Clamp direct position packets to channel precision (fixes #8640) (#10383)

* Fix position precision for direct sends

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Clarify zero position precision logging

* Use const channel reference for position precision

* Use C linkage for position precision test entrypoints

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

src/mesh/PositionPrecision.cpp

@@ -0,0 +1,75 @@
+#include "PositionPrecision.h"
+#include "Channels.h"
+#include "mesh-pb-constants.h"
+
+#include <Arduino.h>
+
+uint32_t getPositionPrecisionForChannel(uint8_t channelIndex)
+{
+    const meshtastic_Channel &channel = channels.getByIndex(channelIndex);
+
+    if (channel.settings.has_module_settings) {
+        return channel.settings.module_settings.position_precision;
+    } else if (channel.role == meshtastic_Channel_Role_PRIMARY) {
+        return 32;
+    } else {
+        return 0;
+    }
+}
+
+static int32_t truncateCoordinate(int32_t coordinate, uint32_t precision)
+{
+    uint32_t coordinateBits = static_cast<uint32_t>(coordinate);
+    uint32_t truncated = coordinateBits & (UINT32_MAX << (32 - precision));
+
+    // Use the middle of the possible location, not the low edge of the bucket.
+    truncated += (1UL << (31 - precision));
+
+    return static_cast<int32_t>(truncated);
+}
+
+void applyPositionPrecision(meshtastic_Position &position, uint32_t precision)
+{
+    if (precision == 0) {
+        uint32_t time = position.time;
+        position = meshtastic_Position_init_default;
+        position.time = time;
+        return;
+    }
+
+    uint32_t effectivePrecision = precision > 32 ? 32 : precision;
+    position.precision_bits = effectivePrecision;
+
+    if (effectivePrecision < 32) {
+        position.latitude_i = truncateCoordinate(position.latitude_i, effectivePrecision);
+        position.longitude_i = truncateCoordinate(position.longitude_i, effectivePrecision);
+    }
+}
+
+bool applyPositionPrecision(meshtastic_MeshPacket &packet, uint32_t precision)
+{
+    if (packet.which_payload_variant != meshtastic_MeshPacket_decoded_tag ||
+        packet.decoded.portnum != meshtastic_PortNum_POSITION_APP) {
+        return true;
+    }
+
+    meshtastic_Position position = meshtastic_Position_init_default;
+    if (!pb_decode_from_bytes(packet.decoded.payload.bytes, packet.decoded.payload.size, &meshtastic_Position_msg, &position)) {
+        return false;
+    }
+
+    applyPositionPrecision(position, precision);
+    packet.decoded.payload.size = pb_encode_to_bytes(packet.decoded.payload.bytes, sizeof(packet.decoded.payload.bytes),
+                                                     &meshtastic_Position_msg, &position);
+    return true;
+}
+
+bool applyPositionPrecisionForChannel(meshtastic_MeshPacket &packet, uint8_t channelIndex)
+{
+    if (packet.which_payload_variant != meshtastic_MeshPacket_decoded_tag ||
+        packet.decoded.portnum != meshtastic_PortNum_POSITION_APP) {
+        return true;
+    }
+
+    return applyPositionPrecision(packet, getPositionPrecisionForChannel(channelIndex));
+}

src/mesh/PositionPrecision.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include "meshtastic/mesh.pb.h"
+#include <stdint.h>
+
+uint32_t getPositionPrecisionForChannel(uint8_t channelIndex);
+void applyPositionPrecision(meshtastic_Position &position, uint32_t precision);
+bool applyPositionPrecision(meshtastic_MeshPacket &packet, uint32_t precision);
+bool applyPositionPrecisionForChannel(meshtastic_MeshPacket &packet, uint8_t channelIndex);

src/mesh/Router.cpp

@@ -4,6 +4,7 @@
 #include "MeshRadio.h"
 #include "MeshService.h"
 #include "NodeDB.h"
+#include "PositionPrecision.h"
 #include "RTC.h"
 
 #include "configuration.h"
@@ -367,6 +368,11 @@ ErrorCode Router::send(meshtastic_MeshPacket *p)
     }
 
     fixPriority(p); // Before encryption, fix the priority if it's unset
+    if (!applyPositionPrecisionForChannel(*p, p->channel)) {
+        LOG_ERROR("Dropping malformed position packet before send");
+        packetPool.release(p);
+        return meshtastic_Routing_Error_BAD_REQUEST;
+    }
 
     // If the packet is not yet encrypted, do so now
     if (p->which_payload_variant == meshtastic_MeshPacket_decoded_tag) {