feat(dockerfile): ability to parse COPY --from=image

Signed-off-by: Furkan <furkan.turkal@trendyol.com>
Co-authored-by: Batuhan <batuhan.apaydin@trendyol.com>

cmd/cosign/cli/dockerfile.go

@@ -118,12 +118,27 @@ func dockerfileResolve() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "resolve",
 		Short: "Resolve the digest of the images and rewrites them with fully qualified image reference",
-		Long:  ``,
+		Long: `Resolve the digest of the images and rewrites them with fully qualified image reference
+
+This command creates a surface which resolves mutable image tags into immutable image digests:
+	- FROM <tag>, it rewrites the Dockerfile to FROM <digest>
+
+Using FROM without <digest> is dangerous because even if what's currently tagged on the registry is signed properly,
+there is a race before the FROM is evaluated (what if it changes!), or (with docker build) it's possible that
+what is in the local cache(!) is what's actually used, and not what was verified! (See issue #648)
+
+This command does NOT do image verification; instead it only rewrites all image tags to corresponding digest(s).
+
+The following image reference definitions are currently supported:
+	-	FROM --platform=linux/amd64 gcr.io/distroless/base AS base
+	-	COPY --from=gcr.io/distroless/base`,
 		Example: `  cosign dockerfile resolve Dockerfile
 		
-		# specify output file
-		cosign dockerfile resolve -o Dockerfile.edited Dockerfile
-`,
+		# print to stdout
+		cosign dockerfile resolve Dockerfile
+
+		# specify a output file
+		cosign dockerfile resolve -o Dockerfile.resolved Dockerfile`,
 		Args: cobra.ExactArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			v := &dockerfile.ResolveDockerfileCommand{

cmd/cosign/cli/dockerfile/resolve.go

@@ -54,9 +54,11 @@ func (c *ResolveDockerfileCommand) Exec(ctx context.Context, args []string) erro
 	}
 
 	if c.Output != "" {
-		_ = os.WriteFile(c.Output, resolvedDockerfile, 0600)
+		if err := os.WriteFile(c.Output, resolvedDockerfile, 0600); err != nil {
+			return fmt.Errorf("failed writing resolved Dockerfile: %w", err)
+		}
 	} else {
-		_, _ = fmt.Fprintln(os.Stdout, string(resolvedDockerfile))
+		fmt.Fprintln(os.Stdout, string(resolvedDockerfile))
 	}
 
 	return nil
@@ -69,8 +71,8 @@ func resolveDigest(dockerfile io.Reader) ([]byte, error) {
 	for fileScanner.Scan() {
 		line := strings.TrimSpace(fileScanner.Text())
 
-		// TODO(developer-guy): support the COPY --from=image:tag cases
-		if strings.HasPrefix(strings.ToUpper(line), "FROM") {
+		if strings.HasPrefix(strings.ToUpper(line), "FROM") ||
+			strings.HasPrefix(strings.ToUpper(line), "COPY") {
 			switch image := getImageFromLine(line); image {
 			case "scratch":
 				tmp.WriteString(line)
@@ -80,6 +82,8 @@ func resolveDigest(dockerfile io.Reader) ([]byte, error) {
 					// we should not return err here since
 					// we can define the image refs smth like
 					// i.e., FROM alpine:$(TAG), FROM $(IMAGE), etc.
+					// TODO: support parameter substitution by passing a `--build-arg` flag
+					fmt.Fprintf(os.Stderr, "WARNING: parameter substitution for images does not supported yet. consider set all environment variables before resolving the Dockerfile. Image: %s.\n", image)
 					tmp.WriteString(line)
 					tmp.WriteString("\n")
 					continue

cmd/cosign/cli/dockerfile/resolve_test.go

@@ -31,6 +31,23 @@ func Test_resolveDigest(t *testing.T) {
 			"happy alpine",
 			`FROM alpine:3.13`,
 			`FROM index.docker.io/library/alpine@sha256:026f721af4cf2843e07bba648e158fb35ecc876d822130633cc49f707f0fc88c
+`,
+			false,
+		},
+		{
+			"happy alpine trim",
+			`   FROM    alpine:3.13   `,
+			`FROM    index.docker.io/library/alpine@sha256:026f721af4cf2843e07bba648e158fb35ecc876d822130633cc49f707f0fc88c
+`,
+			false,
+		},
+		{
+			"happy alpine copy",
+			`FROM alpine:3.13
+COPY --from=alpine:3.13
+`,
+			`FROM index.docker.io/library/alpine@sha256:026f721af4cf2843e07bba648e158fb35ecc876d822130633cc49f707f0fc88c
+COPY --from=index.docker.io/library/alpine@sha256:026f721af4cf2843e07bba648e158fb35ecc876d822130633cc49f707f0fc88c
 `,
 			false,
 		},

cmd/cosign/cli/dockerfile/verify.go

@@ -66,8 +66,11 @@ func getImagesFromDockerfile(dockerfile io.Reader) ([]string, error) {
 	fileScanner := bufio.NewScanner(dockerfile)
 	for fileScanner.Scan() {
 		line := strings.TrimSpace(fileScanner.Text())
-		if strings.HasPrefix(strings.ToUpper(line), "FROM") {
+		if strings.HasPrefix(strings.ToUpper(line), "FROM") ||
+			strings.HasPrefix(strings.ToUpper(line), "COPY") {
 			switch image := getImageFromLine(line); image {
+			case "":
+				continue
 			case "scratch":
 				fmt.Fprintln(os.Stderr, "- scratch image ignored")
 			default:
@@ -82,6 +85,20 @@ func getImagesFromDockerfile(dockerfile io.Reader) ([]string, error) {
 }
 
 func getImageFromLine(line string) string {
+	if strings.HasPrefix(strings.ToUpper(line), "COPY") {
+		line = strings.TrimPrefix(line, "COPY") // Remove "COPY" prefix
+		line = strings.TrimSpace(line)
+		line = strings.TrimPrefix(line, "--from") // Remove "--from" prefix
+		foo := strings.Split(line, "=")           // Get image ref after "="
+		if len(foo) != 2 {
+			return ""
+		}
+		// to support `COPY --from=stage /foo/bar` cases
+		if strings.Contains(foo[1], " ") {
+			return ""
+		}
+		return os.ExpandEnv(foo[1]) // Substitute templated vars
+	}
 	line = strings.TrimPrefix(line, "FROM") // Remove "FROM" prefix
 	line = os.ExpandEnv(line)               // Substitute templated vars
 	fields := strings.Fields(line)

cmd/cosign/cli/dockerfile/verify_test.go

@@ -86,6 +86,27 @@ func TestGetImagesFromDockerfile(t *testing.T) {
 			},
 			expected: []string{"gcr.io/gauntlet/test/one", "gcr.io/gauntlet/test/two:latest", "gcr.io/gauntlet/test/runtime"},
 		},
+		{
+			name: "copy",
+			fileContents: `FROM gcr.io/test/image@sha256:d131624e6f5d8695e9aea7a0439f7bac0fcc50051282e0c3d4d627cab8845ba5
+COPY --from=gcr.io/hello/world`,
+			expected: []string{
+				"gcr.io/test/image@sha256:d131624e6f5d8695e9aea7a0439f7bac0fcc50051282e0c3d4d627cab8845ba5",
+				"gcr.io/hello/world",
+			},
+		},
+		{
+			name: "copy with stage",
+			fileContents: `FROM gcr.io/test/image@sha256:d131624e6f5d8695e9aea7a0439f7bac0fcc50051282e0c3d4d627cab8845ba5
+COPY --from=stage /foo/bar`,
+			expected: []string{"gcr.io/test/image@sha256:d131624e6f5d8695e9aea7a0439f7bac0fcc50051282e0c3d4d627cab8845ba5"},
+		},
+		{
+			name: "copy files",
+			fileContents: `FROM gcr.io/test/image@sha256:d131624e6f5d8695e9aea7a0439f7bac0fcc50051282e0c3d4d627cab8845ba5
+COPY . .`,
+			expected: []string{"gcr.io/test/image@sha256:d131624e6f5d8695e9aea7a0439f7bac0fcc50051282e0c3d4d627cab8845ba5"},
+		},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {

test/e2e_test.sh

@@ -57,8 +57,8 @@ if (./cosign dockerfile verify --key ${DISTROLESS_PUB_KEY} ./test/testdata/unsig
 test_image="gcr.io/distroless/base" ./cosign dockerfile verify --key ${DISTROLESS_PUB_KEY} ./test/testdata/with_arg.Dockerfile
 
 # Test dockerfile resolve and verify
-./cosign dockerfile resolve -o ./test/testdata/fancy_from.Dockerfile.resolved ./test/testdata/fancy_from.Dockerfile
-./cosign dockerfile verify --key ${DISTROLESS_PUB_KEY} ./test/testdata/fancy_from.Dockerfile.resolved
+./cosign dockerfile resolve -o ./test/testdata/with_copy.Dockerfile.resolved ./test/testdata/with_copy.Dockerfile
+./cosign dockerfile verify --key ${DISTROLESS_PUB_KEY} ./test/testdata/with_copy.Dockerfile.resolved
 
 # Image exists, but is unsigned
 if (test_image="ubuntu" ./cosign dockerfile verify --key ${DISTROLESS_PUB_KEY} ./test/testdata/with_arg.Dockerfile); then false; fi

test/testdata/with_copy.Dockerfile

@@ -0,0 +1,18 @@
+# Copyright 2021 The Sigstore 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.
+
+FROM --platform=linux/amd64 gcr.io/distroless/base AS base
+COPY --from=gcr.io/distroless/base /foo /foo
+
+# blah blah