Commits on Jul 17, 2020

1. 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.

   

   alexcrichton committed Jul 17, 2020
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