[antlir2_vm][net] Add option to dump traffic to a pcap file

Summary:
Traffic to/from a qemu netdev can be dumped to a pcap file using the
option:
```
-object filter-dump,id=id,netdev=dev[,file=filename][,maxlen=len][,position=head|tail|id=<id>][,insert=behind|before]
Dump the network traffic on netdev dev to the file specified by filename. At most len bytes (64k by default) per packet are stored. The file format is libpcap, so it can be analyzed with tools such as tcpdump or Wireshark.
```

This diff adds the `dump_file` field to `VirtualNic` struct and a setter.
When dump_file is `None`, no traffic is being dumped. When `Some(path)`,
traffic is dumped to `path`.

Test Plan: buck2 test and upcoming diffs

Reviewed By: wujj123456

Differential Revision: D63498734

fbshipit-source-id: bbcdd602550dbd230418229ef8f8688c943b5f30

Author: chantra

antlir/antlir2/antlir2_vm/src/net.rs

@@ -12,6 +12,7 @@ use std::ffi::OsString;
 use std::net::Ipv6Addr;
 use std::ops::Index;
 use std::ops::IndexMut;
+use std::path::PathBuf;
 use std::process::Command;
 
 use thiserror::Error;
@@ -27,6 +28,8 @@ pub(crate) struct VirtualNIC {
     id: usize,
     /// Max Combined Channels of the NIC. Multi-queue is disabled when set to 1
     max_combined_channels: usize,
+    /// Dump interface traffic to this file. This is not supported for multi-queue NICs.
+    dump_file: Option<PathBuf>,
 }
 
 #[derive(Error, Debug)]
@@ -35,6 +38,8 @@ pub(crate) enum VirtualNICError {
     IPCmdExecError(#[from] std::io::Error),
     #[error("Error from command: `{0}` ")]
     IPCmdReturnError(String),
+    #[error("Traffic is not dumpable: `{0}` ")]
+    TrafficDumpingNotSupported(String),
 }
 
 type Result<T> = std::result::Result<T, VirtualNICError>;
@@ -46,6 +51,7 @@ impl VirtualNIC {
         Self {
             id,
             max_combined_channels,
+            dump_file: None,
         }
     }
 
@@ -92,6 +98,18 @@ impl VirtualNIC {
             })
     }
 
+    /// Set the file to dump interface traffic to.
+    /// Set to None to disable dumping traffic.
+    pub(crate) fn try_dump_file(&mut self, path: Option<PathBuf>) -> Result<&mut Self> {
+        if self.max_combined_channels > 1 {
+            return Err(VirtualNICError::TrafficDumpingNotSupported(
+                "Can not dump traffic for multi-queue NIC: https://fburl.com/dmblggwc".into(),
+            ));
+        }
+        self.dump_file = path;
+        Ok(self)
+    }
+
     /// Host side IP address. It's fd00:<id>::1.
     fn host_ipv6_addr(&self) -> Ipv6Addr {
         let mut ip: u128 = 0xfd00 << (128 - 16);
@@ -116,7 +134,7 @@ impl VirtualNIC {
 
 impl QemuDevice for VirtualNIC {
     fn qemu_args(&self) -> Vec<OsString> {
-        [
+        let mut vec: Vec<_> = [
             "-netdev",
             &format!(
                 "tap,id={dev_id},ifname={dev_name},script=no,downscript=no,queues={queues}",
@@ -141,7 +159,24 @@ impl QemuDevice for VirtualNIC {
         ]
         .iter()
         .map(|x| x.into())
-        .collect()
+        .collect();
+        if let Some(path) = &self.dump_file {
+            vec.extend(
+                [
+                    "-object",
+                    &format!(
+                        "filter-dump,id=dump0,netdev={},file={}",
+                        self.dev_id(),
+                        path.to_string_lossy()
+                    ),
+                ]
+                .iter()
+                .map(|x| x.into())
+                .collect::<Vec<_>>(),
+            );
+        }
+
+        vec
     }
 }
 
@@ -219,6 +254,38 @@ mod test {
         )
     }
 
+    #[test]
+    fn test_qemu_args_with_dump_file() {
+        let dump_file = PathBuf::from("/tmp/dump");
+        let mut nic = VirtualNIC::new(0, 1);
+        nic.try_dump_file(Some(dump_file.clone())).unwrap();
+
+        assert_eq!(
+            nic.qemu_args().join(OsStr::new(" ")),
+            format!(
+                "-netdev tap,id=net0,ifname=vm0,script=no,downscript=no,queues=1 \
+            -device virtio-net-pci,netdev=net0,mac=00:00:00:00:00:01,mq=off,vectors=4 \
+            -object filter-dump,id=dump0,netdev=net0,file={}",
+                dump_file.to_string_lossy()
+            )
+            .as_str()
+        )
+    }
+
+    #[test]
+    // This test is to make sure that the dump file is not added to the qemu args when it's not supported (multi-queue nics)
+    fn test_qemu_args_with_dump_file_not_supported() {
+        let dump_file = PathBuf::from("/tmp/dump");
+        let mut nic = VirtualNIC::new(0, 2);
+        assert!(nic.try_dump_file(Some(dump_file.clone())).is_err());
+
+        assert_eq!(
+            nic.qemu_args().join(OsStr::new(" ")),
+            "-netdev tap,id=net0,ifname=vm0,script=no,downscript=no,queues=2 \
+            -device virtio-net-pci,netdev=net0,mac=00:00:00:00:00:01,mq=on,vectors=6"
+        )
+    }
+
     #[test]
     fn test_qemu_multiqueue_args() {
         assert_eq!(