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Gimli doesn't support iOS #350

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alexcrichton opened this issue Jun 16, 2020 · 8 comments
Closed

Gimli doesn't support iOS #350

alexcrichton opened this issue Jun 16, 2020 · 8 comments
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gimli Related to the gimli implementation OS-ios

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@alexcrichton
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I'm not really sure if libbacktrace supports this and if so how well. This is at least a placeholder to audit what happens tody with libbacktrace and switch gimli over to that.

@alexcrichton alexcrichton added the gimli Related to the gimli implementation label Jun 16, 2020
bors added a commit to rust-lang-ci/rust that referenced this issue Jul 18, 2020
…Simulacrum

std: Switch from libbacktrace to gimli

This commit is a proof-of-concept for switching the standard library's
backtrace symbolication mechanism on most platforms from libbacktrace to
gimli. The standard library's support for `RUST_BACKTRACE=1` requires
in-process parsing of object files and DWARF debug information to
interpret it and print the filename/line number of stack frames as part
of a backtrace.

Historically this support in the standard library has come from a
library called "libbacktrace". The libbacktrace library seems to have
been extracted from gcc at some point and is written in C. We've had a
lot of issues with libbacktrace over time, unfortunately, though. The
library does not appear to be actively maintained since we've had
patches sit for months-to-years without comments. We have discovered a
good number of soundness issues with the library itself, both when
parsing valid DWARF as well as invalid DWARF. This is enough of an issue
that the libs team has previously decided that we cannot feed untrusted
inputs to libbacktrace. This also doesn't take into account the
portability of libbacktrace which has been difficult to manage and
maintain over time. While possible there are lots of exceptions and it's
the main C dependency of the standard library right now.

For years it's been the desire to switch over to a Rust-based solution
for symbolicating backtraces. It's been assumed that we'll be using the
Gimli family of crates for this purpose, which are targeted at safely
and efficiently parsing DWARF debug information. I've been working
recently to shore up the Gimli support in the `backtrace` crate. As of a
few weeks ago the `backtrace` crate, by default, uses Gimli when loaded
from crates.io. This transition has gone well enough that I figured it
was time to start talking seriously about this change to the standard
library.

This commit is a preview of what's probably the best way to integrate
the `backtrace` crate into the standard library with the Gimli feature
turned on. While today it's used as a crates.io dependency, this commit
switches the `backtrace` crate to a submodule of this repository which
will need to be updated manually. This is not done lightly, but is
thought to be the best solution. The primary reason for this is that the
`backtrace` crate needs to do some pretty nontrivial filesystem
interactions to locate debug information. Working without `std::fs` is
not an option, and while it might be possible to do some sort of
trait-based solution when prototyped it was found to be too unergonomic.
Using a submodule allows the `backtrace` crate to build as a submodule
of the `std` crate itself, enabling it to use `std::fs` and such.

Otherwise this adds new dependencies to the standard library. This step
requires extra attention because this means that these crates are now
going to be included with all Rust programs by default. It's important
to note, however, that we're already shipping libbacktrace with all Rust
programs by default and it has a bunch of C code implementing all of
this internally anyway, so we're basically already switching
already-shipping functionality to Rust from C.

* `object` - this crate is used to parse object file headers and
  contents. Very low-level support is used from this crate and almost
  all of it is disabled. Largely we're just using struct definitions as
  well as convenience methods internally to read bytes and such.

* `addr2line` - this is the main meat of the implementation for
  symbolication. This crate depends on `gimli` for DWARF parsing and
  then provides interfaces needed by the `backtrace` crate to turn an
  address into a filename / line number. This crate is actually pretty
  small (fits in a single file almost!) and mirrors most of what
  `dwarf.c` does for libbacktrace.

* `miniz_oxide` - the libbacktrace crate transparently handles
  compressed debug information which is compressed with zlib. This crate
  is used to decompress compressed debug sections.

* `gimli` - not actually used directly, but a dependency of `addr2line`.

* `adler32`- not used directly either, but a dependency of
  `miniz_oxide`.

The goal of this change is to improve the safety of backtrace
symbolication in the standard library, especially in the face of
possibly malformed DWARF debug information. Even to this day we're still
seeing segfaults in libbacktrace which could possibly become security
vulnerabilities. This change should almost entirely eliminate this
possibility whilc also paving the way forward to adding more features
like split debug information.

Some references for those interested are:

* Original addition of libbacktrace - rust-lang#12602
* OOM with libbacktrace - rust-lang#24231
* Backtrace failure due to use of uninitialized value - rust-lang#28447
* Possibility to feed untrusted data to libbacktrace - rust-lang#21889
* Soundness fix for libbacktrace - rust-lang#33729
* Crash in libbacktrace - rust-lang#39468
* Support for macOS, never merged - ianlancetaylor/libbacktrace#2
* Performance issues with libbacktrace - rust-lang#29293, rust-lang#37477
* Update procedure is quite complicated due to how many patches we
  need to carry - rust-lang#50955
* Libbacktrace doesn't work on MinGW with dynamic libs - rust-lang#71060
* Segfault in libbacktrace on macOS - rust-lang#71397

Switching to Rust will not make us immune to all of these issues. The
crashes are expected to go away, but correctness and performance may
still have bugs arise. The gimli and `backtrace` crates, however, are
actively maintained unlike libbacktrace, so this should enable us to at
least efficiently apply fixes as situations come up.

---

I want to note that my purpose for creating a PR here is to start a conversation about this. I think that all the various pieces are in place that this is compelling enough that I think this transition should be talked about seriously. There are a number of items which still need to be addressed before actually merging this PR, however:

* [ ] `gimli` needs to be published to crates.io
* [ ] `addr2line` needs a publish
* [ ] `miniz_oxide` needs a publish
* [ ] Tests probably shouldn't recommend the `gimli` crate's traits for implementing
* [ ] The `backtrace` crate's branch changes need to be merged to the master branch (rust-lang/backtrace-rs#349)
* [ ] The support for `libbacktrace` on some platforms needs to be audited to see if we should support more strategies in the gimli implementation - rust-lang/backtrace-rs#325, rust-lang/backtrace-rs#326, rust-lang/backtrace-rs#350, rust-lang/backtrace-rs#351

Most of the merging/publishing I'm not actively pushing on right now. It's a bit wonky for crates to support libstd so I'm holding off on pulling the trigger everywhere until there's a bit more discussion about how to go through with this. Namely rust-lang/backtrace-rs#349 I'm going to hold off merging until we decide to go through with the submodule strategy.

In any case this is a pretty major change, so I suspect that the compiler team is likely going to be interested in this. I don't mean to force changes by dumping a bunch of code by any means. Integration of external crates into the standard library is so difficult I wanted to have a proof-of-concept to review while talking about whether to do this at all (hence the PR), but I'm more than happy to follow any processes needed to merge this. I must admit though that I'm not entirely sure myself at this time what the process would be to decide to merge this, so I'm hoping others can help me figure that out!
@luser
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luser commented Jul 22, 2020

TL;DR: I don't think libbacktrace works on iOS currently, but adding iOS support to this crate shouldn't be a huge undertaking.

AFAICT libbacktrace relies on _Unwind_Backtrace for unwinding, which doesn't exist on iOS so I don't think libbacktrace works on iOS. Helpfully, the iOS ABIs for both AArch64 as well as ARMv7 mandate the use of a frame pointer, so unwinding is straightforward in practice. Apple's libc implementation of backtrace is pretty simple:

  • Call pthread_self to get the current thread
  • Call pthread_get_stackaddr_np and pthread_get_stacksize_np on the current thread to get the stack bounds
  • Invoke __builtin_frame_address to get the current frame pointer
  • Dereference the frame pointer in a loop to unwind the stack

I'd expect all of the existing Mach-O support in this crate to work fine for symbolication, but in practice backtraces from programs running on iOS devices won't have access to symbol files so it's immaterial. The standard practice for dealing with stack traces from iOS devices is to symbolicate them after the fact. It seems like this could be handled in tooling such as dinghy, although there's already an existing issue there about backtraces not working on iOS.

@tesuji
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tesuji commented Oct 23, 2020

Fixed by #378 ?

@cutsoy
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cutsoy commented Oct 23, 2020

The current state on iOS: #378 restores all functionality that libbacktrace had, but it doesn't show line-numbers or source files. I don't know if that's expected or not? But I don't remember ever seeing line-numbers in backtraces with Rust on iOS, so it's not a regression.

@alexcrichton
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Ok makes sense, thanks for the info! In light of that I think I'll leave this open until we can get line numbers and filenames working on iOS

@fzyzcjy
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fzyzcjy commented Oct 13, 2021

Hi, is there any updates? Can we have backtraces on ios? Thanks!

@bjorn3
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bjorn3 commented Oct 13, 2021

Do .ipa packages include the .dSYM file? If not getting filenames and line numbers is impossible for the app itself and will need synbolication after the fact. As far as I understand #378 already fixed symbol names on iOS.

@fzyzcjy
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fzyzcjy commented Oct 13, 2021

In production environment, there are no symbol names - otherwise it will be easier to reverse-engineering. However, as long as collect information on ios and can symbolize it afterwards, it is completely ok. (So can we do that?)

@workingjubilee
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The current report as of #613 is that we are now getting backtraces like this on visionOS:

backtrace[3e08695382c39b1]::backtrace::trace_unsynchronized::<<backtrace[3e08695382c39b1]::capture::Backtrace>::create::{closure#0}>
   1:        0x104b7d688 - backtrace[3e08695382c39b1]::backtrace::trace::<<backtrace[3e08695382c39b1]::capture::Backtrace>::create::{closure#0}>
   2:        0x104b9c518 - <backtrace[3e08695382c39b1]::capture::Backtrace>::create
   3:        0x104b9c490 - <backtrace[3e08695382c39b1]::capture::Backtrace>::new
   4:        0x104b2dea4 - <object[c239c684900fc29f]::ffi::Obj>::new
   5:        0x104b3a8c8 - object[c239c684900fc29f]::ffi::stream::object_fn_new::{closure#1}

And the implementation for all the Apple targets is virtually identical.

While we don't have adequate testing, I'm declaring this "fixed-ish". That last change will be in 0.3.72 which will be out Very Soon. I am certain there are oodles of bugs left in the support, or feature requests, or whatever, but please open them as individual issues.

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