hypervisor: support user space mounts

This commit introduces the ability to mount custom files and folders
from the host operating system onto a partition.

The hypervisor configuration is adjusted as follows at partition layer:
```yaml
mounts:
  - [ "/dev/urandom", "/dev/urandom"]
  - [ "/dev/urandom", "/dev/random"]
  - [ "/a/file/on/the/host", "/guest" ]
```

Future features could additionally include the ability to mount things
read-only, etc.

Fixes #42

hypervisor/src/hypervisor/config.rs

@@ -38,6 +38,8 @@ pub struct Partition {
     pub devices: Vec<Device>,
     #[serde(default)]
     pub hm_table: PartitionHMTable,
+    #[serde(default)]
+    pub mounts: Vec<(PathBuf, PathBuf)>,
 }
 
 #[derive(Debug, Serialize, Deserialize, Clone)]

hypervisor/src/hypervisor/partition.rs

@@ -5,7 +5,7 @@ use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 use std::time::Duration;
 
-use anyhow::anyhow;
+use anyhow::{anyhow, bail};
 use apex_rs::bindings::PortDirection;
 use apex_rs::prelude::{OperatingMode, StartCondition};
 use clone3::Clone3;
@@ -99,6 +99,36 @@ impl FileMounter {
     }
 }
 
+impl TryFrom<&(PathBuf, PathBuf)> for FileMounter {
+    type Error = anyhow::Error;
+
+    fn try_from(paths: &(PathBuf, PathBuf)) -> Result<Self, Self::Error> {
+        let source = &paths.0;
+        let mut target = paths.1.clone();
+
+        if !source.exists() {
+            bail!("File/Directory {} not existent", source.display())
+        }
+
+        if target.is_absolute() {
+            // Convert absolute paths into relative ones.
+            // Otherwise we will receive a permission error.
+            // TODO: Make this a function?
+            target = target.strip_prefix("/")?.into();
+            assert!(target.is_relative());
+        }
+
+        Ok(Self {
+            source: Some(source.clone()),
+            target: target,
+            fstype: None,
+            flags: MsFlags::MS_BIND,
+            data: None,
+            is_dir: source.is_dir(),
+        })
+    }
+}
+
 impl Run {
     pub fn new(base: &Base, condition: StartCondition, warm_start: bool) -> TypedResult<Run> {
         let cgroup_main = base.cgroup.new("main").typ(SystemError::CGroup)?;
@@ -174,7 +204,7 @@ impl Run {
                 Partition::release_fds(&keep).unwrap();
 
                 // Mount the required mounts
-                let mounts = [
+                let mut mounts = vec![
                     // Mount working directory as tmpfs
                     FileMounter {
                         source: None,
@@ -222,6 +252,12 @@ impl Run {
                     },
                 ];
 
+                for m in &base.mounts {
+                    mounts.push(m.try_into().unwrap());
+                }
+
+                // TODO: Check for duplicate mounts
+
                 for m in mounts {
                     debug!("mounting {:?}", &m);
                     m.mount(base.working_dir.path()).unwrap();
@@ -449,6 +485,7 @@ pub(crate) struct Base {
     hm: PartitionHMTable,
     id: i64,
     bin: PathBuf,
+    mounts: Vec<(PathBuf, PathBuf)>,
     cgroup: CGroup,
     sampling_channel: HashMap<String, SamplingConstant>,
     duration: Duration,
@@ -517,6 +554,7 @@ impl Partition {
             id: config.id,
             cgroup,
             bin: config.image,
+            mounts: config.mounts,
             duration: config.duration,
             period: config.period,
             working_dir,