crypto/rand: add randcrash=0 GODEBUG

For #66821

Change-Id: I525c308d6d6243a2bc805e819dcf40b67e52ade5
Reviewed-on: https://go-review.googlesource.com/c/go/+/608435
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

doc/godebug.md

@@ -168,6 +168,11 @@ For Go 1.24, it now defaults to multipathtcp="2", thus
 enabled by default on listerners. Using multipathtcp="0" reverts to the
 pre-Go 1.24 behavior.
 
+Go 1.24 changed [`crypto/rand.Read`](/pkg/crypto/rand/#Read) to crash the
+program on any error. This setting is controlled by the `randcrash` setting.
+For Go 1.24 it defaults to `randcrash=1`.
+Using `randcrash=0` reverts to the pre-Go 1.24 behavior.
+
 ### Go 1.23
 
 Go 1.23 changed the channels created by package time to be unbuffered

src/crypto/rand/rand.go

@@ -8,6 +8,7 @@ package rand
 
 import (
 	"crypto/internal/boring"
+	"internal/godebug"
 	"io"
 	"os"
 	"sync"
@@ -64,6 +65,8 @@ func (r *reader) Read(b []byte) (n int, err error) {
 //go:linkname fatal
 func fatal(string)
 
+var randcrash = godebug.New("randcrash")
+
 // Read fills b with cryptographically secure random bytes. It never returns an
 // error, and always fills b entirely.
 //
@@ -83,6 +86,10 @@ func Read(b []byte) (n int, err error) {
 		copy(b, bb)
 	}
 	if err != nil {
+		if randcrash.Value() == "0" {
+			randcrash.IncNonDefault()
+			return 0, err
+		}
 		fatal("crypto/rand: failed to read random data (see https://go.dev/issue/66821): " + err.Error())
 		panic("unreachable") // To be sure.
 	}

src/crypto/rand/rand_linux_test.go

@@ -48,20 +48,18 @@ func TestNoGetrandom(t *testing.T) {
 			return
 		}
 
-		buf := &bytes.Buffer{}
 		cmd := testenv.Command(t, os.Args[0], "-test.v")
-		cmd.Stdout = buf
-		cmd.Stderr = buf
 		cmd.Env = append(os.Environ(), "GO_GETRANDOM_DISABLED=1")
-		if err := cmd.Run(); err != nil {
+		out, err := cmd.CombinedOutput()
-			t.Errorf("subprocess failed: %v\n%s", err, buf.Bytes())
+		if err != nil {
+			t.Errorf("subprocess failed: %v\n%s", err, out)
 			return
 		}
 
-		if !bytes.Contains(buf.Bytes(), []byte("GetRandom returned ENOSYS")) {
+		if !bytes.Contains(out, []byte("GetRandom returned ENOSYS")) {
 			t.Errorf("subprocess did not disable getrandom")
 		}
-		if !bytes.Contains(buf.Bytes(), []byte("TestRead")) {
+		if !bytes.Contains(out, []byte("TestRead")) {
 			t.Errorf("subprocess did not run TestRead")
 		}
 	}()

src/crypto/rand/rand_test.go

@@ -8,7 +8,9 @@ import (
 	"bytes"
 	"compress/flate"
 	"crypto/internal/boring"
+	"errors"
 	"internal/race"
+	"internal/testenv"
 	"io"
 	"os"
 	"runtime"
@@ -189,6 +191,44 @@ func TestNoUrandomFallback(t *testing.T) {
 	}
 }
 
+func TestReadError(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode")
+	}
+	testenv.MustHaveExec(t)
+
+	// We run this test in a subprocess because it's expected to crash the
+	// program unless the GODEBUG is set.
+	if os.Getenv("GO_TEST_READ_ERROR") == "1" {
+		defer func(r io.Reader) { Reader = r }(Reader)
+		Reader = readerFunc(func([]byte) (int, error) {
+			return 0, errors.New("error")
+		})
+		if _, err := Read(make([]byte, 32)); err == nil {
+			t.Error("Read did not return error")
+		}
+		return
+	}
+
+	cmd := testenv.Command(t, os.Args[0], "-test.run=TestReadError")
+	cmd.Env = append(os.Environ(), "GO_TEST_READ_ERROR=1")
+	out, err := cmd.CombinedOutput()
+	if err == nil {
+		t.Error("subprocess succeeded unexpectedly")
+	}
+	exp := "fatal error: crypto/rand: failed to read random data"
+	if !bytes.Contains(out, []byte(exp)) {
+		t.Errorf("subprocess output does not contain %q: %s", exp, out)
+	}
+
+	cmd = testenv.Command(t, os.Args[0], "-test.run=TestReadError")
+	cmd.Env = append(os.Environ(), "GO_TEST_READ_ERROR=1", "GODEBUG=randcrash=0")
+	out, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Errorf("subprocess failed: %v\n%s", err, out)
+	}
+}
+
 func BenchmarkRead(b *testing.B) {
 	b.Run("4", func(b *testing.B) {
 		benchmarkRead(b, 4)

src/internal/godebugs/table.go

@@ -47,6 +47,7 @@ var All = []Info{
 	{Name: "netedns0", Package: "net", Changed: 19, Old: "0"},
 	{Name: "panicnil", Package: "runtime", Changed: 21, Old: "1"},
 	{Name: "randautoseed", Package: "math/rand"},
+	{Name: "randcrash", Package: "crypto/rand", Changed: 24, Old: "0"},
 	{Name: "randseednop", Package: "math/rand", Changed: 24, Old: "0"},
 	{Name: "tarinsecurepath", Package: "archive/tar"},
 	{Name: "tls10server", Package: "crypto/tls", Changed: 22, Old: "1"},

src/runtime/metrics/doc.go

@@ -306,6 +306,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 		The number of non-default behaviors executed by the math/rand
 		package due to a non-default GODEBUG=randautoseed=... setting.
 
+	/godebug/non-default-behavior/randcrash:events
+		The number of non-default behaviors executed by the crypto/rand
+		package due to a non-default GODEBUG=randcrash=... setting.
+
 	/godebug/non-default-behavior/randseednop:events
 		The number of non-default behaviors executed by the math/rand
 		package due to a non-default GODEBUG=randseednop=... setting.