test: signing propagates errors

Author: ARR4N

libevm/precompiles/p256verify/p256verify.go

@@ -22,6 +22,7 @@ import (
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/rand"
+	"io"
 	"math/big"
 )
 
@@ -87,7 +88,13 @@ func (in *input) bigWord(index int) *big.Int {
 // argument to [ecdsa.Sign]. It returns a signature payload constructed with
 // [Pack], which can therefore be passed directly to the precompile.
 func Sign(priv *ecdsa.PrivateKey, hash [32]byte) ([]byte, error) {
-	r, s, err := ecdsa.Sign(rand.Reader, priv, hash[:])
+	return signWithRandReader(rand.Reader, priv, hash)
+}
+
+// signWithRandReader is abstracted for testing purposes only and MUST NOT be
+// used directly. Always use [Sign].
+func signWithRandReader(rand io.Reader, priv *ecdsa.PrivateKey, hash [32]byte) ([]byte, error) {
+	r, s, err := ecdsa.Sign(rand, priv, hash[:])
 	if err != nil {
 		return nil, err
 	}

libevm/precompiles/p256verify/p256verify_test.go

@@ -24,6 +24,7 @@ import (
 	"encoding/asn1"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"math/big"
 	"slices"
@@ -44,6 +45,8 @@ import (
 	_ "embed"
 )
 
+var _ vm.PrecompiledContract = Precompile{}
+
 // ulerdoganTestCase is the test case from
 // https://github.com/ulerdogan/go-ethereum/blob/cec0b058115282168c5afc5197de3f6b5479dc4a/core/vm/testdata/precompiles/p256Verify.json,
 // copied under LGPL. See the respective commit for copyright and license
@@ -254,3 +257,22 @@ func TestViaEVM(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, []byte{31: 1}, got)
 }
+
+type brokenReader struct {
+	err error
+}
+
+func (r brokenReader) Read([]byte) (int, error) {
+	return 0, r.err
+}
+
+func TestSignPropagatesReaderError(t *testing.T) {
+	priv, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	require.NoError(t, err, "ecdsa.GenerateKey()")
+
+	r := brokenReader{
+		err: errors.New("uh oh"),
+	}
+	_, err = signWithRandReader(r, priv, [32]byte{})
+	require.Equal(t, r.err, err, "Propagate error from io.Reader entropy source")
+}