ANDROID: ashmem: use strncpy_from_user in set_name

Ashmem expected that it could always read ASHMEM_NAME_LEN bytes from the
provided pointer even if the actual string was shorter. This assumption
turns out to be false.

Bug: 409422661
Change-Id: I617be6c91947d44ec80b894c41fabe0f5c539eae
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

drivers/staging/android/ashmem_rust.rs

@@ -284,17 +284,16 @@ impl MiscDevice for Ashmem {
 }
 
 impl Ashmem {
-    fn set_name(&self, mut reader: UserSliceReader) -> Result<isize> {
+    fn set_name(&self, reader: UserSliceReader) -> Result<isize> {
         let mut local_name = [0u8; ASHMEM_NAME_LEN];
-        reader.read_slice(&mut local_name)?;
+        let mut len = reader.strncpy_from_user(&mut local_name)?;
 
-        // 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);
+        // 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;
+        }
 
         let mut v = KVec::with_capacity(len, GFP_KERNEL)?;
         v.extend_from_slice(&local_name[..len], GFP_KERNEL)?;

rust/kernel/uaccess.rs

@@ -294,6 +294,19 @@ impl UserSliceReader {
         unsafe { buf.set_len(buf.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`].