From 93f6514d8672f2eec78532c3b54bb3f40122586b Mon Sep 17 00:00:00 2001
From: Duke <duke@hush.is>
Date: Thu, 26 Sep 2024 11:13:07 -0400
Subject: [PATCH] Disable absurd fee checks when adding to the mempool

To protect users who are not using opreturn we prevent any use of z_sendmany
with absurd fees if opreturn is not being used, so this change only affects
users who are adding opreturn data. Since there is no way to currently send
opreturn data via a GUI this still protects all GUI users from absurd fees
while allowing CLI users to decide to use higher fees.
---
 src/main.cpp             | 14 ++++++++++++--
 src/wallet/rpcwallet.cpp | 17 ++++++++++++++---
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index 3ddf5bb54..f2f3bf831 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1960,7 +1960,17 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
             dFreeCount += nSize;
         }
         
-        if (!tx.IsCoinImport() && fRejectAbsurdFee && nFees > ::minRelayTxFee.GetFee(nSize) * 10000 && nFees > nValueOut/19)
+        // Disable checks for absurd fees when adding to the mempool. Instead, this check is done
+        // when a user attempts to make a transaction with an absurd fee and only rejects absurd
+        // fees when OP_RETURN data is NOT being used. This means users making normal financial
+        // transactions (z2z) are protected from absurd fees, it is only users who are storing
+        // arbitrary data via a z2t transaction are allowed to (or potentially required) to pay high fees
+        // It would be nice to detect the use of OP_RETURN right here but it seems to only be known
+        // inside of IsStandard() inside of IsStandardTx() and we want to avoid doing expensive checks
+        // multiple times.
+        fRejectAbsurdFee = false;
+
+        if (fRejectAbsurdFee && !tx.IsCoinImport() && nFees > ::minRelayTxFee.GetFee(nSize) * 10000 && nFees > nValueOut/19)
         {
             string errmsg = strprintf("absurdly high fees %s, %d > %d",
                                       hash.ToString(),
@@ -1968,7 +1978,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
             LogPrint("mempool", errmsg.c_str());
             return state.Error("AcceptToMemoryPool: " + errmsg);
         }
-//fprintf(stderr,"addmempool 6\n");
+        //fprintf(stderr,"addmempool 6\n");
 
         // Check against previous transactions
         // This is done last to help prevent CPU exhaustion denial-of-service attacks.
diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index 46a059892..1b6110ec5 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -5155,6 +5155,8 @@ UniValue z_sendmany(const UniValue& params, bool fHelp, const CPubKey& mypk)
     CAmount nTotalOut = 0;
     // Optional OP_RETURN data
     CScript opret;
+    // TODO: enforce that only a single opreturn exists
+    UniValue opretValue;
 
     bool containsSaplingOutput = false;
 
@@ -5189,7 +5191,10 @@ UniValue z_sendmany(const UniValue& params, bool fHelp, const CPubKey& mypk)
         //    throw JSONRPCError(RPC_INVALID_PARAMETER, string("Invalid parameter, duplicated address: ")+address);
         setAddress.insert(address);
 
-        UniValue opretValue = find_value(o, "opreturn");
+        UniValue this_opret = find_value(o, "opreturn");
+        if (!this_opret.isNull()) {
+            opretValue = this_opret;
+        }
 
         // Create the CScript representation of the OP_RETURN
         if (!opretValue.isNull()) {
@@ -5342,12 +5347,18 @@ UniValue z_sendmany(const UniValue& params, bool fHelp, const CPubKey& mypk)
         // or anything less than nDefaultFee instead of being forced to use a custom fee and leak metadata
         if (nTotalOut < nDefaultFee) {
             if (nFee > nDefaultFee) {
-                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Small transaction amount %s has fee %s that is greater than the default fee %s", FormatMoney(nTotalOut), FormatMoney(nFee), FormatMoney(nDefaultFee)));
+                    // Allow large fees if OP_RETURN is being used
+                    if( opretValue.isNull() ) {
+                        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Small transaction amount %s has fee %s that is greater than the default fee %s", FormatMoney(nTotalOut), FormatMoney(nFee), FormatMoney(nDefaultFee)));
+                    }
             }
         } else {
             // Check that the user specified fee is not absurd.
             if (nFee > nTotalOut) {
-                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Fee %s is greater than the sum of outputs %s and also greater than the default fee", FormatMoney(nFee), FormatMoney(nTotalOut)));
+                    // Allow large fees if OP_RETURN is being used
+                    if( opretValue.isNull() ) {
+                        throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Fee %s is greater than the sum of outputs %s and also greater than the default fee", FormatMoney(nFee), FormatMoney(nTotalOut)));
+                    }
             }
 	}
     }