Revert "ANDROID: ashmem: use strncpy_from_user in set_name"

This reverts commit ba971cd36a.

Bug: 414339114
Change-Id: If44d5a7516c7dfdda68272f629ce30d74c84694f
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

drivers/staging/android/ashmem_rust.rs

@@ -297,16 +297,17 @@ impl MiscDevice for Ashmem {
 }
 
 impl Ashmem {
-    fn set_name(&self, reader: UserSliceReader) -> Result<isize> {
+    fn set_name(&self, mut reader: UserSliceReader) -> Result<isize> {
         let mut local_name = [0u8; ASHMEM_NAME_LEN];
-        let mut len = reader.strncpy_from_user(&mut local_name)?;
+        reader.read_slice(&mut local_name)?;
 
-        // If the zero terminator is missing, the string is truncated to `ASHMEM_NAME_LEN-1` so
-        // that `get_name` can return it and has enough space to add a zero terminator.
-        if len == ASHMEM_NAME_LEN {
-            len -= 1;
-            local_name[len] = 0;
-        }
+        // Find the zero terminator. If the zero terminator is missing, the string is truncated to
+        // `ASHMEM_NAME_LEN-1` so that `get_name` can return it and has enough space to add a zero
+        // terminator.
+        let len = local_name
+            .iter()
+            .position(|&c| c == 0)
+            .unwrap_or(local_name.len() - 1);
 
         let mut v = KVec::with_capacity(len, GFP_KERNEL)?;
         v.extend_from_slice(&local_name[..len], GFP_KERNEL)?;

rust/kernel/uaccess.rs

@@ -294,19 +294,6 @@ impl UserSliceReader {
         unsafe { buf.inc_len(len) };
         Ok(())
     }
-
-    /// Reads a nul-terminated string into the provided buffer and returns the length.
-    pub fn strncpy_from_user(self, buf: &mut [u8]) -> Result<usize> {
-        let max = usize::min(self.length, buf.len()) as isize;
-        // SAFETY: `buf` is valid for writing `buf.len()` bytes.
-        let res =
-            unsafe { bindings::strncpy_from_user(buf.as_mut_ptr(), self.ptr as *const u8, max) };
-        if res >= 0 {
-            Ok(res as usize)
-        } else {
-            Err(Error::from_errno(res as i32))
-        }
-    }
 }
 
 /// A writer for [`UserSlice`].