How does wasmtime print Rust backtrace on panics? · general

I'm surprised that if I run a wasm32-wasi Rust program with wasmtime it prints the Rust backtrace (of the Wasm program, not the host) on panic. How does wasmtime implement this? I'd expect it to understand Rust stack implementation on Wasm target for this.. is this the case? Or is it Rust's runtime/core/std that prints the backtrace? If so, why is it printed even when I don't pass RUST_BACKTRACE=1? Example:

$ cat src/main.rs
fn main() {
    panic!()
}

$ cargo build --target=wasm32-wasi
   Compiling wasm_backtrace v0.1.0 (/home/omer/rust/wasm_backtrace)
    Finished dev [unoptimized + debuginfo] target(s) in 0.34s

$ wasmtime target/wasm32-wasi/debug/wasm_backtrace.wasm
thread 'main' panicked at 'explicit panic', src/main.rs:2:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Error: failed to run main module `target/wasm32-wasi/debug/wasm_backtrace.wasm`

Caused by:
    0: failed to invoke command default
    1: wasm trap: unreachable
       wasm backtrace:
         0: 0x6c34 - <unknown>!__rust_start_panic
         1: 0x68df - <unknown>!rust_panic
         2: 0x65b9 - <unknown>!std::panicking::rust_panic_with_hook::h2345fb0909b53e12
         3: 0x10c7 - <unknown>!std::panicking::begin_panic::{{closure}}::h86678b4e45fbd7fa
         4: 0x1ccc - <unknown>!std::sys_common::backtrace::__rust_end_short_backtrace::h8c7fe73fc6112e0e
         5:  0xfb5 - <unknown>!std::panicking::begin_panic::h5a83b3bb18195122
         6: 0x1fed - <unknown>!wasm_backtrace::g::hc5f9230a6e65a92c
         7: 0x1fbf - <unknown>!wasm_backtrace::f::h80747d9c660ceee4
         8: 0x1fb5 - <unknown>!wasm_backtrace::main::h9fc5985544d26b7b
         9: 0x1bee - <unknown>!core::ops::function::FnOnce::call_once::hfd088d5879747b81
         10: 0x1d03 - <unknown>!std::sys_common::backtrace::__rust_begin_short_backtrace::h2d7439e32125816a
         11:  0x4a5 - <unknown>!std::rt::lang_start::{{closure}}::hc9be9987846fa67a
         12: 0x69f0 - <unknown>!std::rt::lang_start_internal::h260050c92cd470af
         13:  0x457 - <unknown>!std::rt::lang_start::hd15e4b73e676a12b
         14: 0x200b - <unknown>!__original_main
         15:  0x397 - <unknown>!_start

Yury Delendik (Dec 10 2020 at 13:18):

It is generated and printed by wasmtime. During the trap, cranelift gets entire backtrace (see backtrace crate) and filters out only wasm addresses, wasmtime just prints out wasm portion, but capable of getting entire stack including host.

osa1 (Dec 10 2020 at 14:02):

How is this done exactly? Does wasmtime/cranelift know about Rust's stack implementation in Wasm linear memory?

osa1 (Dec 10 2020 at 14:03):

So backtrace (the crate) can walk Rust stack.. but for example in my example wasmtime needs to be aware that the Wasm program is Rust and call that crate to obtain the stack trace, no?

Yury Delendik (Dec 10 2020 at 14:05):

nope, the wasm program can be in C, and your will see similar trace but with C names

osa1 (Dec 10 2020 at 14:06):

Yury Delendik (Dec 10 2020 at 14:07):

Yury Delendik (Dec 10 2020 at 14:08):

osa1 (Dec 10 2020 at 14:09):

osa1 (Dec 10 2020 at 14:10):

One more question: the backtrace in my example is obtained on Wasm trap instruction, right?

Yury Delendik (Dec 10 2020 at 14:11):

more exactly from unreachable (multiple instructions can trap for different reasons)

osa1 (Dec 10 2020 at 14:37):

@Yury Delendik thanks, really helpful. Does wasmtime provide an API for getting the Wasm program's backtrace (using the backtrace crate or otherwise) when implementing a host function? As far as I understand host functions have access to Caller but it's not possible to get a backtrace from that type, right?

Yury Delendik (Dec 10 2020 at 14:39):

Peter Huene (Dec 10 2020 at 19:21):

On a related note, I think we should probably default the module name to the filename sans .wasm so that backtraces aren't filled with <unknown>.

Stream: general

Topic: How does wasmtime print Rust backtrace on panics?

osa1 (Dec 10 2020 at 12:58):