Bug 1484988 - Avoid writing past the logical length of a string in Toolkit. r=froydnj

Also remove a useless call to SetCapacity().

MozReview-Commit-ID: LarsWKI3qwz

Differential Revision: https://phabricator.services.mozilla.com/D3882

--HG--
extra : moz-landing-system : lando

services/crypto/component/IdentityCryptoService.cpp

@@ -33,8 +33,7 @@ namespace {
 void
 HexEncode(const SECItem * it, nsACString & result)
 {
-  const char * digits = "0123456789ABCDEF";
-  result.SetCapacity((it->len * 2) + 1);
+  static const char digits[] = "0123456789ABCDEF";
   result.SetLength(it->len * 2);
   char * p = result.BeginWriting();
   for (unsigned int i = 0; i < it->len; ++i) {

toolkit/components/url-classifier/Entries.h

@@ -89,12 +89,9 @@ struct SafebrowsingHash
   void ToString(nsACString& aStr) const {
     uint32_t len = ((sHashSize + 2) / 3) * 4;
 
-    // Capacity should be one greater than length, because PL_Base64Encode
-    // will not be null-terminated, while nsCString requires it.
-    aStr.SetCapacity(len + 1);
-    PL_Base64Encode((char*)buf, sHashSize, aStr.BeginWriting());
-    aStr.BeginWriting()[len] = '\0';
     aStr.SetLength(len);
+    PL_Base64Encode((char*)buf, sHashSize, aStr.BeginWriting());
+    MOZ_ASSERT(aStr.BeginReading()[len] == '\0');
   }
 
   void ToHexString(nsACString& aStr) const {