From 991a628dfeeab80dee38d637e115c004dca11b4c Mon Sep 17 00:00:00 2001
From: Jimi Chen <jimictw@google.com>
Date: Thu, 15 May 2025 12:26:29 +0000
Subject: [PATCH] Add getInterfaceMtu method in ProcfsParsingUtils

Flag: EXEMPT mainline
Bug: 406085682
Test: atest NetworkStackTests:com.android.networkstack.util.ProcfsParsingUtilsTest
Change-Id: I62e129c3903e0c432f4a4d84c7024cfb4e140dd5
---
 .../net/module/util/ProcfsParsingUtils.java   | 59 ++++++++++++++++++-
 1 file changed, 57 insertions(+), 2 deletions(-)

diff --git a/staticlibs/device/com/android/net/module/util/ProcfsParsingUtils.java b/staticlibs/device/com/android/net/module/util/ProcfsParsingUtils.java
index 44fc94121b..151da67440 100644
--- a/staticlibs/device/com/android/net/module/util/ProcfsParsingUtils.java
+++ b/staticlibs/device/com/android/net/module/util/ProcfsParsingUtils.java
@@ -39,6 +39,12 @@ import java.util.List;
 public final class ProcfsParsingUtils {
     public static final String TAG = ProcfsParsingUtils.class.getSimpleName();
 
+    // rfc894 describes the minimum length of the data field of a packet sent over an
+    // Ethernet is 1500 octets, thus the maximum length of an IP datagram
+    // sent over an Ethernet is 1500 octets.
+    public static final int DEFAULT_MTU = 1500;
+    public static final int DEFAULT_TRAFFIC_CLASS = 0;
+
     private static final Inet4Address IPV4_ADDR_ALL_HOST_MULTICAST =
             (Inet4Address) InetAddresses.parseNumericAddress("224.0.0.1");
 
@@ -48,6 +54,7 @@ public final class ProcfsParsingUtils {
     private static final String IPV4_MCAST_PATH = "/proc/net/igmp";
     private static final String IPV6_MCAST_PATH = "/proc/net/igmp6";
     private static final String IPV4_DEFAULT_TTL_PATH = "/proc/sys/net/ipv4/ip_default_ttl";
+    private static final String SYS_CLASS_NET_PATH = "/sys/class/net";
 
     private ProcfsParsingUtils() {
     }
@@ -86,10 +93,15 @@ public final class ProcfsParsingUtils {
     @VisibleForTesting
     public static int parseNdTrafficClass(final List<String> lines) {
         if (lines.size() != 1) {
-            return 0;   // default
+            return DEFAULT_TRAFFIC_CLASS;
         }
 
-        return Integer.parseInt(lines.get(0));
+        try {
+            return Integer.parseInt(lines.get(0));
+        } catch (NumberFormatException e) {
+            Log.e(TAG, "failed to parse ND traffic class.", e);
+            return DEFAULT_TRAFFIC_CLASS;
+        }
     }
 
     /**
@@ -109,6 +121,32 @@ public final class ProcfsParsingUtils {
         }
     }
 
+    /**
+     * Parses an interface's Maximum Transmission Unit (MTU) value from a list of strings.
+     *
+     * This function expects a list containing a single string representing the interface MTU.
+     * If the list is empty or contains multiple lines, it assumes a default value for 1500.
+     *
+     * @param lines A list of strings, expected to contain the MTU value as its sole element.
+     * @return The parsed MTU value as an integer. Returns `IPV4_MIN_MTU` if `ipv4` is true
+     * and the input `lines` size is not 1, otherwise returns `IPV6_MIN_MTU` if `ipv4`
+     * is false and the input `lines` size is not 1. If `lines` has exactly one
+     * element, returns the integer parsed from that string.
+     */
+    @VisibleForTesting
+    public static int parseInterfaceMtu(final List<String> lines) {
+        if (lines.size() != 1) {
+            return DEFAULT_MTU;
+        }
+
+        try {
+            return Integer.parseInt(lines.get(0));
+        } catch (NumberFormatException e) {
+            Log.e(TAG, "failed to parse interface mtu.", e);
+            return DEFAULT_MTU;
+        }
+    }
+
     /**
      * Parses anycast6 addresses associated with a specific interface from a list of strings.
      *
@@ -296,6 +334,23 @@ public final class ProcfsParsingUtils {
         return parseNdTrafficClass(lines);
     }
 
+    /**
+     * Returns the interface MTU for the specified interface.
+     * The function loads the existing interface MTU from the file
+     * `/sys/class/net/{ifname}/mtu`. If the file does not exist, the
+     * function returns the default value 1280.
+     * rfc8200#5 describes IPv6 requires that every link in the Internet have an MTU of 1280
+     * octets or greater. This is known as the IPv6 minimum link MTU.
+     *
+     * @param ifname The name of the interface.
+     * @return The traffic class for the interface.
+     */
+    public static int getInterfaceMtu(final String ifname) {
+        final String mtuPath = SYS_CLASS_NET_PATH + ifname + "/mtu";
+        final List<String> lines = readFile(mtuPath);
+        return parseInterfaceMtu(lines);
+    }
+
     /**
      * The function loads the existing IPv6 anycast address from the file `/proc/net/anycast6`.
      * If the file does not exist or the interface is not found, the function