virtiofs: support read-only mounts

include/libkrun.h

@@ -323,6 +323,26 @@ int32_t krun_add_virtiofs2(uint32_t ctx_id,
                            const char *c_path,
                            uint64_t shm_size);
 
+/**
+ * Adds an independent virtio-fs device pointing to a host's directory with a tag. This
+ * variant allows specifying the size of the DAX window and a read-only flag.
+ *
+ * Arguments:
+ *  "ctx_id"         - the configuration context ID.
+ *  "c_tag"          - tag to identify the filesystem in the guest.
+ *  "c_path"         - full path to the directory in the host to be exposed to the guest.
+ *  "shm_size"       - size of the DAX SHM window in bytes.
+ *  "read_only"      - if true, the filesystem will be exposed as read-only to the guest.
+ *
+ * Returns:
+ *  Zero on success or a negative error number on failure.
+ */
+int32_t krun_add_virtiofs3(uint32_t ctx_id,
+                           const char *c_tag,
+                           const char *c_path,
+                           uint64_t shm_size,
+                           bool read_only);
+
 /* Send the VFKIT magic after establishing the connection,
    as required by gvproxy in vfkit mode. */
 #define NET_FLAG_VFKIT 1 << 0

src/devices/src/virtio/fs/device.rs

@@ -13,7 +13,8 @@ use virtio_bindings::{virtio_config::VIRTIO_F_VERSION_1, virtio_ring::VIRTIO_RIN
 use vm_memory::{ByteValued, GuestMemoryMmap};
 
 use super::super::{
-    ActivateResult, DeviceQueue, DeviceState, FsError, QueueConfig, VirtioDevice, VirtioShmRegion,
+    ActivateError, ActivateResult, DeviceQueue, DeviceState, FsError, QueueConfig, VirtioDevice,
+    VirtioShmRegion,
 };
 use super::passthrough;
 use super::worker::FsWorker;
@@ -46,6 +47,7 @@ pub struct Fs {
     config: VirtioFsConfig,
     shm_region: Option<VirtioShmRegion>,
     passthrough_cfg: passthrough::Config,
+    read_only: bool,
     worker_thread: Option<JoinHandle<()>>,
     worker_stopfd: EventFd,
     exit_code: Arc<AtomicI32>,
@@ -59,6 +61,7 @@ impl Fs {
         shared_dir: String,
         exit_code: Arc<AtomicI32>,
         allow_root_dir_delete: bool,
+        read_only: bool,
     ) -> super::Result<Fs> {
         let avail_features = (1u64 << VIRTIO_F_VERSION_1) | (1u64 << VIRTIO_RING_F_EVENT_IDX);
 
@@ -80,6 +83,7 @@ impl Fs {
             config,
             shm_region: None,
             passthrough_cfg: fs_cfg,
+            read_only,
             worker_thread: None,
             worker_stopfd: EventFd::new(EFD_NONBLOCK).map_err(FsError::EventFd)?,
             exit_code,
@@ -183,11 +187,16 @@ impl VirtioDevice for Fs {
             mem.clone(),
             self.shm_region.clone(),
             self.passthrough_cfg.clone(),
+            self.read_only,
             self.worker_stopfd.try_clone().unwrap(),
             self.exit_code.clone(),
             #[cfg(target_os = "macos")]
             self.map_sender.clone(),
-        );
+        )
+        .map_err(|e| {
+            error!("virtio_fs: failed to create worker: {}", e);
+            ActivateError::BadActivate
+        })?;
         self.worker_thread = Some(worker.run());
 
         self.device_state = DeviceState::Activated(mem, interrupt);

src/devices/src/virtio/fs/macos/passthrough.rs

@@ -2313,6 +2313,12 @@ impl FileSystem for PassthroughFs {
             return Err(linux_error(io::Error::from_raw_os_error(libc::ENOSYS)));
         }
 
+        let open_flags = if (flags & fuse::SetupmappingFlags::WRITE.bits()) != 0 {
+            libc::O_RDWR
+        } else {
+            libc::O_RDONLY
+        };
+
         let prot_flags = if (flags & fuse::SetupmappingFlags::WRITE.bits()) != 0 {
             libc::PROT_READ | libc::PROT_WRITE
         } else {
@@ -2327,7 +2333,7 @@ impl FileSystem for PassthroughFs {
 
         debug!("setupmapping: ino {inode:?} guest_addr={guest_addr:x} len={len}");
 
-        let file = self.open_inode(inode, libc::O_RDWR)?;
+        let file = self.open_inode(inode, open_flags)?;
         let fd = file.as_raw_fd();
 
         let host_addr = unsafe {
@@ -2344,10 +2350,7 @@ impl FileSystem for PassthroughFs {
             return Err(linux_error(io::Error::last_os_error()));
         }
 
-        let ret = unsafe { libc::close(fd) };
-        if ret == -1 {
-            return Err(linux_error(io::Error::last_os_error()));
-        }
+        drop(file);
 
         // We've checked that map_sender is something above.
         let sender = map_sender.as_ref().unwrap();

src/devices/src/virtio/fs/mod.rs

@@ -4,6 +4,7 @@ mod filesystem;
 pub mod fuse;
 #[allow(dead_code)]
 mod multikey;
+mod read_only;
 mod server;
 mod worker;