feat: fall back to userfaultfd syscall on permission errors

Author: vapourismo

CHANGELOG.md

@@ -1,3 +1,7 @@
+### Unreleased
+
+- Fallback to `userfaultfd` syscall when opening `/dev/userfaultfd` fails due to permissions
+
 ### 0.9.0
 
 - Add support for `UFFDIO_CONTINUE` and `UFFDIO_REGISTER_MODE_MINOR` under the new `linux5_13` feature.

src/builder.rs

@@ -147,17 +147,23 @@ impl UffdBuilder {
     // fall back to calling the system call.
     fn open_file_descriptor(&self, flags: i32) -> Result<Uffd> {
         // If `/dev/userfaultfd` exists we'll try to get the file descriptor from it. If the file
-        // doesn't exist we will fall back to calling the system call. This means, that if the
-        // device exists but the calling process does not have access rights to it, this will fail,
-        // i.e. we will not fall back to calling the system call.
+        // doesn't exist or we don't have the permissions to open it, we will fall back to calling
+        // the system call. This means, that if the device exists but the calling process does not
+        // have access rights to it, this will fail, i.e. we will not fall back to calling the
+        // system call.
         match OpenOptions::new()
             .read(true)
             .write(true)
             .open(UFFD_DEVICE_PATH)
         {
             Ok(mut file) => self.uffd_from_dev(&mut file, flags),
-            Err(err) if err.kind() == ErrorKind::NotFound => self.uffd_from_syscall(flags),
-            Err(err) => Err(Error::OpenDevUserfaultfd(err)),
+            Err(err) => {
+                if let ErrorKind::NotFound | ErrorKind::PermissionDenied = err.kind() {
+                    self.uffd_from_syscall(flags)
+                } else {
+                    Err(Error::OpenDevUserfaultfd(err))
+                }
+            }
         }
     }