From f4cff87a7fcb9268f585a8a011df9e7cf9db722c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Pawe=C5=82=20Gronowski?= <pawel.gronowski@docker.com>
Date: Mon, 3 Jun 2024 12:59:37 +0200
Subject: [PATCH] dockerfile/linter: check for nil linter in linter functions
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During parsing, the linter can be nil and linter rules won't succeed at
running due to a nil dereference. Instead of checking for nil
everywhere, this modifies the linter to automatically disable itself if
the linter is nil.

Fixes a nil dereference panic happening when parsing `ENV` and `LABEL`
commands without a linter introduced by
6cfa4599029db7f2e6e83feaaa33984785ddd147.

Co-authored-by: Paweł Gronowski <pawel.gronowski@docker.com>
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
---
 frontend/dockerfile/instructions/parse.go     | 10 ++++----
 .../dockerfile/instructions/parse_test.go     | 24 +++++++++++++++++++
 frontend/dockerfile/linter/linter.go          |  4 ++--
 3 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/frontend/dockerfile/instructions/parse.go b/frontend/dockerfile/instructions/parse.go
index 9c1c7ea379b9..709474ae84f3 100644
--- a/frontend/dockerfile/instructions/parse.go
+++ b/frontend/dockerfile/instructions/parse.go
@@ -80,10 +80,8 @@ func ParseInstructionWithLinter(node *parser.Node, lint *linter.Linter) (v inter
 	case command.Env:
 		return parseEnv(req, lint)
 	case command.Maintainer:
-		if lint != nil {
-			msg := linter.RuleMaintainerDeprecated.Format()
-			lint.Run(&linter.RuleMaintainerDeprecated, node.Location(), msg)
-		}
+		msg := linter.RuleMaintainerDeprecated.Format()
+		lint.Run(&linter.RuleMaintainerDeprecated, node.Location(), msg)
 		return parseMaintainer(req)
 	case command.Label:
 		return parseLabel(req, lint)
@@ -92,11 +90,11 @@ func ParseInstructionWithLinter(node *parser.Node, lint *linter.Linter) (v inter
 	case command.Copy:
 		return parseCopy(req)
 	case command.From:
-		if lint != nil && !isLowerCaseStageName(req.args) {
+		if !isLowerCaseStageName(req.args) {
 			msg := linter.RuleStageNameCasing.Format(req.args[2])
 			lint.Run(&linter.RuleStageNameCasing, node.Location(), msg)
 		}
-		if lint != nil && !doesFromCaseMatchAsCase(req) {
+		if !doesFromCaseMatchAsCase(req) {
 			msg := linter.RuleFromAsCasing.Format(req.command, req.args[1])
 			lint.Run(&linter.RuleFromAsCasing, node.Location(), msg)
 		}
diff --git a/frontend/dockerfile/instructions/parse_test.go b/frontend/dockerfile/instructions/parse_test.go
index 9ac60e2b63cf..111820a5f024 100644
--- a/frontend/dockerfile/instructions/parse_test.go
+++ b/frontend/dockerfile/instructions/parse_test.go
@@ -151,6 +151,30 @@ func TestParseOptInterval(t *testing.T) {
 	require.NoError(t, err)
 }
 
+func TestNilLinter(t *testing.T) {
+	for cmd := range command.Commands {
+		cmd := cmd
+		t.Run(cmd, func(t *testing.T) {
+			t.Parallel()
+
+			for _, tc := range []string{
+				cmd + " foo=bar",
+				cmd + " a",
+				cmd + " a b",
+				cmd + " a b c",
+				cmd + " 0 0",
+			} {
+				t.Run(tc, func(t *testing.T) {
+					ast, err := parser.Parse(strings.NewReader("FROM busybox\n" + tc))
+					if err == nil {
+						_, _, _ = Parse(ast.AST, nil)
+					}
+				})
+			}
+		})
+	}
+}
+
 func TestCommentsDetection(t *testing.T) {
 	dt := `# foo sets foo
 ARG foo=bar
diff --git a/frontend/dockerfile/linter/linter.go b/frontend/dockerfile/linter/linter.go
index 74eebf79ca45..9a4de80e9a0a 100644
--- a/frontend/dockerfile/linter/linter.go
+++ b/frontend/dockerfile/linter/linter.go
@@ -38,7 +38,7 @@ func New(config *Config) *Linter {
 }
 
 func (lc *Linter) Run(rule LinterRuleI, location []parser.Range, txt ...string) {
-	if lc.Warn == nil || lc.SkipAll {
+	if lc == nil || lc.Warn == nil || lc.SkipAll {
 		return
 	}
 	rulename := rule.RuleName()
@@ -50,7 +50,7 @@ func (lc *Linter) Run(rule LinterRuleI, location []parser.Range, txt ...string)
 }
 
 func (lc *Linter) Error() error {
-	if !lc.ReturnAsError {
+	if lc == nil || !lc.ReturnAsError {
 		return nil
 	}
 	if len(lc.CalledRules) == 0 {