rust/kernel/str.rs

// SPDX-License-Identifier: GPL-2.0

//! String representations.

use core::fmt;

/// 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.
    #[allow(dead_code)]
    beg: usize,
    pos: usize,
    end: usize,
}

impl RawFormatter {
    /// 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 _,
        }
    }

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

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(())
    }
}