Ignore more frames on backtrace unwinding. #40264

Yamakaky · Mar 4, 2017

Correctly handles panics in threads and tests. First, the frames after the last __rust_maybe_catch_panic are discarded, then it uses a blacklist that does some more fine-tuning. Since frames after the call to __rust_maybe_catch_panic seem to be platform-independant, BAD_PREFIXES_BOTTOM could probably be cleaned a bit.

#[test]
fn b() {
    None::<()>.unwrap();
}

   0: <core::option::Option<T>>::unwrap
             at /home/yamakaky/dev/rust/rust/src/libcore/macros.rs:21
   1: t::b
             at ./t.rs:12

fn main() {
    ::std::thread::spawn(|| panic!()).join().unwrap();
}

   0: t::main::{{closure}}
             at ./t.rs:2
...
   0: <core::result::Result<T, E>>::unwrap
             at /home/yamakaky/dev/rust/rust/src/libcore/result.rs:737
   1: t::main
             at ./t.rs:2

fn main() {
    let result = ::std::panic::catch_unwind(|| {
        panic!("oh no!");
    });
    assert!(result.is_err());
}

   0: t::main::{{closure}}
             at ./t.rs:10
   1: std::panicking::try::do_call
             at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:454
   2: __rust_try
   3: __rust_maybe_catch_panic
             at /home/yamakaky/dev/rust/rust/src/libpanic_unwind/lib.rs:98
   4: std::panicking::try
             at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:433
   5: std::panic::catch_unwind
             at /home/yamakaky/dev/rust/rust/src/libstd/panic.rs:361
   6: t::main
             at ./t.rs:9

Any idea about use-cases I would have forgotten?

rust-highfive · Mar 4, 2017

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

nagisa · Mar 4, 2017

First, the frames after __rust_maybe_catch_panic are discarded

This may be troublesome due to std::panic::catch_unwind which calls this function as well.

Yamakaky · Mar 4, 2017

Yeah, I fixed that in the latest commit ;) First post updated.

petrochenkov · Mar 4, 2017

Some thoughts:

~~no_std programs don't have rust_maybe_catch_panic~~ D'oh.
~~dropping frames at thread boundary doesn't seem good.~~ I was pretty sure I've seen backtraces spanning across thread boundaries with gdb and C code, but that doesn't seem to be true.
rust_maybe_catch_panic is called as a part of usual catch_unwind, e.g. at boundaries with C libraries, for example. dropping frames from C libraries doesn't seem good as well.

petrochenkov · Mar 4, 2017

petrochenkov reviewed Mar 4, 2017

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petrochenkov · Mar 4, 2017

src/libstd/sys_common/backtrace.rs

+        let mut is_rmcp = false;
+        let _ = resolve_symname(*frame, |symname| {
+            if let Some(mangled_symbol_name) = symname {
+                if mangled_symbol_name == "__rust_maybe_catch_panic" {


This needs to be mangled_symbol_name.contains("rust_maybe_catch_panic") (regardless of other concerns) because the symbol can be named panic_unwind::__rust_maybe_catch_panic or _rust_maybe_catch_panic (on Windows).

Yamakaky · Mar 4, 2017

The backtrace printing code is in libstd, so I think it doesn't matter?
What information would you want? The current behaviour doesn't have much: https://play.rust-lang.org/?gist=7a2efc9f3db3662f7887dc2ce89edc47&version=nightly&backtrace=1. Maybe modify the header to show the thread ID/name?
True, I didn't check that. On it.

Yamakaky · Mar 4, 2017

C-calling-rust works as expected, all the C frames are kept.

petrochenkov · Mar 4, 2017

@Yamakaky
I meant C calling Rust wrapped into catch_unwind, not just C calling Rust.

Yamakaky · Mar 4, 2017

Hum, right. Is it possible to detect if the current crate is a .so or .a?

Also, I'm not sure it's that bad. It's a very specific use case, and we can't clean the C backtrace so it would have the same bottom frames than RUST_BACKTRACE=full. On the other hand, we gain robustness against all the different frame setups on the supported platforms.

alexcrichton · Mar 6, 2017

alexcrichton reviewed Mar 6, 2017

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I'm getting slightly worried about how aggressively we're trying to hide frames while yet having a desire for this to be general purpose. It looks like we're already dropping frames that are otherwise valid and this continues to be very light on tests....

alexcrichton · Mar 6, 2017

src/libstd/sys_common/backtrace.rs

        "_ZN4core6result13unwrap_failed",
        "ZN4core6result13unwrap_failed",
        "core::result::unwrap_failed",
+
        "rust_begin_unwind",
        "_ZN4drop",


Er was this added in the previous PR? This definitely seems like something that we shouldn't be dropping, this is just a normal function?

alexcrichton · Mar 6, 2017

src/libstd/sys_common/backtrace.rs

+         $LT$$LP$$RP$$GT$$GT$9call_once",
+        "ZN91_$LT$std..panic..AssertUnwindSafe$LT$F$GT$$u20$as$u20$core..ops..FnOnce\
+         $LT$$LP$$RP$$GT$$GT$9call_once",
+        "<std::panic::AssertUnwindSafe<F> as core::ops::FnOnce<()>>::call_once",


I don't think that these are correct to have because this is just the beginning of a catch_unwind, right?

So? It's std stuff, we want to remove it.

alexcrichton · Mar 6, 2017

src/libstd/sys_common/backtrace.rs

+
+        "_ZN42_$LT$F$u20$as$u20$test..FnBox$LT$T$GT$$GT$8call_box",
+        "ZN42_$LT$F$u20$as$u20$test..FnBox$LT$T$GT$$GT$8call_box",
+        "<F as test::FnBox<T>>::call_box",


These can happen at any time, right? Not just during tests?

Not sure, but it was fine during my tests.

Yamakaky · Mar 6, 2017

The frames in BAD_PREFIXES_BOTTOM/TOP are only removed if all the frames before (resp. after) were removed. Like if BAD_PREFIXES_TOP = [A, D] and the stackframe is

A
B
C
D
E

it will only remove A and not D.

I can remove the special handling of __rust_maybe_catch_panic, but it will mean BAD_PREFIXES_BOTTOM will stay big, and with platform-specific function names in libstd/sys_common. If we revert it, then maybe split between the platform-specific frames and the common ones?

alexcrichton · Mar 8, 2017

I don't think that's the problem though, if the stack trace as B/C/D/E we'd remove everything but E?

Yamakaky · Mar 8, 2017

if BAD_PREFIXES_TOP = [A, D] and the stacktrace is B/C/D/E, then it's not modified.

alexcrichton · Mar 9, 2017

Ok but the point still stands I think, can we be less aggressive about pruning frames? Many of these patterns are legitimate Rust functions not related to unwinding.

alexcrichton · Mar 9, 2017

alexcrichton reviewed Mar 9, 2017

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alexcrichton · Mar 9, 2017

src/libstd/sys_common/backtrace.rs

-        "__rust_maybe_catch_panic",
-        "_rust_maybe_catch_panic",
-        "__libc_start_main",
+
        "__rust_try",


I don't think that we want this to be the bottom of a frame, this is just one panic::catch_unwind and there could be many on the stack.

alexcrichton · Mar 9, 2017

alexcrichton reviewed Mar 9, 2017

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alexcrichton · Mar 9, 2017

src/libstd/sys_common/backtrace.rs

+
+        "_ZN4core3ops6FnOnce9call_once",
+        "ZN4core3ops6FnOnce9call_once",
+        "core::ops::FnOnce::call_once",


Like above, these could be any rust function, so we shouldn't prune these

I fail to find an example where this frame would be at the beginning of the frame and you would not want to remove it. There will always be the main, or a function if it's a library.

Yamakaky · Mar 9, 2017

Like for this backtrace (rustc without optimisations), what would you want to remove?

thread 'main' panicked at 'explicit panic', t.rs:2
stack backtrace:
   0:     0x562b31ac45f8 - std::sys::imp::backtrace::tracing::imp::unwind_backtrace::hf65740f571b3b062
                               at /home/yamakaky/dev/rust/rust/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1:     0x562b31af430e - std::sys_common::backtrace::_print::h49fe0d471b73c20e
                               at /home/yamakaky/dev/rust/rust/src/libstd/sys_common/backtrace.rs:71
   2:     0x562b31af351e - std::sys_common::backtrace::print::hd65a646efeb9e475
                               at /home/yamakaky/dev/rust/rust/src/libstd/sys_common/backtrace.rs:60
   3:     0x562b31ac704e - std::panicking::default_hook::{{closure}}::h25191b25fd3a12b5
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:355
   4:     0x562b31ac6b1c - std::panicking::default_hook::hf7e171cc5ec0b024
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:371
   5:     0x562b31ac795c - std::panicking::rust_panic_with_hook::h01e0c787710839ce
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:549
   6:     0x562b31ab5343 - std::panicking::begin_panic::h67ddf8e748228f4b
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:511
   7:     0x562b31ab5852 - t::main::hf31397ea67eea730
                               at /tmp/t.rs:2
   8:     0x562b31ac7e85 - core::ops::FnOnce::call_once::h97136135af0c9b86
   9:     0x562b31ac737a - std::panicking::try::do_call::h0b3ba9923f346cc7
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:454
  10:     0x562b31b08347 - __rust_try
  11:     0x562b31b08227 - __rust_maybe_catch_panic
                               at /home/yamakaky/dev/rust/rust/src/libpanic_unwind/lib.rs:98
  12:     0x562b31ac7167 - std::panicking::try::h2b94160ac273c42a
                               at /home/yamakaky/dev/rust/rust/src/libstd/panicking.rs:433
  13:     0x562b31adc66a - std::panic::catch_unwind::h8c1f7b1121d92ea1
                               at /home/yamakaky/dev/rust/rust/src/libstd/panic.rs:361
  14:     0x562b31adf67e - std::rt::lang_start::h29d9bdfd34a5eab2
                               at /home/yamakaky/dev/rust/rust/src/libstd/rt.rs:57
  15:     0x562b31ab5892 - main
  16:     0x7feb00a57290 - __libc_start_main
  17:     0x562b31ab5169 - _start
  18:                0x0 - <unknown>

alexcrichton · Mar 10, 2017

Ideally we'd only show frame 7 in that stack trace, but I think that we need to show 7-13 because we don't know if frames 8-13 are a nested catch panic or not.

I think we can safely assume though that frames 0-6 are panicking machinery which can be ignored by default. Similarly 14-18 are safely system frames.

Yamakaky · Mar 10, 2017

Same question with explicit panic catch (the last example in my first post) https://gist.github.com/Yamakaky/b4c5168b8478311f66b02d47013f7cf1

alexcrichton · Mar 13, 2017

Yes that should only strip frames 0-6 and 14-24 ideally as those weren't written by the user. I don't know of a heuristic which will actually do that though.

Yamakaky · Mar 13, 2017

That's exactly what the current code does ;)

alexcrichton · Mar 27, 2017

alexcrichton reviewed Mar 27, 2017

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alexcrichton · Mar 27, 2017

src/libstd/sys_common/backtrace.rs

        "core::result::unwrap_failed",
+        "ZN4core6result13unwrap_failed",
+
        "rust_begin_unwind",
        "_ZN4drop",
        "mingw_set_invalid_parameter_handler",


This and drop above should be removed

alexcrichton · Mar 27, 2017

alexcrichton reviewed Mar 27, 2017

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alexcrichton · Mar 27, 2017

src/libstd/sys_common/backtrace.rs

+    // Frames to remove from the top of the backtrace.
+    //
+    // The raw form is used so that we don't have to demangle the symbol names.
+    // The `a::b::c` form can show up on Windows/MSVC.
    static BAD_PREFIXES_TOP: &'static [&'static str] = &[


Can we remove this array entirely? Currently rust_panic is the "canonical frame name" of the panic, and if there's a lower frame we should start from then we should just give that a canonical name instead.

What to you think about removing the frames from the core::panicking::panic* functions?

I think it's fine to strive to eliminate them, but we shouldn't have such a long list of what to filter. There should be one frame that we look for and if we find it maybe do a few calculations based off that. Substring searching is basically guarantee to go wrong in the future.

alexcrichton · Apr 11, 2017

@Yamakaky any update on removing the large lists of whitelists to canonical frames to prune?

Yamakaky · Apr 11, 2017

No, sorry. I'm near the end of the school semester so I have a lot of things to do.

alexcrichton · Apr 12, 2017

No worries! Thanks for the update.

arielb1 · Apr 18, 2017

Ping @Yamakaky - are you still working on this PR?

Yamakaky · Apr 18, 2017

Not now, I still have a week before the end of the semester. You can work on it if you want!

bors · Apr 18, 2017

☔️ The latest upstream changes (presumably #41373) made this pull request unmergeable. Please resolve the merge conflicts.

alexcrichton · Apr 27, 2017

Ok I'm going to close this for now, but please feel free to resubmit @Yamakaky if you find the time!

Yamakaky · May 7, 2017

Is it OK if I open a PR with only the bottom cleaning, and leave the top for a later PR?

alexcrichton · May 7, 2017

Certainly!

whitequark · Jun 24, 2017

@Yamakaky Do you think you can take another look on this PR? It would be really great to say goodbye to all the junk in the backtraces.

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Merged
std: Back out backtrace pruning logic #41364

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Rust never lets you know about RUST_BACKTRACE=full #41584

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Improve cleaning of the bottom of the backtrace #41815

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Ignore panic functions in backtraces. Print inlined functions on Windows #45637

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Short backtrace have regressed #47429

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Ignore more frames on backtrace unwinding. #40264

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