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| # Debugging support in the Rust compiler | ||
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| This document explains the state of debugging tools support in the Rust compiler (rustc). | ||
| The document gives an overview of debugging tools like GDB, LLDB etc. and infrastrcture | ||
| around Rust compiler to debug Rust code. If you want to learn how to debug the Rust compiler | ||
| itself, then you must see [Debugging the Compiler] page. | ||
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| The material is gathered from YouTube video [Tom Tromey discusses debugging support in rustc]. | ||
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| ## Preliminaries | ||
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| ### Debuggers | ||
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| According to Wikipedia | ||
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| > A [debugger or debugging tool] is a computer program that is used to test and debug | ||
| > other programs (the "target" program). | ||
| Writing a debugger from scratch for a language requires a lot of work, especially if | ||
| debuggers have to be supported on various platforms. GDB and LLDB, however, can be | ||
| extended to support debugging a language. This is the path that Rust has chosen. | ||
| This document's main goal is to document the said debuggers support in Rust compiler. | ||
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| ### DWARF | ||
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| According to the [DWARF] standard website | ||
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| > DWARF is a debugging file format used by many compilers and debuggers to support source level | ||
| > debugging. It addresses the requirements of a number of procedural languages, | ||
| > such as C, C++, and Fortran, and is designed to be extensible to other languages. | ||
| > DWARF is architecture independent and applicable to any processor or operating system. | ||
| > It is widely used on Unix, Linux and other operating systems, | ||
| > as well as in stand-alone environments. | ||
| DWARF reader is a program that consumes the DWARF format and creates debugger compatible output. | ||
| This program may live in the compiler itself. DWARF uses a data structure called | ||
| Debugging Information Entry (DIE) which stores the information as "tags" to denote functions, | ||
| variables etc., e.g., `DW_TAG_variable`, `DW_TAG_pointer_type`, `DW_TAG_subprogram` etc. | ||
| You can also invent your own tags and attributes. | ||
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| ## Supported debuggers | ||
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| ### GDB | ||
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| We have our own fork of GDB - [https://github.com/rust-dev-tools/gdb] | ||
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| #### Rust expression parser | ||
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| To be able to show debug output we need an expression parser. | ||
| This (GDB) expression parser is written in [Bison] and is only a subset of Rust expressions. | ||
| This means that this parser can parse only a subset of Rust expressions. | ||
| GDB parser was written from scratch and has no relation to any other parser. | ||
| For example, this parser is not related to Rustc's parser. | ||
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| GDB has Rust like value and type output. It can print values and types in a way | ||
| that look like Rust syntax in the output. Or when you print a type as [ptype] in GDB, | ||
| it also looks like Rust source code. Checkout the documentation in the [manual for GDB/Rust]. | ||
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| #### Parser extensions | ||
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| Expression parser has a couple of extensions in it to facilitate features that you cannot do | ||
| with Rust. Some limitations are listed in the [manual for GDB/Rust]. There is some special | ||
| code in the DWARF reader in GDB to support the extensions. | ||
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| A couple of examples of DWARF reader support needed are as follows - | ||
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| 1. Enum: Needed for support for enum types. The Rustc writes the information about enum into | ||
| DWARF and GDB reads the DWARF to understand where is the tag field or is there a tag | ||
| field or is the tag slot shared with non-zero optimization etc. | ||
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| 2. Dissect trait objects: DWARF extension where the trait object's description in the DWARF | ||
| also points to a stub description of the corresponding vtable which in turn points to the | ||
| concrete type for which this trait object exists. This means that you can do a `print *object` | ||
| for that trait object, and GDB will understand how to find the correct type of the payload in | ||
| the trait object. | ||
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| **TODO**: Figure out if the following should be mentioned in the GDB-Rust document rather than | ||
| this guide page so there is no duplication. This is regarding the following comments: | ||
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| [This comment by Tom](https://github.com/rust-lang/rustc-guide/pull/316#discussion_r284027340) | ||
| > gdb's Rust extensions and limitations are documented in the gdb manual: | ||
| https://sourceware.org/gdb/onlinedocs/gdb/Rust.html -- however, this neglects to mention that | ||
| gdb convenience variables and registers follow the gdb $ convention, and that the Rust parser | ||
| implements the gdb @ extension. | ||
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| [This question by Aman](https://github.com/rust-lang/rustc-guide/pull/316#discussion_r285401353) | ||
| > @tromey do you think we should mention this part in the GDB-Rust document rather than this | ||
| document so there is no duplication etc.? | ||
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| #### Developer notes | ||
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| * This work is now upstream. Bugs can be reported in [GDB Bugzilla]. | ||
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| ### LLDB | ||
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| We have our own fork of LLDB - [https://github.com/rust-lang/lldb] | ||
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| Fork of LLVM project - [https://github.com/rust-lang/llvm-project] | ||
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| LLDB currently only works on macOS because of a dependency issue. This issue was easier to | ||
| solve for macOS as compared to Linux. However, Tom has a possible solution which can enable | ||
| us to ship LLDB everywhere. | ||
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| #### Rust expression parser | ||
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| This expression parser is written in C++. It is a type of [Recursive Descent parser]. | ||
| Implements slightly less of the Rust language than GDB. LLDB has Rust like value and type output. | ||
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| #### Parser extensions | ||
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| There is some special code in the DWARF reader in LLDB to support the extensions. | ||
| A couple of examples of DWARF reader support needed are as follows - | ||
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| 1. Enum: Needed for support for enum types. The Rustc writes the information about | ||
| enum into DWARF and LLDB reads the DWARF to understand where is the tag field or | ||
| is there a tag field or is the tag slot shared with non-zero optimization etc. | ||
| In other words, it has enum support as well. | ||
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| #### Developer notes | ||
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| * None of the LLDB work is upstream. This [rust-lang/lldb wiki page] explains a few details. | ||
| * The reason for forking LLDB is that LLDB recently removed all the other language plugins | ||
| due to lack of maintenance. | ||
| * LLDB has a plugin architecture but that does not work for language support. | ||
| * LLDB is available via Rust build (`rustup`). | ||
| * GDB generally works better on Linux. | ||
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| ## DWARF and Rustc | ||
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| [DWARF] is the standard way compilers generate debugging information that debuggers read. | ||
| It is _the_ debugging format on macOS and Linux. It is a multi-language, extensible format | ||
| and is mostly good enough for Rust's purposes. Hence, the current implementation reuses DWARF's | ||
| concepts. This is true even if some of the concepts in DWARF do not align with Rust | ||
| semantically because generally there can be some kind of mapping between the two. | ||
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| We have some DWARF extensions that the Rust compiler emits and the debuggers understand that | ||
| are _not_ in the DWARF standard. | ||
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| * Rust compiler will emit DWARF for a virtual table, and this `vtable` object will have a | ||
| `DW_AT_containing_type` that points to the real type. This lets debuggers dissect a trait object | ||
| pointer to correctly find the payload. E.g., here's such a DIE, from a test case in the gdb | ||
| repository: | ||
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| ```asm | ||
| <1><1a9>: Abbrev Number: 3 (DW_TAG_structure_type) | ||
| <1aa> DW_AT_containing_type: <0x1b4> | ||
| <1ae> DW_AT_name : (indirect string, offset: 0x23d): vtable | ||
| <1b2> DW_AT_byte_size : 0 | ||
| <1b3> DW_AT_alignment : 8 | ||
| ``` | ||
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| * The other extension is that the Rust compiler can emit a tagless discriminated union. | ||
| See [DWARF feature request] for this item. | ||
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| ### Current limitations of DWARF | ||
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| * Traits - require a bigger change than normal to DWARF, on how to represent Traits in DWARF. | ||
| * DWARF provides no way to differentiate between Structs and Tuples. Rust compiler emits | ||
| fields with `__0` and debuggers look for a sequence of such names to overcome this limitation. | ||
| For example, in this case the debugger would look at a field via `x.__0` instead of `x.0`. | ||
| This is resolved via the Rust parser in the debugger so now you can do `x.0`. | ||
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| DWARF relies on debuggers to know some information about platform ABI. | ||
| Rust does not do that all the time. | ||
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| ## Developer notes | ||
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| This section is from the talk about certain aspects of development. | ||
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| ## What is missing | ||
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| ### Shipping GDB in Rustup | ||
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| Tracking issue: [https://github.com/rust-lang/rust/issues/34457] | ||
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| Shipping GDB requires change to Rustup delivery system. To manage Rustup build size and | ||
| times we need to build GDB separately, on its own and somehow provide the artifacts produced | ||
| to be included in the final build. However, if we can ship GDB with rustup, it will simplify | ||
| the development process by having compiler emit new debug info which can be readily consumed. | ||
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| Main issue in achieving this is setting up dependencies. One such dependency is Python. That | ||
| is why we have our own fork of GDB because one of the drivers is patched on Rust's side to | ||
| check the correct version of Python (Python 2.7 in this case. *Note: Python3 is not chosen | ||
| for this purpose because Python's stable ABI is limited and is not sufficient for GDB's needs. | ||
| See [https://docs.python.org/3/c-api/stable.html]*). | ||
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| This is to keep updates to debugger as fast as possible as we make changes to the debugging symbols. | ||
| In essence, to ship the debugger as soon as new debugging info is added. GDB only releases | ||
| every six months or so. However, the changes that are | ||
| not related to Rust itself should ideally be first merged to upstream eventually. | ||
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| ### Code signing for LLDB debug server on macOS | ||
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| According to Wikipedia, [System Integrity Protection] is | ||
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| > System Integrity Protection (SIP, sometimes referred to as rootless) is a security feature | ||
| > of Apple's macOS operating system introduced in OS X El Capitan. It comprises a number of | ||
| > mechanisms that are enforced by the kernel. A centerpiece is the protection of system-owned | ||
| > files and directories against modifications by processes without a specific "entitlement", | ||
| > even when executed by the root user or a user with root privileges (sudo). | ||
| It prevents processes using `ptrace` syscall. If a process wants to use `ptrace` it has to be | ||
| code signed. The certificate that signs it has to be trusted on your machine. | ||
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| See [Apple developer documentation for System Integrity Protection]. | ||
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| We may need to sign up with Apple and get the keys to do this signing. Tom has looked into if | ||
| Mozilla cannot do this because it is at the maximum number of | ||
| keys it is allowed to sign. Tom does not know if Mozilla could get more keys. | ||
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| Alternatively, Tom suggests that maybe a Rust legal entity is needed to get the keys via Apple. | ||
| This problem is not technical in nature. If we had such a key we could sign GDB as well and | ||
| ship that. | ||
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| ### DWARF and Traits | ||
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| Rust traits are not emitted into DWARF at all. The impact of this is calling a method `x.method()` | ||
| does not work as is. The reason being that method is implemented by a trait, as opposed | ||
| to a type. That information is not present so finding trait methods is missing. | ||
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| DWARF has a notion of interface types (possibly added for Java). Tom's idea was to use this | ||
| interface type as traits. | ||
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| DWARF only deals with concrete names, not the reference types. So, a given implementation of a | ||
| trait for a type would be one of these interfaces (`DW_tag_interface` type). Also, the type for | ||
| which it is implemented would describe all the interfaces this type implements. This requires a | ||
| DWARF extension. | ||
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| Issue on Github: [https://github.com/rust-lang/rust/issues/33014] | ||
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| ## Typical process for a Debug Info change (LLVM) | ||
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| LLVM has Debug Info (DI) builders. This is the primary thing that Rust calls into. | ||
| This is why we need to change LLVM first because that is emitted first and not DWARF directly. | ||
| This is a kind of metadata that you construct and hand-off to LLVM. For the Rustc/LLVM hand-off | ||
| some LLVM DI builder methods are called to construct representation of a type. | ||
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| The steps of this process are as follows - | ||
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| 1. LLVM needs changing. | ||
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| LLVM does not emit Interface types at all, so this needs to be implemented in the LLVM first. | ||
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| Get sign off on LLVM maintainers that this is a good idea. | ||
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| 2. Change the DWARF extension. | ||
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| 3. Update the debuggers. | ||
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| Update DWARF readers, expression evaluators. | ||
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| 4. Update Rust compiler. | ||
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| Change it to emit this new information. | ||
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| ### Procedural macro stepping | ||
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| A deeply profound question is that how do you actually debug a procedural macro? | ||
| What is the location you emit for a macro expansion? Consider some of the following cases - | ||
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| * You can emit location of the invocation of the macro. | ||
| * You can emit the location of the definition of the macro. | ||
| * You can emit locations of the content of the macro. | ||
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| RFC: [https://github.com/rust-lang/rfcs/pull/2117] | ||
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| Focus is to let macros decide what to do. This can be achieved by having some kind of attribute | ||
| that lets the macro tell the compiler where the line marker should be. This affects where you | ||
| set the breakpoints and what happens when you step it. | ||
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| ## Future work | ||
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| #### Name mangling changes | ||
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| * New demangler in `libiberty` (gcc source tree). | ||
| * New demangler in LLVM or LLDB. | ||
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| **TODO**: Check the location of the demangler source. | ||
| [Question on Github](https://github.com/rust-lang/rustc-guide/pull/316#discussion_r283062536). | ||
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| #### Reuse Rust compiler for expressions | ||
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| This is an important idea because debuggers by and large do not try to implement type | ||
| inference. You need to be much more explicit when you type into the debugger than your | ||
| actual source code. So, you cannot just copy and paste an expression from your source | ||
| code to debugger and expect the same answer but this would be nice. This can be helped | ||
| by using compiler. | ||
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| It is certainly doable but it is a large project. You certainly need a bridge to the | ||
| debugger because the debugger alone has access to the memory. Both GDB (gcc) and LLDB (clang) | ||
| have this feature. LLDB uses Clang to compile code to JIT and GDB can do the same with GCC. | ||
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| Both debuggers expression evaluation implement both a superset and a subset of Rust. | ||
| They implement just the expression language but they also add some extensions like GDB has | ||
| convenience variables. Therefore, if you are taking this route then you not only need | ||
| to do this bridge but may have to add some mode to let the compiler understand some extensions. | ||
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| #### Windows debugging (PDB) is missing | ||
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| This is a complete unknown. | ||
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| [Tom Tromey discusses debugging support in rustc]: https://www.youtube.com/watch?v=elBxMRSNYr4 | ||
| [Debugging the Compiler]: compiler-debugging.md | ||
| [debugger or debugging tool]: https://en.wikipedia.org/wiki/Debugger | ||
| [Bison]: https://www.gnu.org/software/bison/ | ||
| [ptype]: https://ftp.gnu.org/old-gnu/Manuals/gdb/html_node/gdb_109.html | ||
| [rust-lang/lldb wiki page]: https://github.com/rust-lang/lldb/wiki | ||
| [DWARF]: http://dwarfstd.org | ||
| [manual for GDB/Rust]: https://sourceware.org/gdb/onlinedocs/gdb/Rust.html | ||
| [GDB Bugzilla]: https://sourceware.org/bugzilla/ | ||
| [Recursive Descent parser]: https://en.wikipedia.org/wiki/Recursive_descent_parser | ||
| [System Integrity Protection]: https://en.wikipedia.org/wiki/System_Integrity_Protection | ||
| [https://github.com/rust-dev-tools/gdb]: https://github.com/rust-dev-tools/gdb | ||
| [DWARF feature request]: http://dwarfstd.org/ShowIssue.php?issue=180517.2 | ||
| [https://docs.python.org/3/c-api/stable.html]: https://docs.python.org/3/c-api/stable.html | ||
| [https://github.com/rust-lang/rfcs/pull/2117]: https://github.com/rust-lang/rfcs/pull/2117 | ||
| [https://github.com/rust-lang/rust/issues/33014]: https://github.com/rust-lang/rust/issues/33014 | ||
| [https://github.com/rust-lang/rust/issues/34457]: https://github.com/rust-lang/rust/issues/34457 | ||
| [Apple developer documentation for System Integrity Protection]: https://developer.apple.com/library/archive/releasenotes/MacOSX/WhatsNewInOSX/Articles/MacOSX10_11.html#//apple_ref/doc/uid/TP40016227-SW11 | ||
| [https://github.com/rust-lang/lldb]: https://github.com/rust-lang/lldb | ||
| [https://github.com/rust-lang/llvm-project]: https://github.com/rust-lang/llvm-project |