Add copy_wp() method for UFFDIO_COPY with UFFDIO_COPY_MODE_WP

Add a new method that combines UFFDIO_COPY with UFFDIO_COPY_MODE_WP to
resolve a page fault and enable write-protection tracking in a single
ioctl call, useful for dirty-page tracking scenarios.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vtta

src/lib.rs

@@ -175,6 +175,49 @@ impl Uffd {
         }
     }
 
+    /// Atomically copy a continuous memory chunk into the userfaultfd-registered range and
+    /// register the pages as write-protected, returning the number of bytes successfully copied.
+    ///
+    /// This combines `UFFDIO_COPY` with `UFFDIO_COPY_MODE_WP` to resolve a page fault and
+    /// enable write-protection tracking in a single ioctl.
+    ///
+    /// If `wake` is `true`, wake up the thread waiting for page fault resolution on the memory
+    /// range.
+    #[cfg(feature = "linux5_7")]
+    pub unsafe fn copy_wp(
+        &self,
+        src: *const c_void,
+        dst: *mut c_void,
+        len: usize,
+        wake: bool,
+    ) -> Result<usize> {
+        let mut copy = raw::uffdio_copy {
+            src: src as u64,
+            dst: dst as u64,
+            len: len as u64,
+            mode: raw::UFFDIO_COPY_MODE_WP
+                | if wake {
+                    0
+                } else {
+                    raw::UFFDIO_COPY_MODE_DONTWAKE
+                },
+            copy: 0,
+        };
+
+        let _ =
+            raw::copy(self.as_raw_fd(), &mut copy as *mut raw::uffdio_copy).map_err(|errno| {
+                match errno {
+                    Errno::EAGAIN => Error::PartiallyCopied(copy.copy as usize),
+                    _ => Error::CopyFailed(errno),
+                }
+            })?;
+        if copy.copy < 0 {
+            Err(Error::CopyFailed(Errno::from_i32(-copy.copy as i32)))
+        } else {
+            Ok(copy.copy as usize)
+        }
+    }
+
     /// Zero out a memory address range registered with userfaultfd, and return the number of bytes
     /// that were successfully zeroed.
     ///
@@ -708,4 +751,100 @@ mod test {
 
         Ok(())
     }
+
+    /// Test that `copy_wp()` resolves a missing fault and applies write-protection in one ioctl.
+    ///
+    /// 1. Create a uffd registered for both MISSING and WRITE_PROTECT
+    /// 2. Prepare a source page with value `42`
+    /// 3. Spawn a thread that reads then writes the mapping
+    /// 4. Handle the missing fault with `copy_wp()` — copies source data and sets WP in one ioctl
+    /// 5. The thread's read succeeds (sees `42`), then the write triggers a WP fault
+    /// 6. Handle the WP fault by removing write-protection
+    /// 7. Verify the thread's write (`99`) landed
+    #[cfg(feature = "linux5_7")]
+    #[test]
+    fn test_copy_wp() -> Result<()> {
+        const PAGE_SIZE: usize = 4096;
+
+        unsafe {
+            let uffd = UffdBuilder::new()
+                .require_features(FeatureFlags::PAGEFAULT_FLAG_WP)
+                .close_on_exec(true)
+                .create()?;
+
+            let mapping = libc::mmap(
+                ptr::null_mut(),
+                PAGE_SIZE,
+                libc::PROT_READ | libc::PROT_WRITE,
+                libc::MAP_PRIVATE | libc::MAP_ANON,
+                -1,
+                0,
+            );
+
+            assert!(!mapping.is_null());
+
+            assert!(uffd
+                .register_with_mode(
+                    mapping,
+                    PAGE_SIZE,
+                    RegisterMode::MISSING | RegisterMode::WRITE_PROTECT
+                )?
+                .contains(IoctlFlags::WRITE_PROTECT));
+
+            // Prepare a source page to copy from
+            let src = libc::mmap(
+                ptr::null_mut(),
+                PAGE_SIZE,
+                libc::PROT_READ | libc::PROT_WRITE,
+                libc::MAP_PRIVATE | libc::MAP_ANON,
+                -1,
+                0,
+            );
+            assert!(!src.is_null());
+            *(src as *mut u8) = 42;
+
+            let ptr = mapping as usize;
+            let thread = thread::spawn(move || {
+                let ptr = ptr as *mut u8;
+                // First access triggers missing fault; after copy_wp resolves it
+                // with WP, this read succeeds but the subsequent write triggers
+                // a write-protect fault.
+                assert_eq!(*ptr, 42);
+                *ptr = 99;
+            });
+
+            loop {
+                match uffd.read_event()? {
+                    Some(Event::Pagefault { kind, addr, .. }) => match kind {
+                        FaultKind::Missing => {
+                            assert_eq!(addr, mapping);
+                            // Resolve the missing fault AND set write-protection in one call
+                            let copied =
+                                uffd.copy_wp(src as *const c_void, mapping, PAGE_SIZE, true)?;
+                            assert_eq!(copied, PAGE_SIZE);
+                        }
+                        FaultKind::WriteProtected => {
+                            assert_eq!(addr, mapping);
+                            // Page should have the copied content
+                            assert_eq!(*(addr as *const u8), 42);
+                            uffd.remove_write_protection(mapping, PAGE_SIZE, true)?;
+                            break;
+                        }
+                    },
+                    _ => panic!("unexpected event"),
+                }
+            }
+
+            thread.join().expect("failed to join thread");
+
+            assert_eq!(*(mapping as *const u8), 99);
+
+            uffd.unregister(mapping, PAGE_SIZE)?;
+
+            assert_eq!(libc::munmap(mapping, PAGE_SIZE), 0);
+            assert_eq!(libc::munmap(src, PAGE_SIZE), 0);
+        }
+
+        Ok(())
+    }
 }