Clone the reader to prevent cursor misplacement in ReadCString

kernel/src/context.rs

@@ -204,13 +204,15 @@ impl ReadCString for VmReader<'_, Fallible> {
         );
 
         // Handle the rest of the bytes in bulk
+        let mut cloned_reader = self.clone();
         while (buffer.len() + mem::size_of::<usize>()) <= max_len {
-            let Ok(word) = self.read_val::<usize>() else {
+            let Ok(word) = cloned_reader.read_val::<usize>() else {
                 break;
             };
 
             if has_zero(word) {
                 for byte in word.to_ne_bytes() {
+                    self.skip(1);
                     buffer.push(byte);
                     if byte == 0 {
                         return Ok(CString::from_vec_with_nul(buffer)
@@ -220,6 +222,7 @@ impl ReadCString for VmReader<'_, Fallible> {
                 unreachable!("The branch should never be reached unless `has_zero` has bugs.")
             }
 
+            self.skip(size_of::<usize>());
             buffer.extend_from_slice(&word.to_ne_bytes());
         }
 
@@ -267,3 +270,29 @@ fn check_vaddr(va: Vaddr) -> Result<()> {
 const fn is_addr_aligned(addr: usize) -> bool {
     (addr & (mem::size_of::<usize>() - 1)) == 0
 }
+
+#[cfg(ktest)]
+mod test {
+    use ostd::prelude::*;
+
+    use super::*;
+
+    #[ktest]
+    fn read_multiple_cstring() {
+        let mut buffer = vec![0u8; 100];
+
+        let str1 = CString::new("hello").unwrap();
+        let str2 = CString::new("world!").unwrap();
+
+        let mut writer = VmWriter::from(buffer.as_mut_slice());
+        writer.write(&mut VmReader::from(str1.as_bytes_with_nul()));
+        writer.write(&mut VmReader::from(str2.as_bytes_with_nul()));
+        drop(writer);
+
+        let mut reader = VmReader::from(buffer.as_slice()).to_fallible();
+        let read_str1 = reader.read_cstring().unwrap();
+        assert_eq!(read_str1, str1);
+        let read_str2 = reader.read_cstring().unwrap();
+        assert_eq!(read_str2, str2);
+    }
+}

ostd/src/mm/io.rs

@@ -275,12 +275,13 @@ impl_vm_io_once_pointer!(&mut T, "(**self)");
 impl_vm_io_once_pointer!(Box<T>, "(**self)");
 impl_vm_io_once_pointer!(Arc<T>, "(**self)");
 
-/// A marker structure used for [`VmReader`] and [`VmWriter`],
+/// A marker type used for [`VmReader`] and [`VmWriter`],
 /// representing whether reads or writes on the underlying memory region are fallible.
-pub struct Fallible;
-/// A marker structure used for [`VmReader`] and [`VmWriter`],
+pub enum Fallible {}
+
+/// A marker type used for [`VmReader`] and [`VmWriter`],
 /// representing whether reads or writes on the underlying memory region are infallible.
-pub struct Infallible;
+pub enum Infallible {}
 
 /// Copies `len` bytes from `src` to `dst`.
 ///
@@ -393,6 +394,20 @@ pub struct VmReader<'a, Fallibility = Fallible> {
     phantom: PhantomData<(&'a [u8], Fallibility)>,
 }
 
+// `Clone` can be implemented for `VmReader`
+// because it either points to untyped memory or represents immutable references.
+// Note that we cannot implement `Clone` for `VmWriter`
+// because it can represent mutable references, which must remain exclusive.
+impl<Fallibility> Clone for VmReader<'_, Fallibility> {
+    fn clone(&self) -> Self {
+        Self {
+            cursor: self.cursor,
+            end: self.end,
+            phantom: PhantomData,
+        }
+    }
+}
+
 macro_rules! impl_read_fallible {
     ($reader_fallibility:ty, $writer_fallibility:ty) => {
         impl<'a> FallibleVmRead<$writer_fallibility> for VmReader<'a, $reader_fallibility> {