[PATCH v4 2/3] rust: uaccess: add method to get UserPtr from u64

[Deborah Brouwer <deborah.brouwer@collabora.com>]

Currently a UserPtr can be created from a usize integer representing a
userspace address. To create a UserPtr from a u64 integer, the u64 must
be cast to usize. On 32-bit systems, this cast will discard the upper 32
bits of the address.

Provide a new method to create a UserPtr from a u64 integer. This avoids
the caller having to cast before creating a UserPtr and also clarifies
that the truncation is intended. This is equivalent to the C-side
macro u64_to_user_ptr() that provides typechecking and casting to a __user
pointer.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
---
 rust/kernel/uaccess.rs | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index 8e18285ec44b..a6fd1af155b3 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs
@@ -31,6 +31,15 @@ pub fn from_addr(addr: usize) -> Self {
         Self(addr as *mut c_void)
     }
 
+    /// Create a `UserPtr` from a u64 userspace address.
+    ///
+    /// Deliberately truncate upper bits for 32-bit systems.
+    #[doc(alias = "u64_to_user_ptr")]
+    #[inline]
+    pub fn from_u64(addr: u64) -> Self {
+        Self::from_addr(addr as usize)
+    }
+
     /// Create a `UserPtr` from a pointer representing the userspace address.
     #[inline]
     pub fn from_ptr(addr: *mut c_void) -> Self {

-- 
2.56.0