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Removing use of mem::uninitialized from std::io::util #62657

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Removing use of mem::uninitialized from std::io::util #62657

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282e27a
Removing use of mem::uninitialized from CloudABI and std::io::util
JulianGindi Jul 12, 2019
30e1435
backing out cloudabi changes, and removing uninit memory
JulianGindi Jul 13, 2019
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9 changes: 2 additions & 7 deletions src/libstd/io/util.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,6 @@

use crate::fmt;
use crate::io::{self, Read, Initializer, Write, ErrorKind, BufRead, IoSlice, IoSliceMut};
use crate::mem;

/// Copies the entire contents of a reader into a writer.
///
Expand Down Expand Up @@ -43,12 +42,8 @@ use crate::mem;
pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> io::Result<u64>
where R: Read, W: Write
{
let mut buf = unsafe {
#[allow(deprecated)]
let mut buf: [u8; super::DEFAULT_BUF_SIZE] = mem::uninitialized();
reader.initializer().initialize(&mut buf);
buf
};

let mut buf: [u8; super::DEFAULT_BUF_SIZE] = [0; super::DEFAULT_BUF_SIZE];
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The entire point of this method is to not zero-initialize. See the initializer docs.

You should create a MaybeUninit<[u8; ...]>, and then use &mut *buf.as_ptr_mut() and pass that to initialize. This is, technically, still wrong, but it is less wrong than the previous code. You should add a comment saying that this is UB because we create a reference to uninitialized data, but that we exploit the fact that we are libstd and can rely on more guarantees than "normal" libraries.

Cc rust-lang/unsafe-code-guidelines#90

This is all very subtle. There is a reason I marked the issue as "medium", not "easy". ;)

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You're right, this one is probably outside my skill level currently. I'm sorry for taking up your time, but I do appreciate the bits of knowledge. I'm gonna grab some easier issues to start with. I'm happy to have this PR closed for now.

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That's okay, this happens. :) And no worries, this did not take much time on my end.


let mut written = 0;
loop {
Expand Down