547 lines
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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>
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<ul>
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<li><a href="#preliminaries">Preliminaries</a>
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<ul>
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<li><a href="#debuggers">Debuggers</a></li>
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<li><a href="#dwarf">DWARF</a></li>
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<li><a href="#codeviewpdb">CodeView/PDB</a></li>
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</ul>
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</li>
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<li><a href="#supported-debuggers">Supported debuggers</a>
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<ul>
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<li><a href="#gdb">GDB</a>
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<ul>
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<li><a href="#rust-expression-parser">Rust expression parser</a></li>
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<li><a href="#parser-extensions">Parser extensions</a></li>
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</ul>
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</li>
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<li><a href="#lldb">LLDB</a>
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<ul>
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<li><a href="#rust-expression-parser-1">Rust expression parser</a></li>
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<li><a href="#developer-notes">Developer notes</a></li>
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</ul>
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</li>
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<li><a href="#windbgcdb">WinDbg/CDB</a>
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<ul>
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<li><a href="#natvis">Natvis</a></li>
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</ul>
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</li>
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</ul>
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</li>
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<li><a href="#dwarf-and-rustc">DWARF and <code>rustc</code></a>
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<ul>
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<li><a href="#current-limitations-of-dwarf">Current limitations of DWARF</a></li>
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</ul>
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</li>
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<li><a href="#developer-notes-1">Developer notes</a></li>
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<li><a href="#what-is-missing">What is missing</a>
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<ul>
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<li><a href="#code-signing-for-lldb-debug-server-on-macos">Code signing for LLDB debug server on macOS</a></li>
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<li><a href="#dwarf-and-traits">DWARF and Traits</a></li>
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</ul>
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</li>
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<li><a href="#typical-process-for-a-debug-info-change-llvm">Typical process for a Debug Info change (LLVM)</a>
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<ul>
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<li><a href="#procedural-macro-stepping">Procedural macro stepping</a></li>
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</ul>
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</li>
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<li><a href="#source-file-checksums-in-debug-info">Source file checksums in debug info</a>
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<ul>
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<li><a href="#dwarf-5">DWARF 5</a></li>
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<li><a href="#llvm">LLVM</a></li>
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<li><a href="#microsoft-visual-c-compiler-zh-option">Microsoft Visual C++ Compiler /ZH option</a></li>
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<li><a href="#clang">Clang</a></li>
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</ul>
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</li>
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<li><a href="#future-work">Future work</a>
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<ul>
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<li><a href="#name-mangling-changes">Name mangling changes</a></li>
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<li><a href="#reuse-rust-compiler-for-expressions">Reuse Rust compiler for expressions</a></li>
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</ul>
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</li>
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</ul>
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<p>This document explains the state of debugging tools support in the Rust compiler (rustc).
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It gives an overview of GDB, LLDB, WinDbg/CDB,
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as well as infrastructure around Rust compiler to debug Rust code.
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If you want to learn how to debug the Rust compiler itself,
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see <a href="compiler-debugging.html">Debugging the Compiler</a>.</p>
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<p>The material is gathered from the video,
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<a href="https://www.youtube.com/watch?v=elBxMRSNYr4">Tom Tromey discusses debugging support in rustc</a>.</p>
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<h2 id="preliminaries"><a class="header" href="#preliminaries">Preliminaries</a></h2>
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<h3 id="debuggers"><a class="header" href="#debuggers">Debuggers</a></h3>
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<p>According to Wikipedia</p>
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<blockquote>
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<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
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other programs (the "target" program).</p>
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</blockquote>
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<p>Writing a debugger from scratch for a language requires a lot of work, especially if
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debuggers have to be supported on various platforms. GDB and LLDB, however, can be
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extended to support debugging a language. This is the path that Rust has chosen.
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This document's main goal is to document the said debuggers support in Rust compiler.</p>
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<h3 id="dwarf"><a class="header" href="#dwarf">DWARF</a></h3>
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<p>According to the <a href="http://dwarfstd.org">DWARF</a> standard website</p>
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<blockquote>
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<p>DWARF is a debugging file format used by many compilers and debuggers to support source level
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debugging. It addresses the requirements of a number of procedural languages,
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such as C, C++, and Fortran, and is designed to be extensible to other languages.
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DWARF is architecture independent and applicable to any processor or operating system.
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It is widely used on Unix, Linux and other operating systems,
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as well as in stand-alone environments.</p>
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</blockquote>
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<p>DWARF reader is a program that consumes the DWARF format and creates debugger compatible output.
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This program may live in the compiler itself. DWARF uses a data structure called
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Debugging Information Entry (DIE) which stores the information as "tags" to denote functions,
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variables etc., e.g., <code>DW_TAG_variable</code>, <code>DW_TAG_pointer_type</code>, <code>DW_TAG_subprogram</code> etc.
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You can also invent your own tags and attributes.</p>
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<h3 id="codeviewpdb"><a class="header" href="#codeviewpdb">CodeView/PDB</a></h3>
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<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.
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PDBs can be consumed by debuggers such as WinDbg/CDB and other tools to display debug information.
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A PDB contains multiple streams that describe debug information about a specific binary such
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as types, symbols, and source files used to compile the given binary. CodeView is another
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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
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PDB streams.</p>
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<h2 id="supported-debuggers"><a class="header" href="#supported-debuggers">Supported debuggers</a></h2>
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<h3 id="gdb"><a class="header" href="#gdb">GDB</a></h3>
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<h4 id="rust-expression-parser"><a class="header" href="#rust-expression-parser">Rust expression parser</a></h4>
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<p>To be able to show debug output, we need an expression parser.
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This (GDB) expression parser is written in <a href="https://www.gnu.org/software/bison/">Bison</a>,
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and can parse only a subset of Rust expressions.
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GDB parser was written from scratch and has no relation to any other parser,
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including that of rustc.</p>
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<p>GDB has Rust-like value and type output. It can print values and types in a way
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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,
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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>
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<h4 id="parser-extensions"><a class="header" href="#parser-extensions">Parser extensions</a></h4>
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<p>Expression parser has a couple of extensions in it to facilitate features that you cannot do
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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
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code in the DWARF reader in GDB to support the extensions.</p>
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<p>A couple of examples of DWARF reader support needed are as follows:</p>
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<ol>
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<li>
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<p>Enum: Needed for support for enum types.
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The Rust compiler writes the information about enum into DWARF,
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and GDB reads the DWARF to understand where is the tag field,
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or if there is a tag field,
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or if the tag slot is shared with non-zero optimization etc.</p>
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</li>
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<li>
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<p>Dissect trait objects: DWARF extension where the trait object's description in the DWARF
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also points to a stub description of the corresponding vtable which in turn points to the
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concrete type for which this trait object exists. This means that you can do a <code>print *object</code>
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for that trait object, and GDB will understand how to find the correct type of the payload in
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the trait object.</p>
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</li>
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</ol>
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<p><strong>TODO</strong>: Figure out if the following should be mentioned in the GDB-Rust document rather than
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this guide page so there is no duplication. This is regarding the following comments:</p>
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<p><a href="https://github.com/rust-lang/rustc-dev-guide/pull/316#discussion_r284027340">This comment by Tom</a></p>
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<blockquote>
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<p>gdb's Rust extensions and limitations are documented in the gdb manual:
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https://sourceware.org/gdb/onlinedocs/gdb/Rust.html -- however, this neglects to mention that
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gdb convenience variables and registers follow the gdb $ convention, and that the Rust parser
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implements the gdb @ extension.</p>
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</blockquote>
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<p><a href="https://github.com/rust-lang/rustc-dev-guide/pull/316#discussion_r285401353">This question by Aman</a></p>
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<blockquote>
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<p>@tromey do you think we should mention this part in the GDB-Rust document rather than this
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document so there is no duplication etc.?</p>
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</blockquote>
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<h3 id="lldb"><a class="header" href="#lldb">LLDB</a></h3>
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<h4 id="rust-expression-parser-1"><a class="header" href="#rust-expression-parser-1">Rust expression parser</a></h4>
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<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>.
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It implements slightly less of the Rust language than GDB.
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LLDB has Rust-like value and type output.</p>
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<h4 id="developer-notes"><a class="header" href="#developer-notes">Developer notes</a></h4>
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<ul>
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<li>LLDB has a plugin architecture but that does not work for language support.</li>
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<li>GDB generally works better on Linux.</li>
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</ul>
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<h3 id="windbgcdb"><a class="header" href="#windbgcdb">WinDbg/CDB</a></h3>
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<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
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the Console Debugger (CDB) which both support debugging programs written in Rust. These
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debuggers parse the debug info for a binary from the <code>PDB</code>, if available, to construct a
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visualization to serve up in the debugger.</p>
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<h4 id="natvis"><a class="header" href="#natvis">Natvis</a></h4>
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<p>Both WinDbg and CDB support defining and viewing custom visualizations for any given type
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within the debugger using the Natvis framework. The Rust compiler defines a set of Natvis
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files that define custom visualizations for a subset of types in the standard libraries such
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as, <code>std</code>, <code>core</code>, and <code>alloc</code>. These Natvis files are embedded into <code>PDBs</code> generated by the
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<code>*-pc-windows-msvc</code> target triples to automatically enable these custom visualizations when
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debugging. This default can be overridden by setting the <code>strip</code> rustc flag to either <code>debuginfo</code>
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or <code>symbols</code>.</p>
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<p>Rust has support for embedding Natvis files for crates outside of the standard libraries by
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using the <code>#[debugger_visualizer]</code> attribute.
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For more details on how to embed debugger visualizers,
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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>
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<h2 id="dwarf-and-rustc"><a class="header" href="#dwarf-and-rustc">DWARF and <code>rustc</code></a></h2>
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<p><a href="http://dwarfstd.org">DWARF</a> is the standard way compilers generate debugging information that debuggers read.
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It is <em>the</em> debugging format on macOS and Linux.
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It is a multi-language and extensible format,
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and is mostly good enough for Rust's purposes.
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Hence, the current implementation reuses DWARF's concepts.
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This is true even if some of the concepts in DWARF do not align with Rust semantically because,
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generally, there can be some kind of mapping between the two.</p>
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<p>We have some DWARF extensions that the Rust compiler emits and the debuggers understand that
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are <em>not</em> in the DWARF standard.</p>
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<ul>
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<li>
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<p>Rust compiler will emit DWARF for a virtual table, and this <code>vtable</code> object will have a
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<code>DW_AT_containing_type</code> that points to the real type. This lets debuggers dissect a trait object
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pointer to correctly find the payload. E.g., here's such a DIE, from a test case in the gdb
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repository:</p>
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<pre><code class="language-asm"><1><1a9>: Abbrev Number: 3 (DW_TAG_structure_type)
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<1aa> DW_AT_containing_type: <0x1b4>
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<1ae> DW_AT_name : (indirect string, offset: 0x23d): vtable
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<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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