1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
use core::marker::PhantomData;
use core::mem::{align_of, size_of, transmute};
use core::ptr;
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::finish`"]
pub struct Padder<'a, T> {
ptr: *mut u8,
offset: usize,
_marker: PhantomData<&'a mut T>,
}
impl<'a, T> Padder<'a, T>
where
T: ZeroCopy,
{
#[inline]
pub(crate) fn new(ptr: *mut u8) -> Self {
Self {
ptr,
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 = crate::buf::padding_to(self.offset, align);
// zero out padding.
ptr::write_bytes(self.ptr.add(self.offset), 0, count);
self.offset = self.offset.wrapping_add(count);
if F::PADDED {
let mut padder = Padder::new(self.ptr.wrapping_add(self.offset));
F::pad(&mut padder);
padder.remaining();
}
self.offset = self.offset.wrapping_add(size_of::<F>());
}
/// Only pad a field where the value of the field doesn't matter.
///
/// # Safety
///
/// The caller must ensure that the field type `F` is an actual field in
/// order in the struct being padded and that `F` is a primitive that does
/// not contain any interior padding.
#[inline]
pub unsafe fn pad_discriminator<D>(&mut self) -> D
where
D: ZeroCopy,
{
let count = crate::buf::padding_to(self.offset, align_of::<D>());
// zero out padding.
ptr::write_bytes(self.ptr.add(self.offset), 0, count);
let at = self.offset.wrapping_add(count);
let value = self.ptr.add(at).cast::<D>().read_unaligned();
self.offset = at.wrapping_add(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) {
let count = size_of::<T>() - self.offset;
ptr::write_bytes(self.ptr.wrapping_add(self.offset), 0, count);
}
/// Finalize remaining padding with a custom size.
#[inline]
pub(crate) unsafe fn remaining_with(self, size: usize) {
let count = size - self.offset;
ptr::write_bytes(self.ptr.wrapping_add(self.offset), 0, count);
}
}
#[cfg(test)]
mod tests {
use std::mem::size_of;
use anyhow::Result;
use crate::{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.as_buf_mut().store_unaligned(&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(())
}
}