devices: add on_sandboxed

On sandboxed will be invoked when the device is sandboxed. Device
implementation could do initialization here. It does not need to return
fd opened here to keep fds.

BUG=None
TEST=local build and run

Change-Id: I42c2b3cae3a87dd54f02e77b8cd10766309a0770
Reviewed-on: https://chromium-review.googlesource.com/1327513
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Jingkui Wang <jkwang@google.com>
Reviewed-by: David Tolnay <dtolnay@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>

devices/src/bus.rs

@@ -30,6 +30,8 @@ pub trait BusDevice: Send {
     fn config_register_read(&self, reg_idx: usize) -> u32 {
         0
     }
+    /// Invoked when the device is sandboxed.
+    fn on_sandboxed(&mut self) {}
 }
 
 #[derive(Debug)]

devices/src/pci/pci_device.rs

@@ -61,6 +61,8 @@ pub trait PciDevice: Send {
     /// * `addr` - The guest address inside the BAR.
     /// * `data` - The data to write.
     fn write_bar(&mut self, addr: u64, data: &[u8]);
+    /// Invoked when the device is sandboxed.
+    fn on_device_sandboxed(&mut self) {}
 }
 
 impl<T: PciDevice> BusDevice for T {
@@ -93,6 +95,10 @@ impl<T: PciDevice> BusDevice for T {
     fn config_register_read(&self, reg_idx: usize) -> u32 {
         self.config_registers().read_reg(reg_idx)
     }
+
+    fn on_sandboxed(&mut self) {
+        self.on_device_sandboxed();
+    }
 }
 
 impl<T: PciDevice + ?Sized> PciDevice for Box<T> {
@@ -126,4 +132,8 @@ impl<T: PciDevice + ?Sized> PciDevice for Box<T> {
     fn write_bar(&mut self, addr: u64, data: &[u8]) {
         (**self).write_bar(addr, data)
     }
+    /// Invoked when the device is sandboxed.
+    fn on_device_sandboxed(&mut self) {
+        (**self).on_device_sandboxed()
+    }
 }

devices/src/proxy.rs

@@ -143,6 +143,7 @@ impl ProxyDevice {
         let pid = unsafe {
             match jail.fork(Some(&keep_fds)).map_err(Error::ForkingJail)? {
                 0 => {
+                    device.on_sandboxed();
                     child_proc(child_sock, &mut device);
                     // ! Never returns
                     process::exit(0);