From d1a986698c820415b2e0be12141091a3cbf6fde3 Mon Sep 17 00:00:00 2001
From: Johan Abildskov <randomsort@gmail.com>
Date: Tue, 21 Mar 2023 19:59:28 +0000
Subject: [PATCH] encoding/gob: Extend partially allocated string slices There
 is a bug in the current implementation where very large `[]string` will not
 be extended, causing an index out of bounds panic.

Fixes #59172
---
 src/encoding/gob/codec_test.go  | 14 ++++++++++++++
 src/encoding/gob/dec_helpers.go |  3 +++
 2 files changed, 17 insertions(+)

diff --git a/src/encoding/gob/codec_test.go b/src/encoding/gob/codec_test.go
index 54c356c4644722..28cd6088af16ff 100644
--- a/src/encoding/gob/codec_test.go
+++ b/src/encoding/gob/codec_test.go
@@ -1544,6 +1544,10 @@ type LargeSliceStruct struct {
 	S []StringPair
 }
 
+type LargeSliceString struct {
+	S []string
+}
+
 func testEncodeDecode(t *testing.T, in, out any) {
 	t.Helper()
 	var b bytes.Buffer
@@ -1592,4 +1596,14 @@ func TestLargeSlice(t *testing.T) {
 		rt := &LargeSliceStruct{}
 		testEncodeDecode(t, st, rt)
 	})
+	t.Run("string", func(t *testing.T) {
+		t.Parallel()
+		s := make([]string, 1<<21)
+		for i := range s {
+			s[i] = string(rune(i))
+		}
+		st := &LargeSliceString{S: s}
+		rt := &LargeSliceString{}
+		testEncodeDecode(t, st, rt)
+	})
 }
diff --git a/src/encoding/gob/dec_helpers.go b/src/encoding/gob/dec_helpers.go
index a09ac8fc1afd93..098ba7254a21ed 100644
--- a/src/encoding/gob/dec_helpers.go
+++ b/src/encoding/gob/dec_helpers.go
@@ -358,6 +358,9 @@ func decStringSlice(state *decoderState, v reflect.Value, length int, ovfl error
 		if state.b.Len() == 0 {
 			errorf("decoding string array or slice: length exceeds input size (%d elements)", length)
 		}
+		if i >= len(slice) {
+			growSlice(v, &slice, length)
+		}
 		u := state.decodeUint()
 		n := int(u)
 		if n < 0 || uint64(n) != u || n > state.b.Len() {