devices: ivshmem: Don't remap file on BAR reprogramming

The ivshmem device was remapping the host file every time the BAR was
reprogrammed. This occurs on the vCPU thread and so was falling foul of
our improved seccomp filters. Avoid this by maintaining the existing
region when the device BARs are changed and just adjust the guest GPA.

Assisted-by: Codex:GPT-5.6
Signed-off-by: Rob Bradford <rbradford@meta.com>

devices/src/ivshmem.rs

@@ -49,6 +49,8 @@ pub enum IvshmemError {
     CreateUserspaceMapping(#[source] anyhow::Error),
     #[error("Failed to remove old userspace mapping.")]
     RemoveUserspaceMapping(#[source] anyhow::Error),
+    #[error("Missing memory region")]
+    MissingMemoryRegion,
 }
 
 #[derive(Copy, Clone)]
@@ -70,6 +72,12 @@ pub trait IvshmemOps: Send + Sync {
         backing_file: Option<PathBuf>,
     ) -> Result<(Arc<MmapRegion>, UserspaceMapping), IvshmemError>;
 
+    fn create_userspace_mapping(
+        &mut self,
+        start_addr: u64,
+        region: Arc<MmapRegion>,
+    ) -> Result<UserspaceMapping, IvshmemError>;
+
     fn unmap_ram_region(&mut self, mapping: UserspaceMapping) -> Result<(), IvshmemError>;
 }
 
@@ -94,7 +102,6 @@ pub struct IvshmemDevice {
 
     region_size: u64,
     ivshmem_ops: Arc<Mutex<dyn IvshmemOps>>,
-    backend_file: Option<PathBuf>,
     region: Option<Arc<MmapRegion>>,
     userspace_mapping: Option<UserspaceMapping>,
 }
@@ -111,7 +118,6 @@ impl IvshmemDevice {
     pub fn new(
         id: String,
         region_size: u64,
-        backend_file: Option<PathBuf>,
         ivshmem_ops: Arc<Mutex<dyn IvshmemOps>>,
         snapshot: Option<&Snapshot>,
     ) -> Result<Self, IvshmemError> {
@@ -159,7 +165,6 @@ impl IvshmemDevice {
                 ivshmem_ops,
                 region: None,
                 userspace_mapping: None,
-                backend_file,
             }
         } else {
             IvshmemDevice {
@@ -174,7 +179,6 @@ impl IvshmemDevice {
                 ivshmem_ops,
                 region: None,
                 userspace_mapping: None,
-                backend_file,
             }
         };
         Ok(device)
@@ -353,6 +357,10 @@ impl PciDevice for IvshmemDevice {
 
     fn move_bar(&mut self, old_base: u64, new_base: u64) -> io::Result<()> {
         if new_base == self.data_bar_addr() {
+            let region = self
+                .region
+                .clone()
+                .ok_or_else(|| io::Error::other(IvshmemError::MissingMemoryRegion))?;
             if let Some(old_mapping) = self.userspace_mapping.take() {
                 self.ivshmem_ops
                     .lock()
@@ -360,17 +368,13 @@ impl PciDevice for IvshmemDevice {
                     .unmap_ram_region(old_mapping)
                     .map_err(io::Error::other)?;
             }
-            let (region, new_mapping) = self
+            let new_mapping = self
                 .ivshmem_ops
                 .lock()
                 .unwrap()
-                .map_ram_region(
-                    new_base,
-                    self.region_size as usize,
-                    self.backend_file.clone(),
-                )
+                .create_userspace_mapping(new_base, region)
                 .map_err(io::Error::other)?;
-            self.set_region(region, new_mapping);
+            self.userspace_mapping = Some(new_mapping);
         }
         for bar in self.bar_regions.iter_mut() {
             if bar.addr() == old_base {

vmm/src/device_manager.rs

@@ -4676,7 +4676,6 @@ impl DeviceManager {
             devices::IvshmemDevice::new(
                 id.clone(),
                 ivshmem_cfg.size as u64,
-                Some(ivshmem_cfg.path.clone()),
                 ivshmem_ops.clone(),
                 snapshot,
             )
@@ -5601,6 +5600,16 @@ impl IvshmemOps for IvshmemHandler {
             false,
         )
         .map_err(|e| IvshmemError::CreateUserMemoryRegion(e.into()))?;
+        let region = Arc::new(region);
+        let mapping = self.create_userspace_mapping(start_addr, region.clone())?;
+        Ok((region, mapping))
+    }
+
+    fn create_userspace_mapping(
+        &mut self,
+        start_addr: u64,
+        region: Arc<MmapRegion<AtomicBitmap>>,
+    ) -> Result<UserspaceMapping, IvshmemError> {
         let mem_slot = {
             let mut manager = self.memory_manager.lock().unwrap();
             // SAFETY: guaranteed by MmapRegion invariants
@@ -5616,14 +5625,12 @@ impl IvshmemOps for IvshmemHandler {
             }
         }
         .map_err(|e| IvshmemError::CreateUserspaceMapping(e.into()))?;
-        let region = Arc::new(region);
-        let mapping = UserspaceMapping {
-            mapping: region.clone(),
+        Ok(UserspaceMapping {
+            mapping: region,
             mem_slot,
             addr: GuestAddress(start_addr),
             mergeable: false,
-        };
-        Ok((region, mapping))
+        })
     }
 
     fn unmap_ram_region(&mut self, mapping: UserspaceMapping) -> Result<(), IvshmemError> {