From a3919f117041524042e16c7698a8a08a16d42a4c Mon Sep 17 00:00:00 2001
From: Changyuan Lyu <changyuanl@google.com>
Date: Thu, 9 May 2024 22:42:30 -0700
Subject: [PATCH] feat(net): use a tun/tap interface as a backend

The current virtio-net implementation assumes the tap device path is
referring to a macvtap or ipvtap. This commit allows using a tun/tap
interface pre-created by `ip tuntap add mode tap $IF_NAME` as a
backend.

Usage:

```
--net if=$IF_NAME,mac=$IF_MAC,mtu=$IF_MTU
```

Signed-off-by: Changyuan Lyu <changyuanl@google.com>
---
 alioth-cli/src/main.rs           |  7 +++++--
 alioth/src/virtio/dev/net/net.rs | 19 ++++++++++++++++---
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/alioth-cli/src/main.rs b/alioth-cli/src/main.rs
index 80ad91a..2fd1752 100644
--- a/alioth-cli/src/main.rs
+++ b/alioth-cli/src/main.rs
@@ -101,8 +101,10 @@ fn parse_mem(s: &str) -> Result<usize> {
 fn parse_net<'a>(s: &'a str) -> Result<NetParam> {
     let mut parts = s.trim().splitn(3, ',');
     let splitter = |s: &'a str| s.split_once::<'a, _>('=');
-    let Some(("tap", tap_path)) = parts.next().and_then(splitter) else {
-        bail!("invalid net opt: {s}");
+    let (tap_path, if_name) = match parts.next().and_then(splitter) {
+        Some(("tap", p)) => (p, None),
+        Some(("if", name)) => ("/dev/net/tun", Some(name.to_owned())),
+        _ => bail!("invalid net opt: {s}"),
     };
     let Some(("mac", mac_str)) = parts.next().and_then(splitter) else {
         bail!("invalid net opt: {s}");
@@ -133,6 +135,7 @@ fn parse_net<'a>(s: &'a str) -> Result<NetParam> {
         mtu: mtu_str.parse()?,
         queue_pairs: 1,
         tap: tap_path.into(),
+        if_name,
     })
 }
 
diff --git a/alioth/src/virtio/dev/net/net.rs b/alioth/src/virtio/dev/net/net.rs
index b88201a..051262a 100644
--- a/alioth/src/virtio/dev/net/net.rs
+++ b/alioth/src/virtio/dev/net/net.rs
@@ -15,6 +15,8 @@
 use std::fmt::Debug;
 use std::fs::{self, File};
 use std::io;
+use std::iter::zip;
+use std::mem::MaybeUninit;
 use std::os::fd::AsRawFd;
 use std::os::unix::prelude::OpenOptionsExt;
 use std::path::PathBuf;
@@ -107,6 +109,7 @@ pub struct NetParam {
     pub mtu: u16,
     pub queue_pairs: u16,
     pub tap: PathBuf,
+    pub if_name: Option<String>,
 }
 
 impl DevParam for NetParam {
@@ -125,7 +128,7 @@ impl Net {
             .write(true)
             .open(param.tap)?;
 
-        setup_tap(&mut file)?;
+        setup_tap(&mut file, param.if_name.as_deref())?;
         let net = Net {
             name,
             config: Arc::new(NetConfig {
@@ -244,8 +247,18 @@ pub const TOKEN_TAP: Token = Token(0);
 
 const VNET_HEADER_SIZE: i32 = 12;
 
-fn setup_tap(file: &mut File) -> Result<()> {
-    let mut tap_ifconfig = unsafe { tun_get_iff(file) }?;
+fn setup_tap(file: &mut File, if_name: Option<&str>) -> Result<()> {
+    let mut tap_ifconfig = match if_name {
+        None => unsafe { tun_get_iff(file) }?,
+        Some(name) => {
+            let mut tap_ifconfig = unsafe { MaybeUninit::<libc::ifreq>::zeroed().assume_init() };
+            for (s, d) in zip(name.as_bytes(), tap_ifconfig.ifr_name.as_mut()) {
+                *d = *s as i8;
+            }
+            tap_ifconfig
+        }
+    };
+
     tap_ifconfig.ifr_ifru.ifru_flags = (IFF_TAP | IFF_NO_PI | IFF_VNET_HDR) as i16;
     unsafe { tun_set_iff(file, &tap_ifconfig) }?;
     unsafe { tun_set_vnet_hdr_sz(file, &VNET_HEADER_SIZE) }?;