Implement IPv4 stack and ip app (AI)

- Created src/ipv4.h: IPv4 header struct, protocol numbers, checksum,
  address conversion (ipv4_aton/ntoa), send/receive/routing API,
  protocol handler registration
- Created src/ipv4.c: packet construction with header checksum,
  simple routing (direct subnet + gateway), incoming packet validation
  and dispatch to registered protocol handlers
- Created apps/ip/ip.c: displays network interface config from
  /sys/net (MAC, link, IP, netmask, gateway); supports 'ip set'
  to configure interface via sysfs writes
- Added ipv4.c to kernel build, kernel calls ipv4_init() at boot
- Tested: clean boot, IPv4 initialized, ip app in CPIO

apps/ip/ip.c

@@ -0,0 +1,202 @@
+/**
+ * @file ip.c
+ * @brief Display and configure IPv4 network configuration.
+ *
+ * Reads network interface information from /sys/net and displays
+ * the current IPv4 configuration for all interfaces.
+ *
+ * Usage:
+ *   ip              - Show all interfaces
+ *   ip set <iface> <ip> <netmask> <gateway>  - Configure an interface
+ *
+ * Examples:
+ *   ip
+ *   ip set eth1 192.168.1.100 255.255.255.0 192.168.1.1
+ */
+
+#include "syscalls.h"
+
+/**
+ * Read the contents of a sysfs file into buf.
+ * Returns number of bytes read, or -1 on failure.
+ */
+static int32_t read_sysfs(const char *path, char *buf, uint32_t size) {
+    int32_t fd = open(path, 0);
+    if (fd < 0) return -1;
+
+    int32_t n = read(fd, buf, size - 1);
+    close(fd);
+
+    if (n > 0) {
+        buf[n] = '\0';
+        /* Strip trailing newline */
+        for (int32_t i = n - 1; i >= 0; i--) {
+            if (buf[i] == '\n' || buf[i] == '\r') buf[i] = '\0';
+            else break;
+        }
+    } else {
+        buf[0] = '\0';
+    }
+    return n;
+}
+
+/**
+ * Build a sysfs path: /sys/net/<iface>/<file>
+ */
+static void build_path(char *out, uint32_t out_size,
+                       const char *iface, const char *file) {
+    /* Manual string concatenation */
+    uint32_t pos = 0;
+    const char *prefix = "/sys/net/";
+    while (*prefix && pos < out_size - 1) out[pos++] = *prefix++;
+    while (*iface && pos < out_size - 1) out[pos++] = *iface++;
+    if (pos < out_size - 1) out[pos++] = '/';
+    while (*file && pos < out_size - 1) out[pos++] = *file++;
+    out[pos] = '\0';
+}
+
+/**
+ * Show info for one interface.
+ */
+static void show_iface(const char *name) {
+    char path[128];
+    char val[64];
+
+    puts(name);
+    puts(":\n");
+
+    /* MAC address */
+    build_path(path, sizeof(path), name, "mac");
+    if (read_sysfs(path, val, sizeof(val)) > 0) {
+        puts("  MAC:     ");
+        puts(val);
+        puts("\n");
+    }
+
+    /* Link status */
+    build_path(path, sizeof(path), name, "link");
+    if (read_sysfs(path, val, sizeof(val)) > 0) {
+        puts("  Link:    ");
+        puts(val);
+        puts("\n");
+    }
+
+    /* IP address */
+    build_path(path, sizeof(path), name, "ip");
+    if (read_sysfs(path, val, sizeof(val)) > 0) {
+        puts("  IP:      ");
+        puts(val);
+        puts("\n");
+    }
+
+    /* Netmask */
+    build_path(path, sizeof(path), name, "netmask");
+    if (read_sysfs(path, val, sizeof(val)) > 0) {
+        puts("  Netmask: ");
+        puts(val);
+        puts("\n");
+    }
+
+    /* Gateway */
+    build_path(path, sizeof(path), name, "gateway");
+    if (read_sysfs(path, val, sizeof(val)) > 0) {
+        puts("  Gateway: ");
+        puts(val);
+        puts("\n");
+    }
+}
+
+/**
+ * Write a value to a sysfs file.
+ */
+static int32_t write_sysfs(const char *path, const char *value) {
+    int32_t fd = open(path, 0);
+    if (fd < 0) return -1;
+
+    int32_t n = write(fd, value, strlen(value));
+    close(fd);
+    return n;
+}
+
+int main(void) {
+    char arg1[64];
+
+    /* Check if we have a subcommand */
+    if (getenv("ARG1", arg1, sizeof(arg1)) < 0 || arg1[0] == '\0') {
+        /* No arguments — show all interfaces */
+        char name[128];
+        uint32_t idx = 0;
+        int found = 0;
+
+        while (readdir("/sys/net", idx, name) >= 0) {
+            show_iface(name);
+            found = 1;
+            idx++;
+        }
+
+        if (!found) {
+            puts("No network interfaces found.\n");
+        }
+        return 0;
+    }
+
+    /* Check for "set" subcommand */
+    if (strcmp(arg1, "set") == 0) {
+        char iface_name[32], ip[32], netmask[32], gateway[32];
+        char path[128];
+
+        if (getenv("ARG2", iface_name, sizeof(iface_name)) < 0) {
+            puts("Usage: ip set <iface> <ip> <netmask> <gateway>\n");
+            return 1;
+        }
+        if (getenv("ARG3", ip, sizeof(ip)) < 0) {
+            puts("Usage: ip set <iface> <ip> <netmask> <gateway>\n");
+            return 1;
+        }
+        if (getenv("ARG4", netmask, sizeof(netmask)) < 0) {
+            puts("Usage: ip set <iface> <ip> <netmask> <gateway>\n");
+            return 1;
+        }
+        if (getenv("ARG5", gateway, sizeof(gateway)) < 0) {
+            puts("Usage: ip set <iface> <ip> <netmask> <gateway>\n");
+            return 1;
+        }
+
+        /* Write IP */
+        build_path(path, sizeof(path), iface_name, "ip");
+        if (write_sysfs(path, ip) < 0) {
+            puts("Failed to set IP address\n");
+            return 1;
+        }
+
+        /* Write netmask */
+        build_path(path, sizeof(path), iface_name, "netmask");
+        if (write_sysfs(path, netmask) < 0) {
+            puts("Failed to set netmask\n");
+            return 1;
+        }
+
+        /* Write gateway */
+        build_path(path, sizeof(path), iface_name, "gateway");
+        if (write_sysfs(path, gateway) < 0) {
+            puts("Failed to set gateway\n");
+            return 1;
+        }
+
+        puts("Configured ");
+        puts(iface_name);
+        puts(": ");
+        puts(ip);
+        puts(" / ");
+        puts(netmask);
+        puts(" gw ");
+        puts(gateway);
+        puts("\n");
+        return 0;
+    }
+
+    puts("Unknown command: ");
+    puts(arg1);
+    puts("\nUsage: ip [set <iface> <ip> <netmask> <gateway>]\n");
+    return 1;
+}