Add UserSlicePtrWriter::clear.

Signed-off-by: Wedson Almeida Filho <[email protected]>

Author: wedsonaf

rust/kernel/user_ptr.rs

@@ -4,7 +4,7 @@
 //!
 //! C header: [`include/linux/uaccess.h`](../../../../include/linux/uaccess.h)
 
-use crate::{c_types, error::Error, KernelResult};
+use crate::{bindings, c_types, error::Error, KernelResult};
 use alloc::vec::Vec;
 use core::mem::{size_of, MaybeUninit};
 
@@ -242,6 +242,27 @@ impl UserSlicePtrWriter {
         self.len() == 0
     }
 
+    /// Writes zeroes to the user slice.
+    pub fn clear(&mut self, mut len: usize) -> KernelResult {
+        let mut ret = Ok(());
+        if len > self.1 {
+            ret = Err(Error::EFAULT);
+            len = self.1;
+        }
+
+        // SAFETY: The buffer will be validated by `clear_user`. We ensure that `len` is within
+        // bounds in the check above.
+        let left = unsafe { bindings::clear_user(self.0, len as _) } as usize;
+        if left != 0 {
+            ret = Err(Error::EFAULT);
+            len -= left;
+        }
+
+        self.0 = self.0.wrapping_add(len);
+        self.1 -= len;
+        ret
+    }
+
     /// Writes a byte slice to the user slice.
     ///
     /// Returns `EFAULT` if the byte slice is bigger than the remaining size