cmd/objdump: guard against out-of-range lines from directives.

//line bogo.go:9999999 will cause 'go tool objdump' to crash
unless bogo.go has that many lines.  Guard the array index
and return innocuous values (nil, nil) from the file cache.

Fixes #36683

Change-Id: I4a9f8444dc611654d270cc876e8848dfd2f84770
Reviewed-on: https://go-review.googlesource.com/c/go/+/223081
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/objfile/disasm.go

@@ -175,6 +175,11 @@ func (fc *FileCache) Line(filename string, line int) ([]byte, error) {
 		fc.files.MoveToFront(e)
 	}
 
+	// because //line directives can be out-of-range. (#36683)
+	if line-1 >= len(cf.Lines) || line-1 < 0 {
+		return nil, nil
+	}
+
 	return cf.Lines[line-1], nil
 }
 

src/cmd/objdump/objdump_test.go

@@ -106,8 +106,11 @@ func testDisasm(t *testing.T, printCode bool, flags ...string) {
 	hello := filepath.Join(tmp, fmt.Sprintf("hello-%x.exe", hash))
 	args := []string{"build", "-o", hello}
 	args = append(args, flags...)
-	args = append(args, "testdata/fmthello.go")
-	out, err := exec.Command(testenv.GoToolPath(t), args...).CombinedOutput()
+	args = append(args, "fmthello.go")
+	cmd := exec.Command(testenv.GoToolPath(t), args...)
+	cmd.Dir = "testdata" // "Bad line" bug #36683 is sensitive to being run in the source directory
+	t.Logf("Running %v", cmd.Args)
+	out, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Fatalf("go build fmthello.go: %v\n%s", err, out)
 	}
@@ -139,7 +142,11 @@ func testDisasm(t *testing.T, printCode bool, flags ...string) {
 		args = append([]string{"-S"}, args...)
 	}
 
-	out, err = exec.Command(exe, args...).CombinedOutput()
+	cmd = exec.Command(exe, args...)
+	cmd.Dir = "testdata" // "Bad line" bug #36683 is sensitive to being run in the source directory
+	out, err = cmd.CombinedOutput()
+	t.Logf("Running %v", cmd.Args)
+
 	if err != nil {
 		t.Fatalf("objdump fmthello.exe: %v\n%s", err, out)
 	}

src/cmd/objdump/testdata/fmthello.go

@@ -5,6 +5,8 @@ import "fmt"
 func main() {
 	Println("hello, world")
 	if flag {
+//line fmthello.go:999999
+		Println("bad line")
 		for {
 		}
 	}

test/run.go

@@ -463,7 +463,7 @@ func goGcflags() string {
 }
 
 func goGcflagsIsEmpty() bool {
-       return "" == os.Getenv("GO_GCFLAGS")
+	return "" == os.Getenv("GO_GCFLAGS")
 }
 
 // run runs a test.