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use core::marker::PhantomData;
use core::mem::{align_of, size_of, size_of_val, transmute};
use core::ptr::NonNull;
use crate::buf;
use crate::traits::ZeroCopy;
/// A struct padder as provided to the [`ZeroCopy::pad`] method.
///
/// This knows how to find and initialize padding regions in `repr(C)` types,
/// and provides a builder-like API to doing so.
#[must_use = "For the writer to have an effect on `OwnedBuf` you must call `Padder::remaining` / `Padder::remaining_unsized`"]
pub struct Padder<'a, T: ?Sized> {
data: NonNull<u8>,
offset: usize,
_marker: PhantomData<&'a mut T>,
}
impl<'a, T: ?Sized> Padder<'a, T> {
#[inline]
pub(crate) fn new(data: NonNull<u8>) -> Self {
Self {
data,
offset: 0,
_marker: PhantomData,
}
}
/// Indicate that this validate is transparent over `U`.
//
/// # Safety
///
/// This is only allowed if `T` is `#[repr(transparent)]` over `U`.
#[inline]
pub unsafe fn transparent<U>(&mut self) -> &mut Padder<'a, U> {
transmute(self)
}
/// Pad around the given field with zeros.
///
/// Note that this is necessary to do correctly in order to satisfy the
/// safety requirements by [`remaining()`].
///
/// This is typically not called directly, but rather is implemented by the
/// [`ZeroCopy`] derive.
///
/// [`remaining()`]: Self::remaining
/// [`ZeroCopy`]: derive@crate::ZeroCopy
///
/// # Safety
///
/// The caller must ensure that the field type `F` is an actual field in
/// order in the struct being padded.
#[inline]
pub unsafe fn pad<F>(&mut self)
where
F: ZeroCopy,
{
self.pad_with::<F>(align_of::<F>());
}
/// Pad around the given field with zeros using a custom alignment `align`.
///
/// Note that this is necessary to do correctly in order to satisfy the
/// safety requirements by [`remaining()`].
///
/// This is typically not called directly, but rather is implemented by the
/// [`ZeroCopy`] derive.
///
/// [`remaining()`]: Self::remaining
/// [`ZeroCopy`]: derive@crate::ZeroCopy
///
/// # Safety
///
/// The caller must ensure that the field type `F` is an actual field in
/// order in the struct being padded and that `align` matches the argument
/// provided to `#[repr(packed)]` (note that empty means 1).
#[inline]
pub unsafe fn pad_with<F>(&mut self, align: usize)
where
F: ZeroCopy,
{
let count = buf::padding_to(self.offset, align);
// zero out padding.
self.data.as_ptr().add(self.offset).write_bytes(0, count);
self.offset += count;
if F::PADDED {
let ptr = NonNull::new_unchecked(self.data.as_ptr().add(self.offset));
let mut padder = Padder::new(ptr);
F::pad(&mut padder);
padder.remaining();
}
self.offset += size_of::<F>();
}
/// Specific method to both pad for a discriminant and load it
/// simultaneously for inspection.
///
/// # Safety
///
/// The caller must ensure that the field type `D` is the actual type of the
/// discriminant first in order in the enum being padded and that `D` is a
/// primitive that does not contain any interior padding.
#[inline]
pub unsafe fn pad_discriminant<D>(&mut self) -> D
where
D: ZeroCopy,
{
let count = buf::padding_to(self.offset, align_of::<D>());
// zero out padding.
self.data.as_ptr().add(self.offset).write_bytes(0, count);
let at = self.offset + count;
let value = self.data.as_ptr().add(at).cast::<D>().read_unaligned();
self.offset = at + size_of::<D>();
value
}
/// Finish writing the current buffer.
///
/// This is typically not called directly, but rather is implemented by the
/// [`ZeroCopy`] derive.
///
/// [`ZeroCopy`]: derive@crate::ZeroCopy
///
/// # Safety
///
/// Before calling `remaining()`, the caller must ensure that they've called
/// [`pad::<F>()`] *in order* for every field in a struct being serialized
/// where `F` is the type of the field. Otherwise we might not have written
/// the necessary padding to ensure that all bytes related to the struct are
/// initialized. Failure to do so would result in undefined behavior.
///
/// Fields which are [`ZeroSized`] can be skipped.
///
/// [`pad::<F>()`]: Self::pad
/// [`ZeroSized`]: crate::traits::ZeroSized
#[inline]
pub unsafe fn remaining(self)
where
T: Sized,
{
let count = size_of::<T>() - self.offset;
self.data.as_ptr().add(self.offset).write_bytes(0, count);
}
/// Finalize remaining padding based on the size of an unsized value.
#[inline]
pub(crate) unsafe fn remaining_unsized(self, value: &T) {
let count = size_of_val(value) - self.offset;
self.data.as_ptr().add(self.offset).write_bytes(0, count);
}
}
#[cfg(test)]
mod tests {
use std::mem::size_of;
use anyhow::Result;
use crate::{buf, OwnedBuf, ZeroCopy};
#[test]
fn ensure_padding() -> Result<()> {
#[derive(Debug, PartialEq, Eq, ZeroCopy)]
#[repr(C)]
#[zero_copy(crate)]
struct ZeroPadded(u8, u16, u64);
let padded = ZeroPadded(
0x01u8.to_be(),
0x0203u16.to_be(),
0x0405060708090a0bu64.to_be(),
);
let mut buf = OwnedBuf::new();
// Note: You're responsible for ensuring that the buffer has enough
// capacity.
buf.reserve(size_of::<ZeroPadded>());
// SAFETY: We do not pad beyond known fields and are making sure to
// initialize all of the buffer.
unsafe {
buf::store_unaligned(buf.as_nonnull(), &padded);
buf.advance(size_of::<ZeroPadded>());
}
// Note: The bytes are explicitly convert to big-endian encoding above.
assert_eq!(
buf.as_slice(),
&[1, 0, 2, 3, 0, 0, 0, 0, 4, 5, 6, 7, 8, 9, 10, 11]
);
Ok(())
}
}