lang_start in std/src/rt.rs is unsound in presence of panic payload that panics on drop

[nagisa]

See #86027 for an example of the problem.

fn main() {
    std::panic::panic_any(Bomb);
}

thread 'main' panicked at 'Box<Any>', src/main.rs:12:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'main' panicked at 'explicit panic', src/main.rs:7:9
fatal runtime error: failed to initiate panic, error 5
abort (core dumped)

Here we panic without any landing pads available upstack, because we are already the top-most(-ish) frame in the first place. This is UB.

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-critical

[RalfJung]

Playground link with full code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8042bb4b47c3b1c57f1a0dfe11a3bf41

[RalfJung]

FWIW, Miri does detect this. :)

error: Undefined Behavior: unwinding past the topmost frame of the stack
  --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:43:1
   |
43 | / fn lang_start<T: crate::process::Termination + 'static>(
44 | |     main: fn() -> T,
45 | |     argc: isize,
46 | |     argv: *const *const u8,
...  |
52 | |     )
53 | | }
   | |_^ unwinding past the topmost frame of the stack
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
           
   = note: inside `std::rt::lang_start::<()>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:43:1

error: aborting due to previous error