[PATCH v2 2/2] uaccess: rust: add UserSliceReader::strcpy_into_buf

[Alice Ryhl <aliceryhl@google.com>]

This patch adds a more convenient method for reading C strings from
userspace. Logic is added to NUL-terminate the buffer when necessary so
that a &CStr can be returned.

Note that we treat attempts to read past `self.length` as a fault, so
this returns EFAULT if that limit is exceeded before `buf.len()` is
reached.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/uaccess.rs | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index acb703f074a30e60d42a222dd26aed80d8bdb76a..7cec1b62bd8b816f523c8be12cb29905740789fc 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs
@@ -293,6 +293,41 @@ pub fn read_all<A: Allocator>(mut self, buf: &mut Vec<u8, A>, flags: Flags) -> R
         unsafe { buf.set_len(buf.len() + len) };
         Ok(())
     }
+
+    /// Read a NUL-terminated string from userspace and append it to `dst`.
+    ///
+    /// Fails with [`EFAULT`] if the read happens on a bad address.
+    pub fn strcpy_into_buf<'buf>(&mut self, buf: &'buf mut [u8]) -> Result<&'buf CStr> {
+        if buf.is_empty() {
+            return Err(EINVAL);
+        }
+
+        // SAFETY: The types are compatible and `strncpy_from_user` doesn't write uninitialized
+        // bytes to `buf`.
+        let mut dst = unsafe { &mut *(buf as *mut [u8] as *mut [MaybeUninit<u8>]) };
+
+        // We never read more than `self.length` bytes.
+        if dst.len() > self.length {
+            dst = &mut dst[..self.length];
+        }
+
+        let mut len = raw_strncpy_from_user(self.ptr, dst)?;
+        if len < dst.len() {
+            // Add one to include the NUL-terminator.
+            len += 1;
+        } else if len < buf.len() {
+            // We hit the `self.length` limit before `buf.len()`.
+            return Err(EFAULT);
+        } else {
+            // SAFETY: Due to the check at the beginning, the buffer is not empty.
+            unsafe { *buf.last_mut().unwrap_unchecked() = 0 };
+        }
+        self.skip(len)?;
+
+        // SAFETY: `raw_strncpy_from_user` guarantees that this range of bytes represents a
+        // NUL-terminated string with the only NUL byte being at the end.
+        Ok(unsafe { CStr::from_bytes_with_nul_unchecked(&buf[..len]) })
+    }
 }
 
 /// A writer for [`UserSlice`].

-- 
2.49.0.901.g37484f566f-goog