Merge #20140: Restore compatibility with old CSubNet serialization

886be97af5d4aba338b23a7b20b8560be8156231 Ignore incorrectly-serialized banlist.dat entries (Pieter Wuille)
883cea7dea3cedc9b45b6191f7d4e7be2d9a11ca Restore compatibility with old CSubNet serialization (Pieter Wuille)

Pull request description:

  #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 #19628 but without this PR).

  Reported by Greg Maxwell.

ACKs for top commit:
  laanwj:
    Code review ACK 886be97af5d4aba338b23a7b20b8560be8156231
  vasild:
    ACK 886be97af

Tree-SHA512: d3fb91e8ecd933406e527187974f22770374ee2e12a233e7870363f52ecda471fb0b7bae72420e8ff6b6b1594e3037a5115984c023dbadf38f86aeaffcd681e7

1 files changed, 19 insertions, 1 deletions

diff --git a/src/netaddress.h b/src/netaddress.h
index 9c8148e33e..f35b01d202 100644
--- a/src/netaddress.h
+++ b/src/netaddress.h
@@ -459,6 +459,8 @@ 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);
@@ -476,7 +478,23 @@ 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 */