erofs: fix wrong assumption on the order of on-disk directory entries

For each directory, _all_ the on-disk directory entries are _strictly_
recorded in alphabetical order. There is no special treatment of "." and
"..". Thanks to @hsiangkao for pointing out this bug.

Fixes: eca83ac68c ("Add initial support for EROFS")
Reported-by: Gao Xiang <xiang@kernel.org>
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>

pkg/erofs/erofs.go

@@ -586,8 +586,7 @@ func (i *Inode) blocks() uint64 {
 }
 
 // IterDirents invokes cb on each entry in the directory represented by this inode.
-// The first two directory entries are "." and "..". The remaining directory entries
-// will be iterated in alphabetical order.
+// The directory entries will be iterated in alphabetical order.
 //
 // https://docs.kernel.org/filesystems/erofs.html#directories
 //
@@ -615,7 +614,7 @@ func (i *Inode) blocks() uint64 {
 //
 // [ (metadata block) inode | optional fields | dirent M+2 | dirent M+3 | name M+2 | name M+3 | optional padding ]
 //
-// All directory entries (except the first two: "." and "..") are _strictly_ recorded in alphabetical order.
+// All directory entries are _strictly_ recorded in alphabetical order.
 func (i *Inode) IterDirents(cb func(name string, typ uint8, nid uint64) error) error {
 	if !i.IsDir() {
 		return linuxerr.ENOTDIR

pkg/sentry/fsimpl/erofs/directory.go

@@ -69,9 +69,6 @@ func (i *inode) lookup(name string) (uint64, error) {
 		return 0, err
 	}
 
-	// Skip "." and ".."
-	dirents = dirents[2:]
-
 	// The dirents are sorted in alphabetical order. We do binary search
 	// to find the target.
 	idx := sort.Search(len(dirents), func(i int) bool {

runsc/container/container_test.go

@@ -20,6 +20,7 @@ import (
 	"io"
 	"io/ioutil"
 	"math"
+	"math/rand"
 	"os"
 	"os/exec"
 	"path"
@@ -3158,6 +3159,15 @@ func TestMountEROFS(t *testing.T) {
 	if err := os.Mkdir(sourceDir, 0755); err != nil {
 		t.Fatalf("os.Mkdir() failed: %v", err)
 	}
+	// Create some files with leading non-alphanumeric characters in name. It's helpful
+	// to verify the on-disk directory entries order.
+	for _, c := range []byte("!#$%&()*+,-:;<=>?@[]^_`{|}~") {
+		name := fmt.Sprintf("%s/%c_file", sourceDir, c)
+		// Create the file with random data.
+		if err := ioutil.WriteFile(name, []byte(fmt.Sprintf("%v", rand.Uint64())), 0644); err != nil {
+			t.Fatalf("error creating %q: %v", name, err)
+		}
+	}
 	testApp, err := testutil.FindFile("test/cmd/test_app/test_app")
 	if err != nil {
 		t.Fatalf("error finding test_app: %v", err)