rust/kernel/str.rs - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0

 //! String representations.

 use alloc::vec::Vec;
 use core::fmt::{self, Write};
 use core::ops::{self, Deref, Index};

 use crate::{
     bindings,
     error::{code::*, Error},
 };

 /// Byte string without UTF-8 validity guarantee.
 ///
 /// `BStr` is simply an alias to `[u8]`, but has a more evident semantical meaning.
 pub type BStr = [u8];

 /// Creates a new [`BStr`] from a string literal.
 ///
 /// `b_str!` converts the supplied string literal to byte string, so non-ASCII
 /// characters can be included.
 ///
 /// # Examples
 ///
 /// ```
 /// # use kernel::b_str;
 /// # use kernel::str::BStr;
 /// const MY_BSTR: &BStr = b_str!("My awesome BStr!");
 /// ```
 #[macro_export]
 macro_rules! b_str {
     ($str:literal) => {{
         const S: &'static str = $str;
         const C: &'static $crate::str::BStr = S.as_bytes();
         C
     }};
 }

 /// Possible errors when using conversion functions in [`CStr`].
 #[derive(Debug, Clone, Copy)]
 pub enum CStrConvertError {
     /// Supplied bytes contain an interior `NUL`.
     InteriorNul,

     /// Supplied bytes are not terminated by `NUL`.
     NotNulTerminated,
 }

 impl From<CStrConvertError> for Error {
     #[inline]
     fn from(_: CStrConvertError) -> Error {
         EINVAL
     }
 }

 /// A 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.
 #[repr(transparent)]
 pub struct CStr([u8]);

 impl CStr {
     /// Returns the length of this string excluding `NUL`.
     #[inline]
     pub const fn len(&self) -> usize {
         self.len_with_nul() - 1
     }

     /// Returns the length of this string with `NUL`.
     #[inline]
     pub const fn len_with_nul(&self) -> usize {
         // SAFETY: This is one of the invariant of `CStr`.
         // We add a `unreachable_unchecked` here to hint the optimizer that
         // the value returned from this function is non-zero.
         if self.0.is_empty() {
             unsafe { core::hint::unreachable_unchecked() };
         }
         self.0.len()
     }

     /// Returns `true` if the string only includes `NUL`.
     #[inline]
     pub const fn is_empty(&self) -> bool {
         self.len() == 0
     }

     /// Wraps a raw C string pointer.
     ///
     /// # Safety
     ///
     /// `ptr` must be a valid pointer to a `NUL`-terminated C string, and it must
     /// last at least `'a`. When `CStr` is alive, the memory pointed by `ptr`
     /// must not be mutated.
     #[inline]
     pub unsafe fn from_char_ptr<'a>(ptr: *const core::ffi::c_char) -> &'a Self {
         // SAFETY: The safety precondition guarantees `ptr` is a valid pointer
         // to a `NUL`-terminated C string.
         let len = unsafe { bindings::strlen(ptr) } + 1;
         // SAFETY: Lifetime guaranteed by the safety precondition.
         let bytes = unsafe { core::slice::from_raw_parts(ptr as _, len as _) };
         // SAFETY: As `len` is returned by `strlen`, `bytes` does not contain interior `NUL`.
         // As we have added 1 to `len`, the last byte is known to be `NUL`.
         unsafe { Self::from_bytes_with_nul_unchecked(bytes) }
     }

     /// Creates a [`CStr`] from a `[u8]`.
     ///
     /// The provided slice must be `NUL`-terminated, does not contain any
     /// interior `NUL` bytes.
     pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, CStrConvertError> {
         if bytes.is_empty() {
             return Err(CStrConvertError::NotNulTerminated);
         }
         if bytes[bytes.len() - 1] != 0 {
             return Err(CStrConvertError::NotNulTerminated);
         }
         let mut i = 0;
         // `i + 1 < bytes.len()` allows LLVM to optimize away bounds checking,
         // while it couldn't optimize away bounds checks for `i < bytes.len() - 1`.
         while i + 1 < bytes.len() {
             if bytes[i] == 0 {
                 return Err(CStrConvertError::InteriorNul);
             }
             i += 1;
         }
         // SAFETY: We just checked that all properties hold.
         Ok(unsafe { Self::from_bytes_with_nul_unchecked(bytes) })
     }

     /// Creates a [`CStr`] from a `[u8]` without performing any additional
     /// checks.
     ///
     /// # Safety
     ///
     /// `bytes` *must* end with a `NUL` byte, and should only have a single
     /// `NUL` byte (or the string will be truncated).
     #[inline]
     pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr {
         // SAFETY: Properties of `bytes` guaranteed by the safety precondition.
         unsafe { core::mem::transmute(bytes) }
     }

     /// Returns a C pointer to the string.
     #[inline]
     pub const fn as_char_ptr(&self) -> *const core::ffi::c_char {
         self.0.as_ptr() as _
     }

     /// Convert the string to a byte slice without the trailing 0 byte.
     #[inline]
     pub fn as_bytes(&self) -> &[u8] {
         &self.0[..self.len()]
     }

     /// Convert the string to a byte slice containing the trailing 0 byte.
     #[inline]
     pub const fn as_bytes_with_nul(&self) -> &[u8] {
         &self.0
     }

     /// Yields a [`&str`] slice if the [`CStr`] contains valid UTF-8.
     ///
     /// If the contents of the [`CStr`] are valid UTF-8 data, this
     /// function will return the corresponding [`&str`] slice. Otherwise,
     /// it will return an error with details of where UTF-8 validation failed.
     ///
     /// # Examples
     ///
     /// ```
     /// # use kernel::str::CStr;
     /// let cstr = CStr::from_bytes_with_nul(b"foo\0").unwrap();
     /// assert_eq!(cstr.to_str(), Ok("foo"));
     /// ```
     #[inline]
     pub fn to_str(&self) -> Result<&str, core::str::Utf8Error> {
         core::str::from_utf8(self.as_bytes())
     }

     /// Unsafely convert this [`CStr`] into a [`&str`], without checking for
     /// valid UTF-8.
     ///
     /// # Safety
     ///
     /// The contents must be valid UTF-8.
     ///
     /// # Examples
     ///
     /// ```
     /// # use kernel::c_str;
     /// # use kernel::str::CStr;
     /// // SAFETY: String literals are guaranteed to be valid UTF-8
     /// // by the Rust compiler.
     /// let bar = c_str!("ツ");
     /// assert_eq!(unsafe { bar.as_str_unchecked() }, "ツ");
     /// ```
     #[inline]
     pub unsafe fn as_str_unchecked(&self) -> &str {
         unsafe { core::str::from_utf8_unchecked(self.as_bytes()) }
     }
 }

 impl fmt::Display for CStr {
     /// Formats printable ASCII characters, escaping the rest.
     ///
     /// ```
     /// # use kernel::c_str;
     /// # use kernel::str::CStr;
     /// # use kernel::str::CString;
     /// let penguin = c_str!("🐧");
     /// let s = CString::try_from_fmt(fmt!("{}", penguin)).unwrap();
     /// assert_eq!(s.as_bytes_with_nul(), "\\xf0\\x9f\\x90\\xa7\0".as_bytes());
     ///
     /// let ascii = c_str!("so \"cool\"");
     /// let s = CString::try_from_fmt(fmt!("{}", ascii)).unwrap();
     /// assert_eq!(s.as_bytes_with_nul(), "so \"cool\"\0".as_bytes());
     /// ```
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         for &c in self.as_bytes() {
             if (0x20..0x7f).contains(&c) {
                 // Printable character.
                 f.write_char(c as char)?;
             } else {
                 write!(f, "\\x{:02x}", c)?;
             }
         }
         Ok(())
     }
 }

 impl fmt::Debug for CStr {
     /// Formats printable ASCII characters with a double quote on either end, escaping the rest.
     ///
     /// ```
     /// # use kernel::c_str;
     /// # use kernel::str::CStr;
     /// # use kernel::str::CString;
     /// let penguin = c_str!("🐧");
     /// let s = CString::try_from_fmt(fmt!("{:?}", penguin)).unwrap();
     /// assert_eq!(s.as_bytes_with_nul(), "\"\\xf0\\x9f\\x90\\xa7\"\0".as_bytes());
     ///
     /// // Embedded double quotes are escaped.
     /// let ascii = c_str!("so \"cool\"");
     /// let s = CString::try_from_fmt(fmt!("{:?}", ascii)).unwrap();
     /// assert_eq!(s.as_bytes_with_nul(), "\"so \\\"cool\\\"\"\0".as_bytes());
     /// ```
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str("\"")?;
         for &c in self.as_bytes() {
             match c {
                 // Printable characters.
                 b'\"' => f.write_str("\\\"")?,
                 0x20..=0x7e => f.write_char(c as char)?,
                 _ => write!(f, "\\x{:02x}", c)?,
             }
         }
         f.write_str("\"")
     }
 }

 impl AsRef<BStr> for CStr {
     #[inline]
     fn as_ref(&self) -> &BStr {
         self.as_bytes()
     }
 }

 impl Deref for CStr {
     type Target = BStr;

     #[inline]
     fn deref(&self) -> &Self::Target {
         self.as_bytes()
     }
 }

 impl Index<ops::RangeFrom<usize>> for CStr {
     type Output = CStr;

     #[inline]
     fn index(&self, index: ops::RangeFrom<usize>) -> &Self::Output {
         // Delegate bounds checking to slice.
         // Assign to _ to mute clippy's unnecessary operation warning.
         let _ = &self.as_bytes()[index.start..];
         // SAFETY: We just checked the bounds.
         unsafe { Self::from_bytes_with_nul_unchecked(&self.0[index.start..]) }
     }
 }

 impl Index<ops::RangeFull> for CStr {
     type Output = CStr;

     #[inline]
     fn index(&self, _index: ops::RangeFull) -> &Self::Output {
         self
     }
 }

 mod private {
     use core::ops;

     // Marker trait for index types that can be forward to `BStr`.
     pub trait CStrIndex {}

     impl CStrIndex for usize {}
     impl CStrIndex for ops::Range<usize> {}
     impl CStrIndex for ops::RangeInclusive<usize> {}
     impl CStrIndex for ops::RangeToInclusive<usize> {}
 }

 impl<Idx> Index<Idx> for CStr
 where
     Idx: private::CStrIndex,
     BStr: Index<Idx>,
 {
     type Output = <BStr as Index<Idx>>::Output;

     #[inline]
     fn index(&self, index: Idx) -> &Self::Output {
         &self.as_bytes()[index]
     }
 }

 /// Creates a new [`CStr`] from a string literal.
 ///
 /// The string literal should not contain any `NUL` bytes.
 ///
 /// # Examples
 ///
 /// ```
 /// # use kernel::c_str;
 /// # use kernel::str::CStr;
 /// const MY_CSTR: &CStr = c_str!("My awesome CStr!");
 /// ```
 #[macro_export]
 macro_rules! c_str {
     ($str:expr) => {{
         const S: &str = concat!($str, "\0");
         const C: &$crate::str::CStr = match $crate::str::CStr::from_bytes_with_nul(S.as_bytes()) {
             Ok(v) => v,
             Err(_) => panic!("string contains interior NUL"),
         };
         C
     }};
 }

 #[cfg(test)]
 mod tests {
     use super::*;

     #[test]
     fn test_cstr_to_str() {
         let good_bytes = b"\xf0\x9f\xa6\x80\0";
         let checked_cstr = CStr::from_bytes_with_nul(good_bytes).unwrap();
         let checked_str = checked_cstr.to_str().unwrap();
         assert_eq!(checked_str, "🦀");
     }

     #[test]
     #[should_panic]
     fn test_cstr_to_str_panic() {
         let bad_bytes = b"\xc3\x28\0";
         let checked_cstr = CStr::from_bytes_with_nul(bad_bytes).unwrap();
         checked_cstr.to_str().unwrap();
     }

     #[test]
     fn test_cstr_as_str_unchecked() {
         let good_bytes = b"\xf0\x9f\x90\xA7\0";
         let checked_cstr = CStr::from_bytes_with_nul(good_bytes).unwrap();
         let unchecked_str = unsafe { checked_cstr.as_str_unchecked() };
         assert_eq!(unchecked_str, "🐧");
     }
 }

 /// Allows formatting of [`fmt::Arguments`] into a raw buffer.
 ///
 /// It does not fail if callers write past the end of the buffer so that they can calculate the
 /// size required to fit everything.
 ///
 /// # Invariants
 ///
 /// The memory region between `pos` (inclusive) and `end` (exclusive) is valid for writes if `pos`
 /// is less than `end`.
 pub(crate) struct RawFormatter {
     // Use `usize` to use `saturating_*` functions.
     beg: usize,
     pos: usize,
     end: usize,
 }

 impl RawFormatter {
     /// Creates a new instance of [`RawFormatter`] with an empty buffer.
     fn new() -> Self {
         // INVARIANT: The buffer is empty, so the region that needs to be writable is empty.
         Self {
             beg: 0,
             pos: 0,
             end: 0,
         }
     }

     /// Creates a new instance of [`RawFormatter`] with the given buffer pointers.
     ///
     /// # Safety
     ///
     /// If `pos` is less than `end`, then the region between `pos` (inclusive) and `end`
     /// (exclusive) must be valid for writes for the lifetime of the returned [`RawFormatter`].
     pub(crate) unsafe fn from_ptrs(pos: *mut u8, end: *mut u8) -> Self {
         // INVARIANT: The safety requierments guarantee the type invariants.
         Self {
             beg: pos as _,
             pos: pos as _,
             end: end as _,
         }
     }

     /// Creates a new instance of [`RawFormatter`] with the given buffer.
     ///
     /// # Safety
     ///
     /// The memory region starting at `buf` and extending for `len` bytes must be valid for writes
     /// for the lifetime of the returned [`RawFormatter`].
     pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
         let pos = buf as usize;
         // INVARIANT: We ensure that `end` is never less then `buf`, and the safety requirements
         // guarantees that the memory region is valid for writes.
         Self {
             pos,
             beg: pos,
             end: pos.saturating_add(len),
         }
     }

     /// Returns the current insert position.
     ///
     /// N.B. It may point to invalid memory.
     pub(crate) fn pos(&self) -> *mut u8 {
         self.pos as _
     }

     /// Return the number of bytes written to the formatter.
     pub(crate) fn bytes_written(&self) -> usize {
         self.pos - self.beg
     }
 }

 impl fmt::Write for RawFormatter {
     fn write_str(&mut self, s: &str) -> fmt::Result {
         // `pos` value after writing `len` bytes. This does not have to be bounded by `end`, but we
         // don't want it to wrap around to 0.
         let pos_new = self.pos.saturating_add(s.len());

         // Amount that we can copy. `saturating_sub` ensures we get 0 if `pos` goes past `end`.
         let len_to_copy = core::cmp::min(pos_new, self.end).saturating_sub(self.pos);

         if len_to_copy > 0 {
             // SAFETY: If `len_to_copy` is non-zero, then we know `pos` has not gone past `end`
             // yet, so it is valid for write per the type invariants.
             unsafe {
                 core::ptr::copy_nonoverlapping(
                     s.as_bytes().as_ptr(),
                     self.pos as *mut u8,
                     len_to_copy,
                 )
             };
         }

         self.pos = pos_new;
         Ok(())
     }
 }

 /// Allows formatting of [`fmt::Arguments`] into a raw buffer.
 ///
 /// Fails if callers attempt to write more than will fit in the buffer.
 pub(crate) struct Formatter(RawFormatter);

 impl Formatter {
     /// Creates a new instance of [`Formatter`] with the given buffer.
     ///
     /// # Safety
     ///
     /// The memory region starting at `buf` and extending for `len` bytes must be valid for writes
     /// for the lifetime of the returned [`Formatter`].
     pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
         // SAFETY: The safety requirements of this function satisfy those of the callee.
         Self(unsafe { RawFormatter::from_buffer(buf, len) })
     }
 }

 impl Deref for Formatter {
     type Target = RawFormatter;

     fn deref(&self) -> &Self::Target {
         &self.0
     }
 }

 impl fmt::Write for Formatter {
     fn write_str(&mut self, s: &str) -> fmt::Result {
         self.0.write_str(s)?;

         // Fail the request if we go past the end of the buffer.
         if self.0.pos > self.0.end {
             Err(fmt::Error)
         } else {
             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.
 ///
 /// # Invariants
 ///
 /// The string is always `NUL`-terminated and contains no other `NUL` bytes.
 ///
 /// # Examples
 ///
 /// ```
 /// use kernel::str::CString;
 ///
 /// let s = CString::try_from_fmt(fmt!("{}{}{}", "abc", 10, 20)).unwrap();
 /// assert_eq!(s.as_bytes_with_nul(), "abc1020\0".as_bytes());
 ///
 /// let tmp = "testing";
 /// let s = CString::try_from_fmt(fmt!("{tmp}{}", 123)).unwrap();
 /// assert_eq!(s.as_bytes_with_nul(), "testing123\0".as_bytes());
 ///
 /// // This fails because it has an embedded `NUL` byte.
 /// let s = CString::try_from_fmt(fmt!("a\0b{}", 123));
 /// assert_eq!(s.is_ok(), false);
 /// ```
 pub struct CString {
     buf: Vec<u8>,
 }

 impl CString {
     /// Creates an instance of [`CString`] from the given formatted arguments.
     pub fn try_from_fmt(args: fmt::Arguments<'_>) -> Result<Self, Error> {
         // Calculate the size needed (formatted string plus `NUL` terminator).
         let mut f = RawFormatter::new();
         f.write_fmt(args)?;
         f.write_str("\0")?;
         let size = f.bytes_written();

         // Allocate a vector with the required number of bytes, and write to it.
         let mut buf = Vec::try_with_capacity(size)?;
         // SAFETY: The buffer stored in `buf` is at least of size `size` and is valid for writes.
         let mut f = unsafe { Formatter::from_buffer(buf.as_mut_ptr(), size) };
         f.write_fmt(args)?;
         f.write_str("\0")?;

         // SAFETY: The number of bytes that can be written to `f` is bounded by `size`, which is
         // `buf`'s capacity. The contents of the buffer have been initialised by writes to `f`.
         unsafe { buf.set_len(f.bytes_written()) };

         // Check that there are no `NUL` bytes before the end.
         // SAFETY: The buffer is valid for read because `f.bytes_written()` is bounded by `size`
         // (which the minimum buffer size) and is non-zero (we wrote at least the `NUL` terminator)
         // so `f.bytes_written() - 1` doesn't underflow.
         let ptr = unsafe { bindings::memchr(buf.as_ptr().cast(), 0, (f.bytes_written() - 1) as _) };
         if !ptr.is_null() {
             return Err(EINVAL);
         }

         // INVARIANT: We wrote the `NUL` terminator and checked above that no other `NUL` bytes
         // exist in the buffer.
         Ok(Self { buf })
     }
 }

 impl Deref for CString {
     type Target = CStr;

     fn deref(&self) -> &Self::Target {
         // SAFETY: The type invariants guarantee that the string is `NUL`-terminated and that no
         // other `NUL` bytes exist.
         unsafe { CStr::from_bytes_with_nul_unchecked(self.buf.as_slice()) }
     }
 }

 /// A convenience alias for [`core::format_args`].
 #[macro_export]
 macro_rules! fmt {
     ($($f:tt)*) => ( core::format_args!($($f)*) )
 }
	// SPDX-License-Identifier: GPL-2.0

	//! String representations.

	use alloc::vec::Vec;
	use core::fmt::{self, Write};
	use core::ops::{self, Deref, Index};

	use crate::{
	bindings,
	error::{code::*, Error},
	};

	/// Byte string without UTF-8 validity guarantee.
	///
	/// `BStr` is simply an alias to `[u8]`, but has a more evident semantical meaning.
	pub type BStr = [u8];

	/// Creates a new [`BStr`] from a string literal.
	///
	/// `b_str!` converts the supplied string literal to byte string, so non-ASCII
	/// characters can be included.
	///
	/// # Examples
	///
	/// ```
	/// # use kernel::b_str;
	/// # use kernel::str::BStr;
	/// const MY_BSTR: &BStr = b_str!("My awesome BStr!");
	/// ```
	#[macro_export]
	macro_rules! b_str {
	($str:literal) => {{
	const S: &'static str = $str;
	const C: &'static $crate::str::BStr = S.as_bytes();
	C
	}};
	}

	/// Possible errors when using conversion functions in [`CStr`].
	#[derive(Debug, Clone, Copy)]
	pub enum CStrConvertError {
	/// Supplied bytes contain an interior `NUL`.
	InteriorNul,

	/// Supplied bytes are not terminated by `NUL`.
	NotNulTerminated,
	}

	impl From<CStrConvertError> for Error {
	#[inline]
	fn from(_: CStrConvertError) -> Error {
	EINVAL
	}
	}

	/// A 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.
	#[repr(transparent)]
	pub struct CStr([u8]);

	impl CStr {
	/// Returns the length of this string excluding `NUL`.
	#[inline]
	pub const fn len(&self) -> usize {
	self.len_with_nul() - 1
	}

	/// Returns the length of this string with `NUL`.
	#[inline]
	pub const fn len_with_nul(&self) -> usize {
	// SAFETY: This is one of the invariant of `CStr`.
	// We add a `unreachable_unchecked` here to hint the optimizer that
	// the value returned from this function is non-zero.
	if self.0.is_empty() {
	unsafe { core::hint::unreachable_unchecked() };
	}
	self.0.len()
	}

	/// Returns `true` if the string only includes `NUL`.
	#[inline]
	pub const fn is_empty(&self) -> bool {
	self.len() == 0
	}

	/// Wraps a raw C string pointer.
	///
	/// # Safety
	///
	/// `ptr` must be a valid pointer to a `NUL`-terminated C string, and it must
	/// last at least `'a`. When `CStr` is alive, the memory pointed by `ptr`
	/// must not be mutated.
	#[inline]
	pub unsafe fn from_char_ptr<'a>(ptr: *const core::ffi::c_char) -> &'a Self {
	// SAFETY: The safety precondition guarantees `ptr` is a valid pointer
	// to a `NUL`-terminated C string.
	let len = unsafe { bindings::strlen(ptr) } + 1;
	// SAFETY: Lifetime guaranteed by the safety precondition.
	let bytes = unsafe { core::slice::from_raw_parts(ptr as _, len as _) };
	// SAFETY: As `len` is returned by `strlen`, `bytes` does not contain interior `NUL`.
	// As we have added 1 to `len`, the last byte is known to be `NUL`.
	unsafe { Self::from_bytes_with_nul_unchecked(bytes) }
	}

	/// Creates a [`CStr`] from a `[u8]`.
	///
	/// The provided slice must be `NUL`-terminated, does not contain any
	/// interior `NUL` bytes.
	pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, CStrConvertError> {
	if bytes.is_empty() {
	return Err(CStrConvertError::NotNulTerminated);
	}
	if bytes[bytes.len() - 1] != 0 {
	return Err(CStrConvertError::NotNulTerminated);
	}
	let mut i = 0;
	// `i + 1 < bytes.len()` allows LLVM to optimize away bounds checking,
	// while it couldn't optimize away bounds checks for `i < bytes.len() - 1`.
	while i + 1 < bytes.len() {
	if bytes[i] == 0 {
	return Err(CStrConvertError::InteriorNul);
	}
	i += 1;
	}
	// SAFETY: We just checked that all properties hold.
	Ok(unsafe { Self::from_bytes_with_nul_unchecked(bytes) })
	}

	/// Creates a [`CStr`] from a `[u8]` without performing any additional
	/// checks.
	///
	/// # Safety
	///
	/// `bytes` must end with a `NUL` byte, and should only have a single
	/// `NUL` byte (or the string will be truncated).
	#[inline]
	pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr {
	// SAFETY: Properties of `bytes` guaranteed by the safety precondition.
	unsafe { core::mem::transmute(bytes) }
	}

	/// Returns a C pointer to the string.
	#[inline]
	pub const fn as_char_ptr(&self) -> *const core::ffi::c_char {
	self.0.as_ptr() as _
	}

	/// Convert the string to a byte slice without the trailing 0 byte.
	#[inline]
	pub fn as_bytes(&self) -> &[u8] {
	&self.0[..self.len()]
	}

	/// Convert the string to a byte slice containing the trailing 0 byte.
	#[inline]
	pub const fn as_bytes_with_nul(&self) -> &[u8] {
	&self.0
	}

	/// Yields a [`&str`] slice if the [`CStr`] contains valid UTF-8.
	///
	/// If the contents of the [`CStr`] are valid UTF-8 data, this
	/// function will return the corresponding [`&str`] slice. Otherwise,
	/// it will return an error with details of where UTF-8 validation failed.
	///
	/// # Examples
	///
	/// ```
	/// # use kernel::str::CStr;
	/// let cstr = CStr::from_bytes_with_nul(b"foo\0").unwrap();
	/// assert_eq!(cstr.to_str(), Ok("foo"));
	/// ```
	#[inline]
	pub fn to_str(&self) -> Result<&str, core::str::Utf8Error> {
	core::str::from_utf8(self.as_bytes())
	}

	/// Unsafely convert this [`CStr`] into a [`&str`], without checking for
	/// valid UTF-8.
	///
	/// # Safety
	///
	/// The contents must be valid UTF-8.
	///
	/// # Examples
	///
	/// ```
	/// # use kernel::c_str;
	/// # use kernel::str::CStr;
	/// // SAFETY: String literals are guaranteed to be valid UTF-8
	/// // by the Rust compiler.
	/// let bar = c_str!("ツ");
	/// assert_eq!(unsafe { bar.as_str_unchecked() }, "ツ");
	/// ```
	#[inline]
	pub unsafe fn as_str_unchecked(&self) -> &str {
	unsafe { core::str::from_utf8_unchecked(self.as_bytes()) }
	}
	}

	impl fmt::Display for CStr {
	/// Formats printable ASCII characters, escaping the rest.
	///
	/// ```
	/// # use kernel::c_str;
	/// # use kernel::str::CStr;
	/// # use kernel::str::CString;
	/// let penguin = c_str!("🐧");
	/// let s = CString::try_from_fmt(fmt!("{}", penguin)).unwrap();
	/// assert_eq!(s.as_bytes_with_nul(), "\\xf0\\x9f\\x90\\xa7\0".as_bytes());
	///
	/// let ascii = c_str!("so \"cool\"");
	/// let s = CString::try_from_fmt(fmt!("{}", ascii)).unwrap();
	/// assert_eq!(s.as_bytes_with_nul(), "so \"cool\"\0".as_bytes());
	/// ```
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	for &c in self.as_bytes() {
	if (0x20..0x7f).contains(&c) {
	// Printable character.
	f.write_char(c as char)?;
	} else {
	write!(f, "\\x{:02x}", c)?;
	}
	}
	Ok(())
	}
	}

	impl fmt::Debug for CStr {
	/// Formats printable ASCII characters with a double quote on either end, escaping the rest.
	///
	/// ```
	/// # use kernel::c_str;
	/// # use kernel::str::CStr;
	/// # use kernel::str::CString;
	/// let penguin = c_str!("🐧");
	/// let s = CString::try_from_fmt(fmt!("{:?}", penguin)).unwrap();
	/// assert_eq!(s.as_bytes_with_nul(), "\"\\xf0\\x9f\\x90\\xa7\"\0".as_bytes());
	///
	/// // Embedded double quotes are escaped.
	/// let ascii = c_str!("so \"cool\"");
	/// let s = CString::try_from_fmt(fmt!("{:?}", ascii)).unwrap();
	/// assert_eq!(s.as_bytes_with_nul(), "\"so \\\"cool\\\"\"\0".as_bytes());
	/// ```
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	f.write_str("\"")?;
	for &c in self.as_bytes() {
	match c {
	// Printable characters.
	b'\"' => f.write_str("\\\"")?,
	0x20..=0x7e => f.write_char(c as char)?,
	_ => write!(f, "\\x{:02x}", c)?,
	}
	}
	f.write_str("\"")
	}
	}

	impl AsRef<BStr> for CStr {
	#[inline]
	fn as_ref(&self) -> &BStr {
	self.as_bytes()
	}
	}

	impl Deref for CStr {
	type Target = BStr;

	#[inline]
	fn deref(&self) -> &Self::Target {
	self.as_bytes()
	}
	}

	impl Index<ops::RangeFrom<usize>> for CStr {
	type Output = CStr;

	#[inline]
	fn index(&self, index: ops::RangeFrom<usize>) -> &Self::Output {
	// Delegate bounds checking to slice.
	// Assign to _ to mute clippy's unnecessary operation warning.
	let _ = &self.as_bytes()[index.start..];
	// SAFETY: We just checked the bounds.
	unsafe { Self::from_bytes_with_nul_unchecked(&self.0[index.start..]) }
	}
	}

	impl Index<ops::RangeFull> for CStr {
	type Output = CStr;

	#[inline]
	fn index(&self, _index: ops::RangeFull) -> &Self::Output {
	self
	}
	}

	mod private {
	use core::ops;

	// Marker trait for index types that can be forward to `BStr`.
	pub trait CStrIndex {}

	impl CStrIndex for usize {}
	impl CStrIndex for ops::Range<usize> {}
	impl CStrIndex for ops::RangeInclusive<usize> {}
	impl CStrIndex for ops::RangeToInclusive<usize> {}
	}

	impl<Idx> Index<Idx> for CStr
	where
	Idx: private::CStrIndex,
	BStr: Index<Idx>,
	{
	type Output = <BStr as Index<Idx>>::Output;

	#[inline]
	fn index(&self, index: Idx) -> &Self::Output {
	&self.as_bytes()[index]
	}
	}

	/// Creates a new [`CStr`] from a string literal.
	///
	/// The string literal should not contain any `NUL` bytes.
	///
	/// # Examples
	///
	/// ```
	/// # use kernel::c_str;
	/// # use kernel::str::CStr;
	/// const MY_CSTR: &CStr = c_str!("My awesome CStr!");
	/// ```
	#[macro_export]
	macro_rules! c_str {
	($str:expr) => {{
	const S: &str = concat!($str, "\0");
	const C: &$crate::str::CStr = match $crate::str::CStr::from_bytes_with_nul(S.as_bytes()) {
	Ok(v) => v,
	Err(_) => panic!("string contains interior NUL"),
	};
	C
	}};
	}

	#[cfg(test)]
	mod tests {
	use super::*;

	#[test]
	fn test_cstr_to_str() {
	let good_bytes = b"\xf0\x9f\xa6\x80\0";
	let checked_cstr = CStr::from_bytes_with_nul(good_bytes).unwrap();
	let checked_str = checked_cstr.to_str().unwrap();
	assert_eq!(checked_str, "🦀");
	}

	#[test]
	#[should_panic]
	fn test_cstr_to_str_panic() {
	let bad_bytes = b"\xc3\x28\0";
	let checked_cstr = CStr::from_bytes_with_nul(bad_bytes).unwrap();
	checked_cstr.to_str().unwrap();
	}

	#[test]
	fn test_cstr_as_str_unchecked() {
	let good_bytes = b"\xf0\x9f\x90\xA7\0";
	let checked_cstr = CStr::from_bytes_with_nul(good_bytes).unwrap();
	let unchecked_str = unsafe { checked_cstr.as_str_unchecked() };
	assert_eq!(unchecked_str, "🐧");
	}
	}

	/// Allows formatting of [`fmt::Arguments`] into a raw buffer.
	///
	/// It does not fail if callers write past the end of the buffer so that they can calculate the
	/// size required to fit everything.
	///
	/// # Invariants
	///
	/// The memory region between `pos` (inclusive) and `end` (exclusive) is valid for writes if `pos`
	/// is less than `end`.
	pub(crate) struct RawFormatter {
	// Use `usize` to use `saturating_*` functions.
	beg: usize,
	pos: usize,
	end: usize,
	}

	impl RawFormatter {
	/// Creates a new instance of [`RawFormatter`] with an empty buffer.
	fn new() -> Self {
	// INVARIANT: The buffer is empty, so the region that needs to be writable is empty.
	Self {
	beg: 0,
	pos: 0,
	end: 0,
	}
	}

	/// Creates a new instance of [`RawFormatter`] with the given buffer pointers.
	///
	/// # Safety
	///
	/// If `pos` is less than `end`, then the region between `pos` (inclusive) and `end`
	/// (exclusive) must be valid for writes for the lifetime of the returned [`RawFormatter`].
	pub(crate) unsafe fn from_ptrs(pos: mut u8, end: mut u8) -> Self {
	// INVARIANT: The safety requierments guarantee the type invariants.
	Self {
	beg: pos as _,
	pos: pos as _,
	end: end as _,
	}
	}

	/// Creates a new instance of [`RawFormatter`] with the given buffer.
	///
	/// # Safety
	///
	/// The memory region starting at `buf` and extending for `len` bytes must be valid for writes
	/// for the lifetime of the returned [`RawFormatter`].
	pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
	let pos = buf as usize;
	// INVARIANT: We ensure that `end` is never less then `buf`, and the safety requirements
	// guarantees that the memory region is valid for writes.
	Self {
	pos,
	beg: pos,
	end: pos.saturating_add(len),
	}
	}

	/// Returns the current insert position.
	///
	/// N.B. It may point to invalid memory.
	pub(crate) fn pos(&self) -> *mut u8 {
	self.pos as _
	}

	/// Return the number of bytes written to the formatter.
	pub(crate) fn bytes_written(&self) -> usize {
	self.pos - self.beg
	}
	}

	impl fmt::Write for RawFormatter {
	fn write_str(&mut self, s: &str) -> fmt::Result {
	// `pos` value after writing `len` bytes. This does not have to be bounded by `end`, but we
	// don't want it to wrap around to 0.
	let pos_new = self.pos.saturating_add(s.len());

	// Amount that we can copy. `saturating_sub` ensures we get 0 if `pos` goes past `end`.
	let len_to_copy = core::cmp::min(pos_new, self.end).saturating_sub(self.pos);

	if len_to_copy > 0 {
	// SAFETY: If `len_to_copy` is non-zero, then we know `pos` has not gone past `end`
	// yet, so it is valid for write per the type invariants.
	unsafe {
	core::ptr::copy_nonoverlapping(
	s.as_bytes().as_ptr(),
	self.pos as *mut u8,
	len_to_copy,
	)
	};
	}

	self.pos = pos_new;
	Ok(())
	}
	}

	/// Allows formatting of [`fmt::Arguments`] into a raw buffer.
	///
	/// Fails if callers attempt to write more than will fit in the buffer.
	pub(crate) struct Formatter(RawFormatter);

	impl Formatter {
	/// Creates a new instance of [`Formatter`] with the given buffer.
	///
	/// # Safety
	///
	/// The memory region starting at `buf` and extending for `len` bytes must be valid for writes
	/// for the lifetime of the returned [`Formatter`].
	pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
	// SAFETY: The safety requirements of this function satisfy those of the callee.
	Self(unsafe { RawFormatter::from_buffer(buf, len) })
	}
	}

	impl Deref for Formatter {
	type Target = RawFormatter;

	fn deref(&self) -> &Self::Target {
	&self.0
	}
	}

	impl fmt::Write for Formatter {
	fn write_str(&mut self, s: &str) -> fmt::Result {
	self.0.write_str(s)?;

	// Fail the request if we go past the end of the buffer.
	if self.0.pos > self.0.end {
	Err(fmt::Error)
	} else {
	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.
	///
	/// # Invariants
	///
	/// The string is always `NUL`-terminated and contains no other `NUL` bytes.
	///
	/// # Examples
	///
	/// ```
	/// use kernel::str::CString;
	///
	/// let s = CString::try_from_fmt(fmt!("{}{}{}", "abc", 10, 20)).unwrap();
	/// assert_eq!(s.as_bytes_with_nul(), "abc1020\0".as_bytes());
	///
	/// let tmp = "testing";
	/// let s = CString::try_from_fmt(fmt!("{tmp}{}", 123)).unwrap();
	/// assert_eq!(s.as_bytes_with_nul(), "testing123\0".as_bytes());
	///
	/// // This fails because it has an embedded `NUL` byte.
	/// let s = CString::try_from_fmt(fmt!("a\0b{}", 123));
	/// assert_eq!(s.is_ok(), false);
	/// ```
	pub struct CString {
	buf: Vec<u8>,
	}

	impl CString {
	/// Creates an instance of [`CString`] from the given formatted arguments.
	pub fn try_from_fmt(args: fmt::Arguments<'_>) -> Result<Self, Error> {
	// Calculate the size needed (formatted string plus `NUL` terminator).
	let mut f = RawFormatter::new();
	f.write_fmt(args)?;
	f.write_str("\0")?;
	let size = f.bytes_written();

	// Allocate a vector with the required number of bytes, and write to it.
	let mut buf = Vec::try_with_capacity(size)?;
	// SAFETY: The buffer stored in `buf` is at least of size `size` and is valid for writes.
	let mut f = unsafe { Formatter::from_buffer(buf.as_mut_ptr(), size) };
	f.write_fmt(args)?;
	f.write_str("\0")?;

	// SAFETY: The number of bytes that can be written to `f` is bounded by `size`, which is
	// `buf`'s capacity. The contents of the buffer have been initialised by writes to `f`.
	unsafe { buf.set_len(f.bytes_written()) };

	// Check that there are no `NUL` bytes before the end.
	// SAFETY: The buffer is valid for read because `f.bytes_written()` is bounded by `size`
	// (which the minimum buffer size) and is non-zero (we wrote at least the `NUL` terminator)
	// so `f.bytes_written() - 1` doesn't underflow.
	let ptr = unsafe { bindings::memchr(buf.as_ptr().cast(), 0, (f.bytes_written() - 1) as _) };
	if !ptr.is_null() {
	return Err(EINVAL);
	}

	// INVARIANT: We wrote the `NUL` terminator and checked above that no other `NUL` bytes
	// exist in the buffer.
	Ok(Self { buf })
	}
	}

	impl Deref for CString {
	type Target = CStr;

	fn deref(&self) -> &Self::Target {
	// SAFETY: The type invariants guarantee that the string is `NUL`-terminated and that no
	// other `NUL` bytes exist.
	unsafe { CStr::from_bytes_with_nul_unchecked(self.buf.as_slice()) }
	}
	}

	/// A convenience alias for [`core::format_args`].
	#[macro_export]
	macro_rules! fmt {
	($($f:tt)) => ( core::format_args!($($f)) )
	}