context: follow symlink roots during Walk

context.go

@@ -587,10 +587,7 @@ func (c *context) Walk(fn filepath.WalkFunc) error {
 	root := c.root
 	fi, err := c.driver.Lstat(c.root)
 	if err == nil && fi.Mode()&os.ModeSymlink != 0 {
-		root, err = c.driver.Readlink(c.root)
-		if err != nil {
-			return err
-		}
+		root += string(c.pathDriver.Separator())
 	}
 	return c.pathDriver.Walk(root, func(p string, fi os.FileInfo, _ error) error {
 		contained, err := c.containWithRoot(p, root)

context_unix_test.go

@@ -0,0 +1,101 @@
+//go:build !windows
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package continuity
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"reflect"
+	"testing"
+)
+
+func TestContextWalkSymlinkRoot(t *testing.T) {
+	root := t.TempDir()
+
+	target := filepath.Join(root, "target")
+	if err := os.MkdirAll(filepath.Join(target, "subdir"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(target, "subdir", "file"), []byte("content"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	links := filepath.Join(root, "links")
+	if err := os.Mkdir(links, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	link := filepath.Join(links, "root")
+	if err := os.Symlink("../target", link); err != nil {
+		t.Skipf("symlink not supported: %v", err)
+	}
+	absoluteLink := filepath.Join(links, "absolute-root")
+	if err := os.Symlink(target, absoluteLink); err != nil {
+		t.Skipf("absolute symlink not supported: %v", err)
+	}
+	chain := filepath.Join(links, "chain")
+	if err := os.Symlink("root", chain); err != nil {
+		t.Skipf("symlink-to-symlink not supported: %v", err)
+	}
+
+	for _, tc := range []struct {
+		name string
+		root string
+	}{
+		{
+			name: "relative_symlink",
+			root: link,
+		},
+		{
+			name: "absolute_symlink",
+			root: absoluteLink,
+		},
+		{
+			name: "symlink_to_symlink",
+			root: chain,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			ctx, err := NewContext(tc.root)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			var got []string
+			err = ctx.Walk(func(p string, fi os.FileInfo, err error) error {
+				if err != nil {
+					return err
+				}
+				if fi == nil {
+					return fmt.Errorf("missing file info for %q", p)
+				}
+				got = append(got, filepath.ToSlash(p))
+				return nil
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			want := []string{"/", "/subdir", "/subdir/file"}
+			if !reflect.DeepEqual(got, want) {
+				t.Fatalf("paths mismatch\nwant: %v\n got: %v", want, got)
+			}
+		})
+	}
+}