[PATCH v4 1/2] uaccess: rust: add strncpy_from_user

[Alice Ryhl <aliceryhl@google.com>]

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.

Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/uaccess.rs | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
index 80a9782b1c6e98ed6eae308ade8551afa7adc188..9b1e4016fca2c25a44a8417c7e35e0fcf08aa959 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 +369,36 @@ pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {
         Ok(())
     }
 }
+
+/// Reads a nul-terminated string into `dst` and returns the length.
+///
+/// This reads from userspace until a NUL byte is encountered, or until `dst.len()` bytes have been
+/// read. Fails with [`EFAULT`] if a read happens on a bad address (some data may have been
+/// copied). 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(dst.len())` is returned.
+///
+/// # Guarantees
+///
+/// When this function returns `Ok(len)`, it is guaranteed that the first `len` bytes of `dst` are
+/// initialized and non-zero. Furthermore, if `len < dst.len()`, then `dst[len]` is a NUL byte.
+/// Unsafe code may rely on these guarantees.
+#[inline]
+#[expect(dead_code)]
+fn raw_strncpy_from_user(dst: &mut [MaybeUninit<u8>], src: UserPtr) -> Result<usize> {
+    // CAST: Slice lengths are guaranteed to be `<= isize::MAX`.
+    let len = dst.len() as isize;
+
+    // SAFETY: `dst` is valid for writing `dst.len()` bytes.
+    let res = unsafe {
+        bindings::strncpy_from_user(dst.as_mut_ptr().cast::<c_char>(), src 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)
+}

-- 
2.49.0.1151.ga128411c76-goog