Segfault in `Report::downcast()` introduced in 0.6.9

[stevepryde]

I recently had a test segfault after updating to the latest eyre (0.6.11). The same test passes on 0.6.8 but segfaults on 0.6.9 or later.

error: test failed, to rerun pass `--bin ...`

Caused by:
  process didn't exit successfully: ... (signal: 11, SIGSEGV: invalid memory reference)

Here is a minimal reproduction:

use eyre::Context;

fn main() {
    let err = trigger().unwrap_err();
    let _ = err.downcast::<MyError>();
}

#[derive(Debug, thiserror::Error)]
enum MyError {
    #[error("error building http client")]
    ClientConstruction,
    #[error(transparent)]
    SsmError(#[from] aws_sdk_ssm::Error),
}

fn trigger() -> eyre::Result<()> {
    do_thing().wrap_err("failed")?;
    Ok(())
}

fn do_thing() -> Result<(), MyError> {
    Err(MyError::ClientConstruction)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_me() {
        let err = trigger().unwrap_err();
        let _ = err.downcast::<MyError>();
    }
}

Cargo.toml deps

[dependencies]
eyre = "0.6.9" # or later
thiserror = "1"
aws-sdk-ssm = "1"

I included the test because cargo test gives slightly more info.

Something about the second error variant is related, even though that variant isn't constructed. I used aws_sdk_ssm::Error in the error variant because that's how I hit the issue.

The issue seems to be in src/error.rs, in context_drop_rest(), which was changed in 0.6.9. Any reason why this code differs from anyhow ?

[stevepryde]

Additional details:

• Mac M1 (macOS 14.2.1)
• Rust 1.74.1 (also tried on 1.74.0 and 1.73.0, same result)

[stevepryde]

I get the same segfault also on latest Manjaro Linux (x86-64), using Rust 1.74.1.

[abusch]

I've managed to simplify the reproduction case to remove dependencies (thiserror and aws-sdk-ssm are off the hook):

#![allow(dead_code)]
use std::{error::Error, fmt::Display};

use eyre::Context;

fn main() {
    let err = trigger().unwrap_err();
    let _ = err.downcast::<MyError>();
    println!("it worked!");
}

#[derive(Debug)]
enum MyError {
    ClientConstruction,
    Other(String),
}

impl Display for MyError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "MyError error")
    }
}
impl Error for MyError {}

fn trigger() -> eyre::Result<()> {
    Err(MyError::ClientConstruction).wrap_err("failed")
}

The failure I get is slightly different (on a Mac M1 Sonoma 14.2) :

eyre_test(48377,0x1e194d000) malloc: *** error for object 0x1025c23ff: pointer being freed was not allocated
eyre_test(48377,0x1e194d000) malloc: *** set a breakpoint in malloc_error_break to debug

[Process exited 255]

Some things I've noticed:

• replacing the Other variant with Other(u32) works fine. So does Other (empty variant). But Other(Box<u32>) fails
• removing .wrap_err() (i.e returning Err(MyError::ClientConstruction.into()) in trigger()) makes it work

[ten3roberts]

Thank you for opening the issue.

I will look into it and see what may be causing it

[ten3roberts]

I believe it is that the inner error is incorrectly freed even though it was downcasted, we still need to free the wrapped error.

The content of the Other variant is likely because it gives the enum a Drop impl if any fields needs drop. Without it, the box deallocation will take shortcuts when freeing and not drop the contents, but just free

[stevepryde]

Any reason why this fix hasn't been released yet (and affected versions yanked)? It's a known soundness issue in a popular crate.