Merge bitcoin#20140: Restore compatibility with old CSubNet serialization

886be97 Ignore incorrectly-serialized banlist.dat entries (Pieter Wuille)
883cea7 Restore compatibility with old CSubNet serialization (Pieter Wuille)

Pull request description:

  bitcoin#19628 changed CSubNet for IPv4 netmasks, using the first 4 bytes of `netmask` rather than the last 4 to store the actual mask. Unfortunately, CSubNet objects are serialized on disk in banlist.dat, breaking compatibility with existing banlists (and bringing them into an inconsistent state where entries reported in `listbanned` cannot be removed).

  Fix this by reverting to the old format (just for serialization). Also add a sanity check to the deserializer so that nonsensical banlist.dat entries are ignored (which would otherwise be possible if someone added IPv4 entries after bitcoin#19628 but without this PR).

  Reported by Greg Maxwell.

ACKs for top commit:
  laanwj:
    Code review ACK 886be97
  vasild:
    ACK 886be97

Tree-SHA512: d3fb91e8ecd933406e527187974f22770374ee2e12a233e7870363f52ecda471fb0b7bae72420e8ff6b6b1594e3037a5115984c023dbadf38f86aeaffcd681e7

Author: laanwj

src/banman.cpp

@@ -192,7 +192,7 @@ void BanMan::SweepBanned()
         while (it != m_banned.end()) {
             CSubNet sub_net = (*it).first;
             CBanEntry ban_entry = (*it).second;
-            if (now > ban_entry.nBanUntil) {
+            if (!sub_net.IsValid() || now > ban_entry.nBanUntil) {
                 m_banned.erase(it++);
                 m_is_dirty = true;
                 notify_ui = true;

src/netaddress.cpp

@@ -1076,6 +1076,17 @@ bool CSubNet::IsValid() const
     return valid;
 }
 
+bool CSubNet::SanityCheck() const
+{
+    if (!(network.IsIPv4() || network.IsIPv6())) return false;
+
+    for (size_t x = 0; x < network.m_addr.size(); ++x) {
+        if (network.m_addr[x] & ~netmask[x]) return false;
+    }
+
+    return true;
+}
+
 bool operator==(const CSubNet& a, const CSubNet& b)
 {
     return a.valid == b.valid && a.network == b.network && !memcmp(a.netmask, b.netmask, 16);

src/netaddress.h

@@ -461,6 +461,8 @@ class CSubNet
         /// Is this value valid? (only used to signal parse errors)
         bool valid;
 
+        bool SanityCheck() const;
+
     public:
         CSubNet();
         CSubNet(const CNetAddr& addr, uint8_t mask);
@@ -478,7 +480,23 @@ class CSubNet
         friend bool operator!=(const CSubNet& a, const CSubNet& b) { return !(a == b); }
         friend bool operator<(const CSubNet& a, const CSubNet& b);
 
-        SERIALIZE_METHODS(CSubNet, obj) { READWRITE(obj.network, obj.netmask, obj.valid); }
+        SERIALIZE_METHODS(CSubNet, obj)
+        {
+            READWRITE(obj.network);
+            if (obj.network.IsIPv4()) {
+                // Before commit 102867c587f5f7954232fb8ed8e85cda78bb4d32, CSubNet used the last 4 bytes of netmask
+                // to store the relevant bytes for an IPv4 mask. For compatiblity reasons, keep doing so in
+                // serialized form.
+                unsigned char dummy[12] = {0};
+                READWRITE(dummy);
+                READWRITE(MakeSpan(obj.netmask).first(4));
+            } else {
+                READWRITE(obj.netmask);
+            }
+            READWRITE(obj.valid);
+            // Mark invalid if the result doesn't pass sanity checking.
+            SER_READ(obj, if (obj.valid) obj.valid = obj.SanityCheck());
+        }
 };
 
 /** A combination of a network address (CNetAddr) and a (TCP) port */