linux-kernel - [PATCH v3 05/16] rust: str: introduce `NullTerminatedFormatter`

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Message-Id: <20250711-rnull-up-v6-16-v3-5-3a262b4e2921@kernel.org>
Date: Fri, 11 Jul 2025 13:43:06 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <ojeda@...nel.org>, 
 Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
 Benno Lossin <lossin@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
 Jens Axboe <axboe@...nel.dk>
Cc: linux-block@...r.kernel.org, rust-for-linux@...r.kernel.org, 
 linux-kernel@...r.kernel.org, Andreas Hindborg <a.hindborg@...nel.org>
Subject: [PATCH v3 05/16] rust: str: introduce `NullTerminatedFormatter`

Add `NullTerminatedFormatter`, a formatter that writes a null terminated
string to an array or slice buffer. Because this type needs to manage the
trailing null marker, the existing formatters cannot be used to implement
this type.

Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
---
 rust/kernel/str.rs | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index b1bc584803b0..c58925438c6e 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -838,6 +838,56 @@ fn write_str(&mut self, s: &str) -> fmt::Result {
     }
 }
 
+/// A mutable reference to a byte buffer where a string can be written into.
+///
+/// The buffer will be automatically null terminated after the last written character.
+///
+/// # Invariants
+///
+/// `buffer` is always null terminated.
+pub(crate) struct NullTerminatedFormatter<'a> {
+    buffer: &'a mut [u8],
+}
+
+impl<'a> NullTerminatedFormatter<'a> {
+    /// Create a new [`Self`] instance.
+    pub(crate) fn new(buffer: &'a mut [u8]) -> Option<NullTerminatedFormatter<'a>> {
+        *(buffer.first_mut()?) = 0;
+
+        // INVARIANT: We null terminated the buffer above.
+        Some(Self { buffer })
+    }
+
+    #[expect(dead_code)]
+    pub(crate) fn from_array<const N: usize>(
+        buffer: &'a mut [crate::ffi::c_char; N],
+    ) -> Option<NullTerminatedFormatter<'a>> {
+        Self::new(buffer)
+    }
+}
+
+impl Write for NullTerminatedFormatter<'_> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        let bytes = s.as_bytes();
+        let len = bytes.len();
+
+        // We want space for a null terminator. Buffer length is always at least 1, so no overflow.
+        if len > self.buffer.len() - 1 {
+            return Err(fmt::Error);
+        }
+
+        let buffer = core::mem::take(&mut self.buffer);
+        // We break the null termination invariant for a short while.
+        buffer[..len].copy_from_slice(bytes);
+        self.buffer = &mut buffer[len..];
+
+        // INVARIANT: We null terminate the buffer.
+        self.buffer[0] = 0;
+
+        Ok(())
+    }
+}
+
 /// An owned string that is guaranteed to have exactly one `NUL` byte, which is at the end.
 ///
 /// Used for interoperability with kernel APIs that take C strings.

-- 
2.47.2