fix: use the gas limit a site sends as gas (closes #207)

Sites send the gas limit under its JSON-RPC name, `gas`. ethers only
reads `gasLimit`, so the field was dropped and the node's estimate was
shown and signed instead. The page's request now takes `gas` as the gas
limit, and the existing ceiling, display and comparison with the signed
transaction cover it as they already did `gasLimit`. A request that
gives both with different values is refused.

Model: opus-5-5

src/shared/approvalTx.js

@@ -1,13 +1,14 @@
 // Preparation of the transaction an approval screen displays.
 //
-// A dApp's eth_sendTransaction normally fixes only `to`, `value` and `data`.
-// The nonce, the gas limit and the fees have to be filled in from the network
-// before anything can be signed, and whoever fills them in decides what the
-// user is shown. That work used to happen in the popup, after the user had
-// already approved: the numbers on the approval screen came from the popup and
-// were compared against nothing, so a compromised popup could display one fee
-// and sign another, and the ceilings in approvalVerify.js were all that stood
-// between the user and a fee that hands the validator the balance.
+// A dApp's eth_sendTransaction normally fixes `to`, `value` and `data`, and
+// sometimes the gas limit. The nonce, the fees, and the gas limit when the
+// site gives none have to be filled in from the network before anything can
+// be signed, and whoever fills them in decides what the user is shown. That
+// work used to happen in the popup, after the user had already approved: the
+// numbers on the approval screen came from the popup and were compared against
+// nothing, so a compromised popup could display one fee and sign another, and
+// the ceilings in approvalVerify.js were all that stood between the user and a
+// fee that hands the validator the balance.
 //
 // So it happens here instead, in the background, before the approval window is
 // opened. The background populates the transaction, shows that object, and
@@ -31,6 +32,7 @@ const {
     VoidSigner,
     accessListify,
     getAddress,
+    getBigInt,
     getBytes,
     hexlify,
     toQuantity,
@@ -47,10 +49,11 @@ const {
 // minutes long, which is not a wait anyone will sit through.
 const POPULATE_TIMEOUT_MS = 20000;
 
-// The request fields taken from the page. Anything else is dropped rather than
-// passed to ethers: the object is page-controlled, and a future ethers that
-// learns to carry a new transaction field must not start picking one up out of
-// it without this module knowing.
+// The request fields taken from the page, plus `gas`, which requestFrom()
+// takes as the gas limit. Anything else is dropped rather than passed to
+// ethers: the object is page-controlled, and a future ethers that learns to
+// carry a new transaction field must not start picking one up out of it
+// without this module knowing.
 //
 // The nonce is not taken from the page; it is always the account's next nonce
 // from the network. A page that chose it could replace one of the user's
@@ -84,6 +87,16 @@ function present(v) {
     return v !== null && v !== undefined && v !== "";
 }
 
+// Whether two quantities from the page are the same number. One that is not a
+// number matches nothing.
+function sameQuantity(a, b) {
+    try {
+        return getBigInt(a) === getBigInt(b);
+    } catch {
+        return false;
+    }
+}
+
 // These strings reach the user through the requesting page, so they are full
 // sentences even when the tail of one came from ethers or from the node.
 function sentence(text) {
@@ -111,6 +124,20 @@ function requestFrom(txParams, from) {
     for (const key of REQUEST_FIELDS) {
         if (present(txParams[key])) request[key] = txParams[key];
     }
+    // JSON-RPC calls the gas limit `gas`, and that is the name sites send.
+    // ethers only reads `gasLimit`: left as `gas` it would be dropped and an
+    // estimate signed in its place.
+    if (present(txParams.gas)) {
+        if (
+            present(request.gasLimit) &&
+            !sameQuantity(request.gasLimit, txParams.gas)
+        ) {
+            throw fail(
+                "The site asked for two different gas limits for this transaction.",
+            );
+        }
+        request.gasLimit = txParams.gas;
+    }
     if (
         present(request.type) &&
         !ALLOWED_TX_TYPES.includes(Number(request.type))