ICE using -Zmiri-tag-raw-pointers: 'we should never pop more than one frame at once'

[y86-dev]

I tried this code (havent tried to reduce this yet, because i discovered this while trying to solve another bug):

Code

#![deny(unsafe_op_in_unsafe_fn)]
use core::{
    marker::PhantomPinned,
    mem::{self, ManuallyDrop, MaybeUninit},
    pin::Pin,
    ptr::{addr_of, addr_of_mut},
};

#[repr(transparent)]
struct UnsafeAliasCell<T> {
    value: T,
    _pin: PhantomPinned,
}

impl<T> UnsafeAliasCell<T> {
    pub fn new(value: T) -> Self {
        Self {
            value,
            _pin: PhantomPinned,
        }
    }

    pub fn get(&mut self) -> *mut T {
        addr_of_mut!(self.value)
    }
}

#[repr(C)]
pub struct PtrBufUninit<T, const N: usize> {
    idx: MaybeUninit<*const T>,
    end: MaybeUninit<*const T>,
    buf: UnsafeAliasCell<[ManuallyDrop<T>; N]>,
}

struct PtrBufUninitProj<'pin, T, const N: usize> {
    idx: &'pin mut MaybeUninit<*const T>,
    end: &'pin mut MaybeUninit<*const T>,
    buf: &'pin mut UnsafeAliasCell<[ManuallyDrop<T>; N]>,
}

#[repr(C)]
pub struct PtrBuf<T, const N: usize> {
    idx: *const T,
    end: *const T,
    buf: UnsafeAliasCell<[ManuallyDrop<T>; N]>,
}

struct PtrBufProj<'pin, T, const N: usize> {
    idx: &'pin mut *const T,
    end: &'pin mut *const T,
    #[allow(dead_code)]
    buf: &'pin mut UnsafeAliasCell<[ManuallyDrop<T>; N]>,
}

impl<T, const N: usize> PtrBufUninit<T, N> {
    pub fn init(mut self: Pin<Box<Self>>) -> Pin<Box<PtrBuf<T, N>>> {
        let this = self.as_mut().project();
        let ptr = this.buf.get() as *const T;
        this.idx.write(ptr);
        let ptr = unsafe {
            //ptr.offset(N as isize - 1)
            // SAFETY: buf is a valid pointer
            addr_of!((*(this.buf.get() as *mut [ManuallyDrop<T>; N]))[N - 1])
                as *const ManuallyDrop<T> as *const T
        };
        this.end.write(ptr);
        unsafe {
            // SAFETY: we do not move the contents of the box
            let this = Pin::into_inner_unchecked(self);
            let this = Box::from_raw(Box::into_raw(this) as *mut PtrBuf<T, N>);
            Pin::new_unchecked(this)
        }
    }

    fn project(self: Pin<&mut Self>) -> PtrBufUninitProj<'_, T, N> {
        // SAFETY: we do not structually pin anything, we only need buf to stay where it is.
        let this = unsafe { Pin::get_unchecked_mut(self) };
        PtrBufUninitProj {
            idx: &mut this.idx,
            end: &mut this.end,
            buf: &mut this.buf,
        }
    }
}

impl<T, const N: usize> PtrBuf<T, N> {
    fn project(self: Pin<&mut Self>) -> PtrBufProj<'_, T, N> {
        // SAFETY: we do not structually pin anything, we only need buf to stay where it is.
        let this = unsafe { Pin::get_unchecked_mut(self) };
        PtrBufProj {
            idx: &mut this.idx,
            end: &mut this.end,
            buf: &mut this.buf,
        }
    }
}

impl<T, const N: usize> From<[T; N]> for PtrBufUninit<T, N> {
    fn from(arr: [T; N]) -> Self {
        assert_ne!(N, 0);
        assert!(
            N < isize::MAX as usize,
            "N cannot be bigger than isize::MAX"
        );
        Self {
            idx: MaybeUninit::uninit(),
            end: MaybeUninit::uninit(),
            buf: unsafe {
                // SAFETY: T and ManuallyDrop<T> have the same layout, so [T; N] and
                // [ManuallyDrop<T>; N] also have the same layout
                let ptr = addr_of!(arr) as *const [T; N] as *const [ManuallyDrop<T>; N];
                mem::forget(arr);
                UnsafeAliasCell::new(ptr.read())
            },
        }
    }
}

impl<T, const N: usize> PtrBuf<T, N> {
    pub fn next(self: Pin<&mut Self>) -> Option<T> {
        let this = self.project();
        if this.idx > this.end {
            None
        } else {
            let val = unsafe {
                // SAFETY: we checked if idx is in bounds before and
                // we read the value at idx and offset idx, this value is never read again
                this.idx.read()
            };
            unsafe {
                // SAFETY: we are still in bounds of buf (end stores the end of buf)
                // and adding size_of::<T>() will only land us at most one byte of buf.
                *this.idx = this.idx.offset(1);
            }
            Some(val)
        }
    }
}

impl<T, const N: usize> Drop for PtrBuf<T, N> {
    fn drop(&mut self) {
        fn pinned_drop<T, const N: usize>(mut this: Pin<&mut PtrBuf<T, N>>) {
            while let Some(x) = this.as_mut().next() {
                drop(x);
            }
        }
        pinned_drop(unsafe { Pin::new_unchecked(self) });
    }
}

impl<T, const N: usize> Drop for PtrBufUninit<T, N> {
    fn drop(&mut self) {
        let slice = unsafe {
            // We did not access buf anywhere yet
            &mut *self.buf.get()
        };
        for x in slice {
            unsafe {
                // SAFETY: we are dropping ourselves and thus this is subject to the same
                // invariants
                ManuallyDrop::drop(x);
            }
        }
    }
}

fn main() {
    let buf: PtrBufUninit<i32, 5> = [42, -42, 1337, 0, 6].into();
    let mut buf = Box::pin(buf).init();
    assert_eq!(buf.as_mut().next(), Some(42));
    assert_eq!(buf.as_mut().next(), Some(-42));
    assert_eq!(buf.as_mut().next(), Some(1337));
    assert_eq!(buf.as_mut().next(), Some(0));
    assert_eq!(buf.as_mut().next(), Some(6));
    assert_eq!(buf.as_mut().next(), None);
}

Then when running under miri with

MIRIFLAGS="-Zmiri-tag-raw-pointers -Zmiri-track-pointer-tag=2500" cargo miri run

Stacktrace

note: tracking was triggered
   --> ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/maybe_uninit.rs:634:13
    |
634 |             ManuallyDrop::into_inner(self.value)
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ created tag 2500
    |
    = note: inside `std::mem::MaybeUninit::<[std::mem::ManuallyDrop<i32>; 5]>::assume_init` at ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/maybe_uninit.rs:634:13
    = note: inside `std::ptr::read::<[std::mem::ManuallyDrop<i32>; 5]>` at ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1097:9
    = note: inside `std::ptr::const_ptr::<impl *const [std::mem::ManuallyDrop<i32>; 5]>::read` at ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/const_ptr.rs:941:18
note: inside `<PtrBufUninit<i32, 5_usize> as std::convert::From<[i32; 5]>>::from` at src/main.rs:113:38
   --> src/main.rs:113:38
    |
113 |                 UnsafeAliasCell::new(ptr.read())
    |                                      ^^^^^^^^^^
    = note: inside `<[i32; 5] as std::convert::Into<PtrBufUninit<i32, 5_usize>>>::into` at ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/convert/mod.rs:550:9
note: inside `main` at src/main.rs:168:37
   --> src/main.rs:168:37
    |
168 |     let buf: PtrBufUninit<i32, 5> = [42, -42, 1337, 0, 6].into();
    |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at 'we should never pop more than one frame at once', src/tools/miri/src/diagnostics.rs:377:17
stack backtrace:
   0:     0x7f4767bec85d - std::backtrace_rs::backtrace::libunwind::trace::hd79e9b51bb0b02a3
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f4767bec85d - std::backtrace_rs::backtrace::trace_unsynchronized::hc4b2624d11f57391
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f4767bec85d - std::sys_common::backtrace::_print_fmt::h5b920b6df28041d5
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/sys_common/backtrace.rs:66:5
   3:     0x7f4767bec85d - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h01f2db48eea34166
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/sys_common/backtrace.rs:45:22
   4:     0x7f4767c47efc - core::fmt::write::h743b8fce003c331c
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/core/src/fmt/mod.rs:1194:17
   5:     0x7f4767bde041 - std::io::Write::write_fmt::h55edc38b905db9b5
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/io/mod.rs:1655:15
   6:     0x7f4767bef575 - std::sys_common::backtrace::_print::h72c54a6b7a86b7bf
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/sys_common/backtrace.rs:48:5
   7:     0x7f4767bef575 - std::sys_common::backtrace::print::h8b541992f5fa33c9
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/sys_common/backtrace.rs:35:9
   8:     0x7f4767bef575 - std::panicking::default_hook::{{closure}}::h47e8a61e5844dea4
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/panicking.rs:295:22
   9:     0x7f4767bef1e9 - std::panicking::default_hook::h65ae1796882c178c
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/panicking.rs:314:9
  10:     0x7f476838d5a1 - rustc_driver[caca827775d68846]::DEFAULT_HOOK::{closure#0}::{closure#0}
  11:     0x7f4767befd46 - std::panicking::rust_panic_with_hook::h1c3eee211b989bad
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/panicking.rs:702:17
  12:     0x7f4767befb09 - std::panicking::begin_panic_handler::{{closure}}::h653627205f5b2cdc
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/panicking.rs:586:13
  13:     0x7f4767becd14 - std::sys_common::backtrace::__rust_end_short_backtrace::h36d845a914b6aae7
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/sys_common/backtrace.rs:138:18
  14:     0x7f4767bef879 - rust_begin_unwind
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/panicking.rs:584:5
  15:     0x7f4767bb4bd3 - core::panicking::panic_fmt::hb6389d787a80a806
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/core/src/panicking.rs:142:14
  16:     0x5646cd103156 - <std[fd2a1eaf3e081d4d]::thread::local::LocalKey<core[864d7ae58f623181]::cell::RefCell<alloc[642b2cd0ec1e1a81]::vec::Vec<miri[11bda64e92b8a89f]::diagnostics::NonHaltingDiagnostic>>>>::with::<<rustc_const_eval[ec16a9026af29337]::interpret::eval_context::InterpCx<miri[11bda64e92b8a89f]::machine::Evaluator> as miri[11bda64e92b8a89f]::diagnostics::EvalContextExt>::process_diagnostics::{closure#0}, ()>
  17:     0x5646cd0f7159 - miri[11bda64e92b8a89f]::eval::eval_entry
  18:     0x5646cd06a902 - <rustc_interface[dacacaf3651fe347]::passes::QueryContext>::enter::<<miri[aaa733ecb7479b45]::MiriCompilerCalls as rustc_driver[caca827775d68846]::Callbacks>::after_analysis::{closure#0}, ()>
  19:     0x5646cd06346e - <miri[aaa733ecb7479b45]::MiriCompilerCalls as rustc_driver[caca827775d68846]::Callbacks>::after_analysis
  20:     0x7f476a3a1ab5 - <rustc_interface[dacacaf3651fe347]::interface::Compiler>::enter::<rustc_driver[caca827775d68846]::run_compiler::{closure#1}::{closure#2}, core[864d7ae58f623181]::result::Result<core[864d7ae58f623181]::option::Option<rustc_interface[dacacaf3651fe347]::queries::Linker>, rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>>
  21:     0x7f476a3cb3ff - rustc_span[ff646084812721ee]::with_source_map::<core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>, rustc_interface[dacacaf3651fe347]::interface::create_compiler_and_run<core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>, rustc_driver[caca827775d68846]::run_compiler::{closure#1}>::{closure#1}>
  22:     0x7f476a3b5d64 - rustc_interface[dacacaf3651fe347]::interface::create_compiler_and_run::<core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>, rustc_driver[caca827775d68846]::run_compiler::{closure#1}>
  23:     0x7f476a39efa1 - <scoped_tls[d6203f3b3010b98e]::ScopedKey<rustc_span[ff646084812721ee]::SessionGlobals>>::set::<rustc_interface[dacacaf3651fe347]::interface::run_compiler<core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>, rustc_driver[caca827775d68846]::run_compiler::{closure#1}>::{closure#0}, core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>>
  24:     0x7f476a3b854f - std[fd2a1eaf3e081d4d]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[dacacaf3651fe347]::util::run_in_thread_pool_with_globals<rustc_interface[dacacaf3651fe347]::interface::run_compiler<core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>, rustc_driver[caca827775d68846]::run_compiler::{closure#1}>::{closure#0}, core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>>::{closure#0}, core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>>
  25:     0x7f476a3b8689 - <<std[fd2a1eaf3e081d4d]::thread::Builder>::spawn_unchecked_<rustc_interface[dacacaf3651fe347]::util::run_in_thread_pool_with_globals<rustc_interface[dacacaf3651fe347]::interface::run_compiler<core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>, rustc_driver[caca827775d68846]::run_compiler::{closure#1}>::{closure#0}, core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>>::{closure#0}, core[864d7ae58f623181]::result::Result<(), rustc_errors[14f27e81a88a2c7]::ErrorGuaranteed>>::{closure#1} as core[864d7ae58f623181]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  26:     0x7f4767bf9bf3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h211dae6d8dec3611
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/alloc/src/boxed.rs:1866:9
  27:     0x7f4767bf9bf3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hb54334e8a7bb1f12
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/alloc/src/boxed.rs:1866:9
  28:     0x7f4767bf9bf3 - std::sys::unix::thread::Thread::new::thread_start::hf8b57655f2f2e68e
                               at /rustc/4ca19e09d302a4cbde14f9cb1bc109179dc824cd/library/std/src/sys/unix/thread.rs:108:17
  29:     0x7f47678b6b1a - start_thread
  30:     0x7f476793b660 - __GI___clone3
  31:                0x0 - <unknown>

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.62.0-nightly (4ca19e09d 2022-04-19) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C incremental -Z miri-tag-raw-pointers -Z miri-track-pointer-tag=2500

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack

It also occures on the current master branch (3ac7ca4).

Removing the -Zmiri-track-raw-pointers produces no ICE.

[y86-dev]

It also occurs with:

• 79
• 82-87
• 309
• 312-317
• 410
• 413-418
• 2251
• 2257
• 2501
• 3633-3638
• 3645-3655
• 3662
• 3663
• 3666-3676
• 3680-3688
• 3771-3775
• 3778-3781

(tested up to 6000)

[y86-dev]

The errors at 79 and 82-87 also seem to occur with a new project.

[RalfJung]

This is probably basically the same as #2076 -- our diagnostics infrastructure was never tested for what happens when diagnostics are created in the "setup" phase of the interpreter, before any code runs. (We also have a tear-down phase after all code runs, which leads to issues like #2020.)

[saethlin]

I tried and I can't reproduce this based on the report here 😕 I swear I've seen the referenced ICE before, and it's not obvious to me that any change has been made that should have fixed this.

[y86-dev]

Interesting, I also fail to reproduce this on the latest version...

[y86-dev]

hmm I did a quick git bisect and found 5a6c4a6 to be the responsible commit (only tested for 79). This seems strange, as it is only a version bump and a single line change in src/machine.rs. I do not know if that is the reason, but maybe something in rustc changed?

[RalfJung]

This is the rustc range (assuming you always tested the version given in rust-version): rust-lang/rust@9a25164...8fbd92d.

I can't see anything obvious there, either... strange.

I am inclined to close this when #2076 is closed. We can always re-investigate if/when it happens again...

[RalfJung]

This might also have been caused by rust-lang/rust#87000, which I am working on fixing.