WASI panics in fd_readdir when trying to read a directory with the Cyrillic name Действие ("Action")

[XX]

I'm reading dir with subdirs from wasm WASI module. Some subdirs have names in Cyrillic. Most names work well, but a dir named Действие (translated as "Action") produces a panic:

thread 'tokio-runtime-worker' panicked at 'assertion failed: self.is_char_boundary(new_len)', /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/alloc/src/string.rs:1279:13
stack backtrace:
   0:     0x55c946fb46c1 - std::backtrace_rs::backtrace::libunwind::trace::he648b5c8dd376705
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x55c946fb46c1 - std::backtrace_rs::backtrace::trace_unsynchronized::h5da3e203eef39e9f
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x55c946fb46c1 - std::sys_common::backtrace::_print_fmt::h8d28d3f20588ae4c
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x55c946fb46c1 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hd9a5b0c9c6b058c0
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x55c946fe282f - core::fmt::rt::Argument::fmt::h0afc04119f252b53
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/fmt/rt.rs:138:9
   5:     0x55c946fe282f - core::fmt::write::h50b1b3e73851a6fe
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/fmt/mod.rs:1094:21
   6:     0x55c946fb0377 - std::io::Write::write_fmt::h184eaf275e4484f0
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/io/mod.rs:1714:15
   7:     0x55c946fb44d5 - std::sys_common::backtrace::_print::hf58c3a5a25090e71
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x55c946fb44d5 - std::sys_common::backtrace::print::hb9cf0a7c7f077819
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x55c946fb5a03 - std::panicking::default_hook::{{closure}}::h066adb2e3f3e2c07
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:269:22
  10:     0x55c946fb5794 - std::panicking::default_hook::h277fa2776900ff14
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:288:9
  11:     0x55c946fb5f89 - std::panicking::rust_panic_with_hook::hceaf38da6d9db792
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:705:13
  12:     0x55c946fb5e41 - std::panicking::begin_panic_handler::{{closure}}::h2bce3ed2516af7df
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:595:13
  13:     0x55c946fb4b26 - std::sys_common::backtrace::__rust_end_short_backtrace::h090f3faf8f98a395
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/sys_common/backtrace.rs:151:18
  14:     0x55c946fb5bd2 - rust_begin_unwind
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:593:5
  15:     0x55c943fe56a3 - core::panicking::panic_fmt::h4ec8274704d163a3
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/panicking.rs:67:14
  16:     0x55c943fe5733 - core::panicking::panic::hee69a8315e4031d6
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/panicking.rs:117:5
  17:     0x55c9443f35b5 - alloc::string::String::truncate::h82d02a6707fc8e5e
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/alloc/src/string.rs:1279:13
  18:     0x55c944014ebc - wasmtime_wasi::preview2::preview1::<impl wasmtime_wasi::preview2::preview1::wasi_snapshot_preview1::WasiSnapshotPreview1 for T>::fd_readdir::{{closure}}::{{closure}}::h97aab24454c0467a
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasi/src/preview2/preview1.rs:1614:17
  19:     0x55c944013582 - wasmtime_wasi::preview2::preview1::<impl wasmtime_wasi::preview2::preview1::wasi_snapshot_preview1::WasiSnapshotPreview1 for T>::fd_readdir::{{closure}}::h71c012aa997b73d6
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasi/src/preview2/preview1.rs:1512:5
  20:     0x55c9443e2fcd - <core::pin::Pin<P> as core::future::future::Future>::poll::hb4af1694529eb86d
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/future/future.rs:125:9
  21:     0x55c9440f07fd - wasmtime_wasi::preview2::preview1::wasi_snapshot_preview1::fd_readdir::{{closure}}::hf7116fbdd0addf6e
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasi/src/preview2/preview1.rs:409:1
  22:     0x55c9443ac11b - <tracing::instrument::Instrumented<T> as core::future::future::Future>::poll::h936b7cf29985876e
                               at /home/xx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tracing-0.1.37/src/instrument.rs:272:9
  23:     0x55c944119dc9 - wasmtime_wasi::preview2::preview1::wasi_snapshot_preview1::add_to_linker::{{closure}}::{{closure}}::h63b0a11ab91ebc68
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasi/src/preview2/preview1.rs:409:1
  24:     0x55c9442febf6 - wasmtime::store::AsyncCx::block_on::h82bda0230fa2549f
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasmtime/src/store.rs:1939:17
  25:     0x55c9443d5257 - wasmtime::linker::Linker<T>::func_wrap5_async::{{closure}}::h9344d71184333eb2
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasmtime/src/linker.rs:165:32
  26:     0x55c94437a6cb - <F as wasmtime::func::IntoFunc<T,(wasmtime::func::Caller<T>,A1,A2,A3,A4,A5),R>>::into_func::native_call_shim::{{closure}}::{{closure}}::h3f9721c1a8eb8bfc
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasmtime/src/func.rs:1983:41
  27:     0x55c9441f5141 - core::ops::function::FnOnce::call_once::hce93171fad2be346
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/ops/function.rs:250:5
  28:     0x55c943fec4da - <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::hba0591f268e936b6
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/core/src/panic/unwind_safe.rs:271:9
  29:     0x55c944195178 - std::panicking::try::do_call::hb281efb5695617a3
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:500:40
  30:     0x55c9441a752b - __rust_try
  31:     0x55c944185419 - std::panicking::try::h6b63c45588efcce6
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panicking.rs:464:19
  32:     0x55c9441bc2fd - std::panic::catch_unwind::h64a7c0ca6df50084
                               at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/panic.rs:142:14
  33:     0x55c9443791ee - <F as wasmtime::func::IntoFunc<T,(wasmtime::func::Caller<T>,A1,A2,A3,A4,A5),R>>::into_func::native_call_shim::{{closure}}::h9fdd439159cc9c6d
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasmtime/src/func.rs:1978:29
  34:     0x55c94438911b - wasmtime::func::Caller<T>::with::{{closure}}::h5736c62974799fb0
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasmtime/src/func.rs:1778:13
  35:     0x55c94432c7e3 - wasmtime_runtime::instance::Instance::from_vmctx::hc0687768455fff47
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/runtime/src/instance.rs:240:9
  36:     0x55c944386701 - wasmtime::func::Caller<T>::with::h54bd907b8742a763
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasmtime/src/func.rs:1776:9
  37:     0x55c944378063 - <F as wasmtime::func::IntoFunc<T,(wasmtime::func::Caller<T>,A1,A2,A3,A4,A5),R>>::into_func::native_call_shim::h4d07b8819148efa0
                               at /home/xx/.cargo/git/checkouts/wasmtime-41807828cb3a7a7e/186c3ec/crates/wasmtime/src/func.rs:1967:34

Versions and Environment

Wasmtime version or commit: actual from master branch
Operating system: Kubuntu
Architecture: x86_64 GNU/Linux

[bjorn3]

This is the issue I think

wasmtime/crates/wasi/src/preview2/preview1.rs

Line 1614 in f46d5a3

path.truncate(cap);

path is a string, so not every length of truncation would leave behind a valid UTF-8 string. The truncation should probably be done on a Vec<u8> instead.

[alexcrichton]

Thanks for tracking this down @bjorn3!

I believe this should be fixed by #7011, but @XX can you test and confirm?

[XX]

@alexcrichton I built the app with your fix ( #7011 ) and all directories are processed well. Thank You!

[pchickey]

Thanks for the report @XX and thanks for the fix @alexcrichton !