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tracing: use ManuallyDrop instead of mem::forget (#2765)

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The current code is UB and LLVM could choose to reuse the stack slot causing a UAF.

## Motivation

UB is bad.

## Solution

Don't do that.
This commit is contained in:
Manish Goregaokar 2023-10-18 16:44:33 -07:00 committed by Eliza Weisman
parent 4b99457c87
commit 20a1762b3f
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GPG Key ID: F9C1A595C3814436
1 changed files with 6 additions and 7 deletions
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13
tracing/src/instrument.rs
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@ -5,7 +5,7 @@ use crate::{
use core::{
future::Future,
marker::Sized,
mem::{self, ManuallyDrop},
mem::ManuallyDrop,
pin::Pin,
task::{Context, Poll},
};
@ -359,12 +359,11 @@ impl<T> Instrumented<T> {
///
/// Note that this drops the span.
pub fn into_inner(self) -> T {
// To manually destructure `Instrumented` without `Drop`, we save
// pointers to the fields and use `mem::forget` to leave those pointers
// valid.
let span: *const Span = &self.span;
let inner: *const ManuallyDrop<T> = &self.inner;
mem::forget(self);
// To manually destructure `Instrumented` without `Drop`, we
// move it into a ManuallyDrop and use pointers to its fields
let this = ManuallyDrop::new(self);
let span: *const Span = &this.span;
let inner: *const ManuallyDrop<T> = &this.inner;
// SAFETY: Those pointers are valid for reads, because `Drop` didn't
// run, and properly aligned, because `Instrumented` isn't
// `#[repr(packed)]`.