linux-kernel - [PATCH v2 1/2] uaccess: rust: add strncpy_from

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Message-ID: <20250429-strncpy-from-user-v2-1-7e6facac0bf0@google.com>
Date: Tue, 29 Apr 2025 09:02:22 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH v2 1/2] uaccess: rust: add strncpy_from_user

This patch adds a direct wrapper around the C function of the same name.
It's not really intended for direct use by Rust code since
strncpy_from_user has a somewhat unfortunate API where it only
nul-terminates the buffer if there's space for the nul-terminator. This
means that a direct Rust wrapper around it could not return a &CStr
since the buffer may not be a cstring. However, we still add the method
to build more convenient APIs on top of it, which will happen in
subsequent patches.

Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
 rust/kernel/uaccess.rs | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index 80a9782b1c6e98ed6eae308ade8551afa7adc188..acb703f074a30e60d42a222dd26aed80d8bdb76a 100644
--- a/rust/kernel/uaccess.rs
+++ b/rust/kernel/uaccess.rs
@@ -8,7 +8,7 @@
     alloc::{Allocator, Flags},
     bindings,
     error::Result,
-    ffi::c_void,
+    ffi::{c_char, c_void},
     prelude::*,
     transmute::{AsBytes, FromBytes},
 };
@@ -369,3 +369,35 @@ pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {
         Ok(())
     }
 }
+
+/// Reads a nul-terminated string into `buf` and returns the length.
+///
+/// This reads from userspace until a NUL byte is encountered, or until `buf.len()` bytes have been
+/// read. Fails with [`EFAULT`] if a read happens on a bad address. When the end of the buffer is
+/// encountered, no NUL byte is added, so the string is *not* guaranteed to be NUL-terminated when
+/// `Ok(buf.len())` is returned.
+///
+/// # Guarantees
+///
+/// When this function returns `Ok(len)`, it is guaranteed that the first `len` of `buf` bytes are
+/// initialized and non-zero. Furthermore, if `len < buf.len()`, then `buf[len]` is a NUL byte.
+/// Unsafe code may rely on these guarantees.
+#[inline]
+pub fn raw_strncpy_from_user(ptr: UserPtr, buf: &mut [MaybeUninit<u8>]) -> Result<usize> {
+    // CAST: Slice lengths are guaranteed to be `<= isize::MAX`.
+    let len = buf.len() as isize;
+
+    // SAFETY: `buf` is valid for writing `buf.len()` bytes.
+    let res = unsafe {
+        bindings::strncpy_from_user(buf.as_mut_ptr().cast::<c_char>(), ptr as *const c_char, len)
+    };
+
+    if res < 0 {
+        return Err(Error::from_errno(res as i32));
+    }
+
+    #[cfg(CONFIG_RUST_OVERFLOW_CHECKS)]
+    assert!(res <= len);
+
+    Ok(res as usize)
+}

-- 
2.49.0.901.g37484f566f-goog