TunDevice: add read/write, cleaner error handling

TunDevice.cpp

@@ -4,18 +4,23 @@
 #include <sys/ioctl.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <errno.h>
 #include <cstdlib>
 #include <cstring>
 
 #include "TunDevice.hpp"
 
+static const size_t TUN_MTU = 1500;  // TODO determine this cleanly
+
 TunDevice::TunDevice(const std::string& dev)
 {
     struct ifreq ifr;
-    int fd, err;
+    int fd;
 
-    if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )
-        throw TunDevice::InitializationError("Cannot open /dev/net/tun", fd);
+    if( (fd = open("/dev/net/tun", O_RDWR)) < 0 ) {
+        throw TunDevice::InitializationError(
+                "Cannot open /dev/net/tun", errno, true);
+    }
 
     memset(&ifr, 0, sizeof(ifr));
 
@@ -31,16 +36,50 @@ TunDevice::TunDevice(const std::string& dev)
         strncpy(ifr.ifr_name, dev.c_str(), IFNAMSIZ-1);
     }
 
-    if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
+    if(ioctl(fd, TUNSETIFF, (void *) &ifr) < 0){
         close(fd);
         throw TunDevice::InitializationError(
-                "Tunnel interface failed [TUNSETIFF]", err
-                );
+                "Tunnel interface failed [TUNSETIFF]", errno, true);
     }
     _dev_name = ifr.ifr_name;
     _fd = fd;
+
+    // The device is now fully set up
+    _poll_fd.fd = _fd;
+    _poll_fd.events = POLLIN;
 }
 
 TunDevice::~TunDevice() {
     close(_fd);
 }
+
+size_t TunDevice::read_packet(char* read_buffer, size_t buf_size, int timeout) {
+    int poll_rc = poll(&_poll_fd, 1, timeout);
+    if(poll_rc < 0) {
+        if(errno == EINTR) // Interrupt.
+            return 0;
+        throw TunDevice::NetError(
+                "Error polling from interface", errno, true);
+    }
+    else if(poll_rc == 0 || (_poll_fd.revents & POLLIN) == 0) {
+        // Nothing to read
+        return 0;
+    }
+
+    int nread = read(_fd, read_buffer, buf_size);
+    if(nread < 0) {
+        throw TunDevice::NetError(
+                "Error reading from interface", errno, true);
+    }
+    _last_read_size = nread;
+    return _last_read_size;
+}
+
+size_t TunDevice::write_packet(const char* data, size_t len) {
+    int nwritten = write(_fd, data, len);
+    if(nwritten < 0) {
+        throw TunDevice::NetError(
+                "Error writing to interface: ", errno, true);
+    }
+    return nwritten;
+}

TunDevice.hpp

@@ -1,14 +1,26 @@
 #pragma once
 
 #include <string>
+#include <poll.h>
 #include "util.hpp"
 
 class TunDevice {
     public:
         class InitializationError : public MsgException {
             public:
-                InitializationError(const std::string& msg, int code=0)
-                    : MsgException(msg, code) {}
+                InitializationError(
+                        const std::string& msg,
+                        int code=0,
+                        bool is_perror=false)
+                    : MsgException(msg, code, is_perror) {}
+        };
+        class NetError : public MsgException {
+            public:
+                NetError(
+                        const std::string& msg,
+                        int code=0,
+                        bool is_perror=false)
+                    : MsgException(msg, code, is_perror) {}
         };
 
         TunDevice(const std::string& dev);
@@ -16,7 +28,18 @@ class TunDevice {
 
         const std::string& get_dev_name() const { return _dev_name; }
         int get_fd() const { return _fd; }
+
+        /* Reads a packet from the device.
+         * Timeouts after `timeout` ms, or never if `timeout < 0`.
+         * Upon timeout, returns 0.
+         */
+        size_t read_packet(char* read_buffer, size_t buf_size, int timeout=-1);
+
+        size_t write_packet(const char* data, size_t len);
+
     private:
         int _fd;
         std::string _dev_name;
+        struct pollfd _poll_fd;
+        size_t _last_read_size;
 };

util.cpp

@@ -1,13 +1,19 @@
 #include <cstdio>
+#include <string.h>
 
 #include "util.hpp"
 
-MsgException::MsgException(const std::string& msg, int code)
+MsgException::MsgException(const std::string& msg, int code, bool is_perror)
     : _msg(msg), _code(code)
 {
     _what = _msg;
 
     if(_code != 0) {
+        if(is_perror) {
+            _what += ": ";
+            _what += strerror(errno);
+        }
+
         char remainder[20];
         sprintf(remainder, " (code %d)", _code);
         _what += remainder;

util.hpp

@@ -3,10 +3,14 @@
 #include <exception>
 #include <string>
 
-// MsgException -- an exception bearing a passed explanation message
+/** MsgException -- an exception bearing a passed explanation message
+ *
+ * If `is_perror` is true, then the `strerror` corresponding message is appened
+ * to the message in `what()`.
+ */
 class MsgException : public std::exception {
     public:
-        MsgException(const std::string& msg, int code=0);
+        MsgException(const std::string& msg, int code=0, bool is_perror=false);
         int errcode() const noexcept { return _code; }
         const char* what() const noexcept { return _what.c_str(); };