv("PODMAN_ALLOW_SINGLE_ID_MAPPING_IN_USERNS") != ""
-
 	var uids, gids []idtools.IDMap
 	username := os.Getenv("USER")
 	if username == "" {
 		user, err := user.LookupId(fmt.Sprintf("%d", os.Getuid()))
-		if err != nil && !allowSingleIDMapping {
-			if os.IsNotExist(err) {
-				return false, 0, errors.Wrapf(err, "/etc/subuid or /etc/subgid does not exist, see subuid/subgid man pages for information on these files")
-			}
-			return false, 0, errors.Wrapf(err, "could not find user by UID nor USER env was set")
-		}
 		if err == nil {
 			username = user.Username
 		}
 	}
 	mappings, err := idtools.NewIDMappings(username, username)
-	if !allowSingleIDMapping {
+	if err != nil {
-		if err != nil {
+		logrus.Warnf("cannot find mappings for user %s: %v", username, err)
-			return false, -1, err
+	} else {
-		}
-
-		availableGIDs, availableUIDs := 0, 0
-		for _, i := range mappings.UIDs() {
-			availableUIDs += i.Size
-		}
-
-		minUIDs := getMinimumIDs("/proc/sys/kernel/overflowuid")
-		if availableUIDs < minUIDs {
-			return false, 0, fmt.Errorf("not enough UIDs available for the user, at least %d are needed", minUIDs)
-		}
-
-		for _, i := range mappings.GIDs() {
-			availableGIDs += i.Size
-		}
-		minGIDs := getMinimumIDs("/proc/sys/kernel/overflowgid")
-		if availableGIDs < minGIDs {
-			return false, 0, fmt.Errorf("not enough GIDs available for the user, at least %d are needed", minGIDs)
-		}
-	}
-	if err == nil {
 		uids = mappings.UIDs()
 		gids = mappings.GIDs()
 	}
@@ -285,12 +239,10 @@ func BecomeRootInUserNS() (bool, int, error) {
 	uidsMapped := false
 	if mappings != nil && uids != nil {
 		err := tryMappingTool("newuidmap", pid, os.Getuid(), uids)
-		if !allowSingleIDMapping && err != nil {
-			return false, 0, err
-		}
 		uidsMapped = err == nil
 	}
 	if !uidsMapped {
+		logrus.Warnf("using rootless single mapping into the namespace. This might break some images. Check /etc/subuid and /etc/subgid for adding subids")
 		setgroups := fmt.Sprintf("/proc/%d/setgroups", pid)
 		err = ioutil.WriteFile(setgroups, []byte("deny\n"), 0666)
 		if err != nil {
@@ -307,9 +259,6 @@ func BecomeRootInUserNS() (bool, int, error) {
 	gidsMapped := false
 	if mappings != nil && gids != nil {
 		err := tryMappingTool("newgidmap", pid, os.Getgid(), gids)
-		if !allowSingleIDMapping && err != nil {
-			return false, 0, err
-		}
 		gidsMapped = err == nil
 	}
 	if !gidsMapped {

test/e2e/rootless_test.go

@@ -126,7 +126,6 @@ var _ = Describe("Podman rootless", func() {
 			env := os.Environ()
 			env = append(env, fmt.Sprintf("XDG_RUNTIME_DIR=%s", xdgRuntimeDir))
 			env = append(env, fmt.Sprintf("HOME=%s", home))
-			env = append(env, "PODMAN_ALLOW_SINGLE_ID_MAPPING_IN_USERNS=1")
 			env = append(env, "USER=foo")
 
 			cmd := rootlessTest.PodmanAsUser([]string{"pod", "create", "--infra=false"}, 1000, 1000, "", env)
@@ -171,7 +170,6 @@ var _ = Describe("Podman rootless", func() {
 			env := os.Environ()
 			env = append(env, fmt.Sprintf("XDG_RUNTIME_DIR=%s", xdgRuntimeDir))
 			env = append(env, fmt.Sprintf("HOME=%s", home))
-			env = append(env, "PODMAN_ALLOW_SINGLE_ID_MAPPING_IN_USERNS=1")
 			env = append(env, "USER=foo")
 
 			allArgs := append([]string{"run"}, args...)