1 //! Reading of the rustc metadata for rlibs and dylibs
7 use object::write::{self, StandardSegment, Symbol, SymbolSection};
9 elf, pe, Architecture, BinaryFormat, Endianness, FileFlags, Object, ObjectSection,
10 SectionFlags, SectionKind, SymbolFlags, SymbolKind, SymbolScope,
13 use snap::write::FrameEncoder;
15 use rustc_data_structures::memmap::Mmap;
16 use rustc_data_structures::owning_ref::OwningRef;
17 use rustc_data_structures::rustc_erase_owner;
18 use rustc_data_structures::sync::MetadataRef;
19 use rustc_metadata::fs::METADATA_FILENAME;
20 use rustc_metadata::EncodedMetadata;
21 use rustc_session::cstore::MetadataLoader;
22 use rustc_session::Session;
23 use rustc_target::abi::Endian;
24 use rustc_target::spec::{RelocModel, Target};
26 /// The default metadata loader. This is used by cg_llvm and cg_clif.
28 /// # Metadata location
32 /// <dd>The metadata can be found in the `lib.rmeta` file inside of the ar archive.</dd>
34 /// <dd>The metadata can be found in the `.rustc` section of the shared library.</dd>
36 pub struct DefaultMetadataLoader;
38 fn load_metadata_with(
40 f: impl for<'a> FnOnce(&'a [u8]) -> Result<&'a [u8], String>,
41 ) -> Result<MetadataRef, String> {
43 File::open(path).map_err(|e| format!("failed to open file '{}': {}", path.display(), e))?;
44 let data = unsafe { Mmap::map(file) }
45 .map_err(|e| format!("failed to mmap file '{}': {}", path.display(), e))?;
46 let metadata = OwningRef::new(data).try_map(f)?;
47 return Ok(rustc_erase_owner!(metadata.map_owner_box()));
50 impl MetadataLoader for DefaultMetadataLoader {
51 fn get_rlib_metadata(&self, _target: &Target, path: &Path) -> Result<MetadataRef, String> {
52 load_metadata_with(path, |data| {
53 let archive = object::read::archive::ArchiveFile::parse(&*data)
54 .map_err(|e| format!("failed to parse rlib '{}': {}", path.display(), e))?;
56 for entry_result in archive.members() {
57 let entry = entry_result
58 .map_err(|e| format!("failed to parse rlib '{}': {}", path.display(), e))?;
59 if entry.name() == METADATA_FILENAME.as_bytes() {
62 .map_err(|e| format!("failed to parse rlib '{}': {}", path.display(), e))?;
63 return search_for_metadata(path, data, ".rmeta");
67 Err(format!("metadata not found in rlib '{}'", path.display()))
71 fn get_dylib_metadata(&self, _target: &Target, path: &Path) -> Result<MetadataRef, String> {
72 load_metadata_with(path, |data| search_for_metadata(path, data, ".rustc"))
76 fn search_for_metadata<'a>(
80 ) -> Result<&'a [u8], String> {
81 let Ok(file) = object::File::parse(bytes) else {
82 // The parse above could fail for odd reasons like corruption, but for
83 // now we just interpret it as this target doesn't support metadata
84 // emission in object files so the entire byte slice itself is probably
85 // a metadata file. Ideally though if necessary we could at least check
86 // the prefix of bytes to see if it's an actual metadata object and if
87 // not forward the error along here.
90 file.section_by_name(section)
91 .ok_or_else(|| format!("no `{}` section in '{}'", section, path.display()))?
93 .map_err(|e| format!("failed to read {} section in '{}': {}", section, path.display(), e))
96 pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static>> {
97 let endianness = match sess.target.options.endian {
98 Endian::Little => Endianness::Little,
99 Endian::Big => Endianness::Big,
101 let architecture = match &sess.target.arch[..] {
102 "arm" => Architecture::Arm,
103 "aarch64" => Architecture::Aarch64,
104 "x86" => Architecture::I386,
105 "s390x" => Architecture::S390x,
106 "mips" => Architecture::Mips,
107 "mips64" => Architecture::Mips64,
109 if sess.target.pointer_width == 32 {
110 Architecture::X86_64_X32
115 "powerpc" => Architecture::PowerPc,
116 "powerpc64" => Architecture::PowerPc64,
117 "riscv32" => Architecture::Riscv32,
118 "riscv64" => Architecture::Riscv64,
119 "sparc64" => Architecture::Sparc64,
120 // Unsupported architecture.
123 let binary_format = if sess.target.is_like_osx {
125 } else if sess.target.is_like_windows {
131 let mut file = write::Object::new(binary_format, architecture, endianness);
132 let e_flags = match architecture {
133 Architecture::Mips => {
134 let arch = match sess.target.options.cpu.as_ref() {
135 "mips1" => elf::EF_MIPS_ARCH_1,
136 "mips2" => elf::EF_MIPS_ARCH_2,
137 "mips3" => elf::EF_MIPS_ARCH_3,
138 "mips4" => elf::EF_MIPS_ARCH_4,
139 "mips5" => elf::EF_MIPS_ARCH_5,
140 s if s.contains("r6") => elf::EF_MIPS_ARCH_32R6,
141 _ => elf::EF_MIPS_ARCH_32R2,
143 // The only ABI LLVM supports for 32-bit MIPS CPUs is o32.
144 let mut e_flags = elf::EF_MIPS_CPIC | elf::EF_MIPS_ABI_O32 | arch;
145 if sess.target.options.relocation_model != RelocModel::Static {
146 e_flags |= elf::EF_MIPS_PIC;
148 if sess.target.options.cpu.contains("r6") {
149 e_flags |= elf::EF_MIPS_NAN2008;
153 Architecture::Mips64 => {
154 // copied from `mips64el-linux-gnuabi64-gcc foo.c -c`
155 let e_flags = elf::EF_MIPS_CPIC
157 | if sess.target.options.cpu.contains("r6") {
158 elf::EF_MIPS_ARCH_64R6 | elf::EF_MIPS_NAN2008
160 elf::EF_MIPS_ARCH_64R2
164 Architecture::Riscv64 if sess.target.options.features.contains("+d") => {
165 // copied from `riscv64-linux-gnu-gcc foo.c -c`, note though
166 // that the `+d` target feature represents whether the double
167 // float abi is enabled.
168 let e_flags = elf::EF_RISCV_RVC | elf::EF_RISCV_FLOAT_ABI_DOUBLE;
173 // adapted from LLVM's `MCELFObjectTargetWriter::getOSABI`
174 let os_abi = match sess.target.options.os.as_ref() {
175 "hermit" => elf::ELFOSABI_STANDALONE,
176 "freebsd" => elf::ELFOSABI_FREEBSD,
177 "solaris" => elf::ELFOSABI_SOLARIS,
178 _ => elf::ELFOSABI_NONE,
181 file.flags = FileFlags::Elf { os_abi, abi_version, e_flags };
185 pub enum MetadataPosition {
190 // For rlibs we "pack" rustc metadata into a dummy object file.
192 // Historically it was needed because rustc linked rlibs as whole-archive in some cases.
193 // In that case linkers try to include all files located in an archive, so if metadata is stored
194 // in an archive then it needs to be of a form that the linker is able to process.
195 // Now it's not clear whether metadata still needs to be wrapped into an object file or not.
197 // Note, though, that we don't actually want this metadata to show up in any
198 // final output of the compiler. Instead this is purely for rustc's own
199 // metadata tracking purposes.
201 // With the above in mind, each "flavor" of object format gets special
202 // handling here depending on the target:
204 // * MachO - macos-like targets will insert the metadata into a section that
205 // is sort of fake dwarf debug info. Inspecting the source of the macos
206 // linker this causes these sections to be skipped automatically because
207 // it's not in an allowlist of otherwise well known dwarf section names to
208 // go into the final artifact.
210 // * WebAssembly - we actually don't have any container format for this
211 // target. WebAssembly doesn't support the `dylib` crate type anyway so
212 // there's no need for us to support this at this time. Consequently the
213 // metadata bytes are simply stored as-is into an rlib.
215 // * COFF - Windows-like targets create an object with a section that has
216 // the `IMAGE_SCN_LNK_REMOVE` flag set which ensures that if the linker
217 // ever sees the section it doesn't process it and it's removed.
219 // * ELF - All other targets are similar to Windows in that there's a
220 // `SHF_EXCLUDE` flag we can set on sections in an object file to get
221 // automatically removed from the final output.
222 pub fn create_rmeta_file(sess: &Session, metadata: &[u8]) -> (Vec<u8>, MetadataPosition) {
223 let Some(mut file) = create_object_file(sess) else {
224 // This is used to handle all "other" targets. This includes targets
225 // in two categories:
227 // * Some targets don't have support in the `object` crate just yet
228 // to write an object file. These targets are likely to get filled
231 // * Targets like WebAssembly don't support dylibs, so the purpose
232 // of putting metadata in object files, to support linking rlibs
233 // into dylibs, is moot.
235 // In both of these cases it means that linking into dylibs will
236 // not be supported by rustc. This doesn't matter for targets like
237 // WebAssembly and for targets not supported by the `object` crate
238 // yet it means that work will need to be done in the `object` crate
239 // to add a case above.
240 return (metadata.to_vec(), MetadataPosition::Last);
242 let section = file.add_section(
243 file.segment_name(StandardSegment::Debug).to_vec(),
247 match file.format() {
248 BinaryFormat::Coff => {
249 file.section_mut(section).flags =
250 SectionFlags::Coff { characteristics: pe::IMAGE_SCN_LNK_REMOVE };
252 BinaryFormat::Elf => {
253 file.section_mut(section).flags =
254 SectionFlags::Elf { sh_flags: elf::SHF_EXCLUDE as u64 };
258 file.append_section_data(section, metadata, 1);
259 (file.write().unwrap(), MetadataPosition::First)
264 // When using link.exe it was seen that the section name `.note.rustc`
265 // was getting shortened to `.note.ru`, and according to the PE and COFF
268 // > Executable images do not use a string table and do not support
269 // > section names longer than 8 characters
271 // https://docs.microsoft.com/en-us/windows/win32/debug/pe-format
273 // As a result, we choose a slightly shorter name! As to why
274 // `.note.rustc` works on MinGW, see
275 // https://github.com/llvm/llvm-project/blob/llvmorg-12.0.0/lld/COFF/Writer.cpp#L1190-L1197
276 pub fn create_compressed_metadata_file(
278 metadata: &EncodedMetadata,
281 let mut compressed = rustc_metadata::METADATA_HEADER.to_vec();
282 FrameEncoder::new(&mut compressed).write_all(metadata.raw_data()).unwrap();
283 let Some(mut file) = create_object_file(sess) else {
284 return compressed.to_vec();
286 let section = file.add_section(
287 file.segment_name(StandardSegment::Data).to_vec(),
289 SectionKind::ReadOnlyData,
291 match file.format() {
292 BinaryFormat::Elf => {
293 // Explicitly set no flags to avoid SHF_ALLOC default for data section.
294 file.section_mut(section).flags = SectionFlags::Elf { sh_flags: 0 };
298 let offset = file.append_section_data(section, &compressed, 1);
300 // For MachO and probably PE this is necessary to prevent the linker from throwing away the
301 // .rustc section. For ELF this isn't necessary, but it also doesn't harm.
302 file.add_symbol(Symbol {
303 name: symbol_name.as_bytes().to_vec(),
305 size: compressed.len() as u64,
306 kind: SymbolKind::Data,
307 scope: SymbolScope::Dynamic,
309 section: SymbolSection::Section(section),
310 flags: SymbolFlags::None,
313 file.write().unwrap()