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| <h1 id="debugging-support-in-the-rust-compiler"><a class="header" href="#debugging-support-in-the-rust-compiler">Debugging support in the Rust compiler</a></h1> |
| <ul> |
| <li><a href="#preliminaries">Preliminaries</a> |
| <ul> |
| <li><a href="#debuggers">Debuggers</a></li> |
| <li><a href="#dwarf">DWARF</a></li> |
| <li><a href="#codeviewpdb">CodeView/PDB</a></li> |
| </ul> |
| </li> |
| <li><a href="#supported-debuggers">Supported debuggers</a> |
| <ul> |
| <li><a href="#gdb">GDB</a> |
| <ul> |
| <li><a href="#rust-expression-parser">Rust expression parser</a></li> |
| <li><a href="#parser-extensions">Parser extensions</a></li> |
| </ul> |
| </li> |
| <li><a href="#lldb">LLDB</a> |
| <ul> |
| <li><a href="#rust-expression-parser-1">Rust expression parser</a></li> |
| <li><a href="#developer-notes">Developer notes</a></li> |
| </ul> |
| </li> |
| <li><a href="#windbgcdb">WinDbg/CDB</a> |
| <ul> |
| <li><a href="#natvis">Natvis</a></li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| <li><a href="#dwarf-and-rustc">DWARF and <code>rustc</code></a> |
| <ul> |
| <li><a href="#current-limitations-of-dwarf">Current limitations of DWARF</a></li> |
| </ul> |
| </li> |
| <li><a href="#developer-notes-1">Developer notes</a></li> |
| <li><a href="#what-is-missing">What is missing</a> |
| <ul> |
| <li><a href="#code-signing-for-lldb-debug-server-on-macos">Code signing for LLDB debug server on macOS</a></li> |
| <li><a href="#dwarf-and-traits">DWARF and Traits</a></li> |
| </ul> |
| </li> |
| <li><a href="#typical-process-for-a-debug-info-change-llvm">Typical process for a Debug Info change (LLVM)</a> |
| <ul> |
| <li><a href="#procedural-macro-stepping">Procedural macro stepping</a></li> |
| </ul> |
| </li> |
| <li><a href="#source-file-checksums-in-debug-info">Source file checksums in debug info</a> |
| <ul> |
| <li><a href="#dwarf-5">DWARF 5</a></li> |
| <li><a href="#llvm">LLVM</a></li> |
| <li><a href="#microsoft-visual-c-compiler-zh-option">Microsoft Visual C++ Compiler /ZH option</a></li> |
| <li><a href="#clang">Clang</a></li> |
| </ul> |
| </li> |
| <li><a href="#future-work">Future work</a> |
| <ul> |
| <li><a href="#name-mangling-changes">Name mangling changes</a></li> |
| <li><a href="#reuse-rust-compiler-for-expressions">Reuse Rust compiler for expressions</a></li> |
| </ul> |
| </li> |
| </ul> |
| <p>This document explains the state of debugging tools support in the Rust compiler (rustc). |
| It gives an overview of GDB, LLDB, WinDbg/CDB, |
| as well as infrastructure around Rust compiler to debug Rust code. |
| If you want to learn how to debug the Rust compiler itself, |
| see <a href="compiler-debugging.html">Debugging the Compiler</a>.</p> |
| <p>The material is gathered from the video, |
| <a href="https://www.youtube.com/watch?v=elBxMRSNYr4">Tom Tromey discusses debugging support in rustc</a>.</p> |
| <h2 id="preliminaries"><a class="header" href="#preliminaries">Preliminaries</a></h2> |
| <h3 id="debuggers"><a class="header" href="#debuggers">Debuggers</a></h3> |
| <p>According to Wikipedia</p> |
| <blockquote> |
| <p>A <a href="https://en.wikipedia.org/wiki/Debugger">debugger or debugging tool</a> is a computer program that is used to test and debug |
| other programs (the "target" program).</p> |
| </blockquote> |
| <p>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.</p> |
| <h3 id="dwarf"><a class="header" href="#dwarf">DWARF</a></h3> |
| <p>According to the <a href="http://dwarfstd.org">DWARF</a> standard website</p> |
| <blockquote> |
| <p>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.</p> |
| </blockquote> |
| <p>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., <code>DW_TAG_variable</code>, <code>DW_TAG_pointer_type</code>, <code>DW_TAG_subprogram</code> etc. |
| You can also invent your own tags and attributes.</p> |
| <h3 id="codeviewpdb"><a class="header" href="#codeviewpdb">CodeView/PDB</a></h3> |
| <p><a href="https://llvm.org/docs/PDB/index.html">PDB</a> (Program Database) is a file format created by Microsoft that contains debug information. |
| PDBs can be consumed by debuggers such as WinDbg/CDB and other tools to display debug information. |
| A PDB contains multiple streams that describe debug information about a specific binary such |
| as types, symbols, and source files used to compile the given binary. CodeView is another |
| format which defines the structure of <a href="https://llvm.org/docs/PDB/CodeViewSymbols.html">symbol records</a> and <a href="https://llvm.org/docs/PDB/CodeViewTypes.html">type records</a> that appear within |
| PDB streams.</p> |
| <h2 id="supported-debuggers"><a class="header" href="#supported-debuggers">Supported debuggers</a></h2> |
| <h3 id="gdb"><a class="header" href="#gdb">GDB</a></h3> |
| <h4 id="rust-expression-parser"><a class="header" href="#rust-expression-parser">Rust expression parser</a></h4> |
| <p>To be able to show debug output, we need an expression parser. |
| This (GDB) expression parser is written in <a href="https://www.gnu.org/software/bison/">Bison</a>, |
| and can parse only a subset of Rust expressions. |
| GDB parser was written from scratch and has no relation to any other parser, |
| including that of rustc.</p> |
| <p>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 <a href="https://ftp.gnu.org/old-gnu/Manuals/gdb/html_node/gdb_109.html">ptype</a> in GDB, |
| it also looks like Rust source code. Checkout the documentation in the <a href="https://sourceware.org/gdb/onlinedocs/gdb/Rust.html">manual for GDB/Rust</a>.</p> |
| <h4 id="parser-extensions"><a class="header" href="#parser-extensions">Parser extensions</a></h4> |
| <p>Expression parser has a couple of extensions in it to facilitate features that you cannot do |
| with Rust. Some limitations are listed in the <a href="https://sourceware.org/gdb/onlinedocs/gdb/Rust.html">manual for GDB/Rust</a>. There is some special |
| code in the DWARF reader in GDB to support the extensions.</p> |
| <p>A couple of examples of DWARF reader support needed are as follows:</p> |
| <ol> |
| <li> |
| <p>Enum: Needed for support for enum types. |
| The Rust compiler writes the information about enum into DWARF, |
| and GDB reads the DWARF to understand where is the tag field, |
| or if there is a tag field, |
| or if the tag slot is shared with non-zero optimization etc.</p> |
| </li> |
| <li> |
| <p>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 <code>print *object</code> |
| for that trait object, and GDB will understand how to find the correct type of the payload in |
| the trait object.</p> |
| </li> |
| </ol> |
| <p><strong>TODO</strong>: 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:</p> |
| <p><a href="https://github.com/rust-lang/rustc-dev-guide/pull/316#discussion_r284027340">This comment by Tom</a></p> |
| <blockquote> |
| <p>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.</p> |
| </blockquote> |
| <p><a href="https://github.com/rust-lang/rustc-dev-guide/pull/316#discussion_r285401353">This question by Aman</a></p> |
| <blockquote> |
| <p>@tromey do you think we should mention this part in the GDB-Rust document rather than this |
| document so there is no duplication etc.?</p> |
| </blockquote> |
| <h3 id="lldb"><a class="header" href="#lldb">LLDB</a></h3> |
| <h4 id="rust-expression-parser-1"><a class="header" href="#rust-expression-parser-1">Rust expression parser</a></h4> |
| <p>This expression parser is written in C++. It is a type of <a href="https://en.wikipedia.org/wiki/Recursive_descent_parser">Recursive Descent parser</a>. |
| It implements slightly less of the Rust language than GDB. |
| LLDB has Rust-like value and type output.</p> |
| <h4 id="developer-notes"><a class="header" href="#developer-notes">Developer notes</a></h4> |
| <ul> |
| <li>LLDB has a plugin architecture but that does not work for language support.</li> |
| <li>GDB generally works better on Linux.</li> |
| </ul> |
| <h3 id="windbgcdb"><a class="header" href="#windbgcdb">WinDbg/CDB</a></h3> |
| <p>Microsoft provides <a href="https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/">Windows Debugging Tools</a> such as the Windows Debugger (WinDbg) and |
| the Console Debugger (CDB) which both support debugging programs written in Rust. These |
| debuggers parse the debug info for a binary from the <code>PDB</code>, if available, to construct a |
| visualization to serve up in the debugger.</p> |
| <h4 id="natvis"><a class="header" href="#natvis">Natvis</a></h4> |
| <p>Both WinDbg and CDB support defining and viewing custom visualizations for any given type |
| within the debugger using the Natvis framework. The Rust compiler defines a set of Natvis |
| files that define custom visualizations for a subset of types in the standard libraries such |
| as, <code>std</code>, <code>core</code>, and <code>alloc</code>. These Natvis files are embedded into <code>PDBs</code> generated by the |
| <code>*-pc-windows-msvc</code> target triples to automatically enable these custom visualizations when |
| debugging. This default can be overridden by setting the <code>strip</code> rustc flag to either <code>debuginfo</code> |
| or <code>symbols</code>.</p> |
| <p>Rust has support for embedding Natvis files for crates outside of the standard libraries by |
| using the <code>#[debugger_visualizer]</code> attribute. |
| For more details on how to embed debugger visualizers, |
| please refer to the section on the <a href="https://doc.rust-lang.org/nightly/reference/attributes/debugger.html#the-debugger_visualizer-attribute"><code>debugger_visualizer</code> attribute</a>.</p> |
| <h2 id="dwarf-and-rustc"><a class="header" href="#dwarf-and-rustc">DWARF and <code>rustc</code></a></h2> |
| <p><a href="http://dwarfstd.org">DWARF</a> is the standard way compilers generate debugging information that debuggers read. |
| It is <em>the</em> debugging format on macOS and Linux. |
| It is a multi-language and 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.</p> |
| <p>We have some DWARF extensions that the Rust compiler emits and the debuggers understand that |
| are <em>not</em> in the DWARF standard.</p> |
| <ul> |
| <li> |
| <p>Rust compiler will emit DWARF for a virtual table, and this <code>vtable</code> object will have a |
| <code>DW_AT_containing_type</code> 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:</p> |
| <pre><code class="language-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 |
| </code></pre> |
| </li> |
| <li> |
| <p>The other extension is that the Rust compiler can emit a tagless discriminated union. |
| See <a href="http://dwarfstd.org/ShowIssue.php?issue=180517.2">DWARF feature request</a> for this item.</p> |
| </li> |
| </ul> |
| <h3 id="current-limitations-of-dwarf"><a class="header" href="#current-limitations-of-dwarf">Current limitations of DWARF</a></h3> |
| <ul> |
| <li>Traits - require a bigger change than normal to DWARF, on how to represent Traits in DWARF.</li> |
| <li>DWARF provides no way to differentiate between Structs and Tuples. Rust compiler emits |
| fields with <code>__0</code> 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 <code>x.__0</code> instead of <code>x.0</code>. |
| This is resolved via the Rust parser in the debugger so now you can do <code>x.0</code>.</li> |
| </ul> |
| <p>DWARF relies on debuggers to know some information about platform ABI. |
| Rust does not do that all the time.</p> |
| <h2 id="developer-notes-1"><a class="header" href="#developer-notes-1">Developer notes</a></h2> |
| <p>This section is from the talk about certain aspects of development.</p> |
| <h2 id="what-is-missing"><a class="header" href="#what-is-missing">What is missing</a></h2> |
| <h3 id="code-signing-for-lldb-debug-server-on-macos"><a class="header" href="#code-signing-for-lldb-debug-server-on-macos">Code signing for LLDB debug server on macOS</a></h3> |
| <p>According to Wikipedia, <a href="https://en.wikipedia.org/wiki/System_Integrity_Protection">System Integrity Protection</a> is</p> |
| <blockquote> |
| <p>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).</p> |
| </blockquote> |
| <p>It prevents processes using <code>ptrace</code> syscall. If a process wants to use <code>ptrace</code> it has to be |
| code signed. The certificate that signs it has to be trusted on your machine.</p> |
| <p>See <a href="https://developer.apple.com/library/archive/releasenotes/MacOSX/WhatsNewInOSX/Articles/MacOSX10_11.html#//apple_ref/doc/uid/TP40016227-SW11">Apple developer documentation for System Integrity Protection</a>.</p> |
| <p>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.</p> |
| <p>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.</p> |
| <h3 id="dwarf-and-traits"><a class="header" href="#dwarf-and-traits">DWARF and Traits</a></h3> |
| <p>Rust traits are not emitted into DWARF at all. The impact of this is calling a method <code>x.method()</code> |
| 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.</p> |
| <p>DWARF has a notion of interface types (possibly added for Java). Tom's idea was to use this |
| interface type as traits.</p> |
| <p>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 (<code>DW_tag_interface</code> type). Also, the type for |
| which it is implemented would describe all the interfaces this type implements. This requires a |
| DWARF extension.</p> |
| <p>Issue on Github: <a href="https://github.com/rust-lang/rust/issues/33014">https://github.com/rust-lang/rust/issues/33014</a></p> |
| <h2 id="typical-process-for-a-debug-info-change-llvm"><a class="header" href="#typical-process-for-a-debug-info-change-llvm">Typical process for a Debug Info change (LLVM)</a></h2> |
| <p>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.</p> |
| <p>The steps of this process are as follows:</p> |
| <ol> |
| <li> |
| <p>LLVM needs changing.</p> |
| <p>LLVM does not emit Interface types at all, so this needs to be implemented in the LLVM first.</p> |
| <p>Get sign off on LLVM maintainers that this is a good idea.</p> |
| </li> |
| <li> |
| <p>Change the DWARF extension.</p> |
| </li> |
| <li> |
| <p>Update the debuggers.</p> |
| <p>Update DWARF readers, expression evaluators.</p> |
| </li> |
| <li> |
| <p>Update Rust compiler.</p> |
| <p>Change it to emit this new information.</p> |
| </li> |
| </ol> |
| <h3 id="procedural-macro-stepping"><a class="header" href="#procedural-macro-stepping">Procedural macro stepping</a></h3> |
| <p>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 -</p> |
| <ul> |
| <li>You can emit location of the invocation of the macro.</li> |
| <li>You can emit the location of the definition of the macro.</li> |
| <li>You can emit locations of the content of the macro.</li> |
| </ul> |
| <p>RFC: <a href="https://github.com/rust-lang/rfcs/pull/2117">https://github.com/rust-lang/rfcs/pull/2117</a></p> |
| <p>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.</p> |
| <h2 id="source-file-checksums-in-debug-info"><a class="header" href="#source-file-checksums-in-debug-info">Source file checksums in debug info</a></h2> |
| <p>Both DWARF and CodeView (PDB) support embedding a cryptographic hash of each source file that |
| contributed to the associated binary.</p> |
| <p>The cryptographic hash can be used by a debugger to verify that the source file matches the |
| executable. If the source file does not match, the debugger can provide a warning to the user.</p> |
| <p>The hash can also be used to prove that a given source file has not been modified since it was |
| used to compile an executable. Because MD5 and SHA1 both have demonstrated vulnerabilities, |
| using SHA256 is recommended for this application.</p> |
| <p>The Rust compiler stores the hash for each source file in the corresponding <code>SourceFile</code> in |
| the <code>SourceMap</code>. The hashes of input files to external crates are stored in <code>rlib</code> metadata.</p> |
| <p>A default hashing algorithm is set in the target specification. This allows the target to |
| specify the best hash available, since not all targets support all hash algorithms.</p> |
| <p>The hashing algorithm for a target can also be overridden with the <code>-Z source-file-checksum=</code> |
| command-line option.</p> |
| <h4 id="dwarf-5"><a class="header" href="#dwarf-5">DWARF 5</a></h4> |
| <p>DWARF version 5 supports embedding an MD5 hash to validate the source file version in use. |
| DWARF 5 - Section 6.2.4.1 opcode DW_LNCT_MD5</p> |
| <h4 id="llvm"><a class="header" href="#llvm">LLVM</a></h4> |
| <p>LLVM IR supports MD5 and SHA1 (and SHA256 in LLVM 11+) source file checksums in the DIFile node.</p> |
| <p><a href="https://llvm.org/docs/LangRef.html#difile">LLVM DIFile documentation</a></p> |
| <h4 id="microsoft-visual-c-compiler-zh-option"><a class="header" href="#microsoft-visual-c-compiler-zh-option">Microsoft Visual C++ Compiler /ZH option</a></h4> |
| <p>The MSVC compiler supports embedding MD5, SHA1, or SHA256 hashes in the PDB using the <code>/ZH</code> |
| compiler option.</p> |
| <p><a href="https://docs.microsoft.com/en-us/cpp/build/reference/zh">MSVC /ZH documentation</a></p> |
| <h4 id="clang"><a class="header" href="#clang">Clang</a></h4> |
| <p>Clang always embeds an MD5 checksum, though this does not appear in documentation.</p> |
| <h2 id="future-work"><a class="header" href="#future-work">Future work</a></h2> |
| <h4 id="name-mangling-changes"><a class="header" href="#name-mangling-changes">Name mangling changes</a></h4> |
| <ul> |
| <li>New demangler in <code>libiberty</code> (gcc source tree).</li> |
| <li>New demangler in LLVM or LLDB.</li> |
| </ul> |
| <p><strong>TODO</strong>: Check the location of the demangler source. <a href="https://github.com/rust-lang/rustc-dev-guide/issues/1157">#1157</a></p> |
| <h4 id="reuse-rust-compiler-for-expressions"><a class="header" href="#reuse-rust-compiler-for-expressions">Reuse Rust compiler for expressions</a></h4> |
| <p>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.</p> |
| <p>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.</p> |
| <p>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.</p> |
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