1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
//! Reading of the rustc metadata for rlibs and dylibs

use std::fs::File;
use std::io::Write;
use std::path::Path;

use object::write::{self, StandardSegment, Symbol, SymbolSection};
use object::{
    elf, pe, Architecture, BinaryFormat, Endianness, FileFlags, Object, ObjectSection,
    SectionFlags, SectionKind, SymbolFlags, SymbolKind, SymbolScope,
};

use snap::write::FrameEncoder;

use rustc_data_structures::memmap::Mmap;
use rustc_data_structures::owning_ref::OwningRef;
use rustc_data_structures::rustc_erase_owner;
use rustc_data_structures::sync::MetadataRef;
use rustc_metadata::fs::METADATA_FILENAME;
use rustc_metadata::EncodedMetadata;
use rustc_session::cstore::MetadataLoader;
use rustc_session::Session;
use rustc_target::abi::Endian;
use rustc_target::spec::{RelocModel, Target};

/// The default metadata loader. This is used by cg_llvm and cg_clif.
///
/// # Metadata location
///
/// <dl>
/// <dt>rlib</dt>
/// <dd>The metadata can be found in the `lib.rmeta` file inside of the ar archive.</dd>
/// <dt>dylib</dt>
/// <dd>The metadata can be found in the `.rustc` section of the shared library.</dd>
/// </dl>
pub struct DefaultMetadataLoader;

fn load_metadata_with(
    path: &Path,
    f: impl for<'a> FnOnce(&'a [u8]) -> Result<&'a [u8], String>,
) -> Result<MetadataRef, String> {
    let file =
        File::open(path).map_err(|e| format!("failed to open file '{}': {}", path.display(), e))?;
    let data = unsafe { Mmap::map(file) }
        .map_err(|e| format!("failed to mmap file '{}': {}", path.display(), e))?;
    let metadata = OwningRef::new(data).try_map(f)?;
    return Ok(rustc_erase_owner!(metadata.map_owner_box()));
}

impl MetadataLoader for DefaultMetadataLoader {
    fn get_rlib_metadata(&self, _target: &Target, path: &Path) -> Result<MetadataRef, String> {
        load_metadata_with(path, |data| {
            let archive = object::read::archive::ArchiveFile::parse(&*data)
                .map_err(|e| format!("failed to parse rlib '{}': {}", path.display(), e))?;

            for entry_result in archive.members() {
                let entry = entry_result
                    .map_err(|e| format!("failed to parse rlib '{}': {}", path.display(), e))?;
                if entry.name() == METADATA_FILENAME.as_bytes() {
                    let data = entry
                        .data(data)
                        .map_err(|e| format!("failed to parse rlib '{}': {}", path.display(), e))?;
                    return search_for_metadata(path, data, ".rmeta");
                }
            }

            Err(format!("metadata not found in rlib '{}'", path.display()))
        })
    }

    fn get_dylib_metadata(&self, _target: &Target, path: &Path) -> Result<MetadataRef, String> {
        load_metadata_with(path, |data| search_for_metadata(path, data, ".rustc"))
    }
}

fn search_for_metadata<'a>(
    path: &Path,
    bytes: &'a [u8],
    section: &str,
) -> Result<&'a [u8], String> {
    let Ok(file) = object::File::parse(bytes) else {
        // The parse above could fail for odd reasons like corruption, but for
        // now we just interpret it as this target doesn't support metadata
        // emission in object files so the entire byte slice itself is probably
        // a metadata file. Ideally though if necessary we could at least check
        // the prefix of bytes to see if it's an actual metadata object and if
        // not forward the error along here.
        return Ok(bytes);
    };
    file.section_by_name(section)
        .ok_or_else(|| format!("no `{}` section in '{}'", section, path.display()))?
        .data()
        .map_err(|e| format!("failed to read {} section in '{}': {}", section, path.display(), e))
}

pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static>> {
    let endianness = match sess.target.options.endian {
        Endian::Little => Endianness::Little,
        Endian::Big => Endianness::Big,
    };
    let architecture = match &sess.target.arch[..] {
        "arm" => Architecture::Arm,
        "aarch64" => Architecture::Aarch64,
        "x86" => Architecture::I386,
        "s390x" => Architecture::S390x,
        "mips" => Architecture::Mips,
        "mips64" => Architecture::Mips64,
        "x86_64" => {
            if sess.target.pointer_width == 32 {
                Architecture::X86_64_X32
            } else {
                Architecture::X86_64
            }
        }
        "powerpc" => Architecture::PowerPc,
        "powerpc64" => Architecture::PowerPc64,
        "riscv32" => Architecture::Riscv32,
        "riscv64" => Architecture::Riscv64,
        "sparc64" => Architecture::Sparc64,
        // Unsupported architecture.
        _ => return None,
    };
    let binary_format = if sess.target.is_like_osx {
        BinaryFormat::MachO
    } else if sess.target.is_like_windows {
        BinaryFormat::Coff
    } else {
        BinaryFormat::Elf
    };

    let mut file = write::Object::new(binary_format, architecture, endianness);
    let e_flags = match architecture {
        Architecture::Mips => {
            let arch = match sess.target.options.cpu.as_ref() {
                "mips1" => elf::EF_MIPS_ARCH_1,
                "mips2" => elf::EF_MIPS_ARCH_2,
                "mips3" => elf::EF_MIPS_ARCH_3,
                "mips4" => elf::EF_MIPS_ARCH_4,
                "mips5" => elf::EF_MIPS_ARCH_5,
                s if s.contains("r6") => elf::EF_MIPS_ARCH_32R6,
                _ => elf::EF_MIPS_ARCH_32R2,
            };
            // The only ABI LLVM supports for 32-bit MIPS CPUs is o32.
            let mut e_flags = elf::EF_MIPS_CPIC | elf::EF_MIPS_ABI_O32 | arch;
            if sess.target.options.relocation_model != RelocModel::Static {
                e_flags |= elf::EF_MIPS_PIC;
            }
            if sess.target.options.cpu.contains("r6") {
                e_flags |= elf::EF_MIPS_NAN2008;
            }
            e_flags
        }
        Architecture::Mips64 => {
            // copied from `mips64el-linux-gnuabi64-gcc foo.c -c`
            let e_flags = elf::EF_MIPS_CPIC
                | elf::EF_MIPS_PIC
                | if sess.target.options.cpu.contains("r6") {
                    elf::EF_MIPS_ARCH_64R6 | elf::EF_MIPS_NAN2008
                } else {
                    elf::EF_MIPS_ARCH_64R2
                };
            e_flags
        }
        Architecture::Riscv64 if sess.target.options.features.contains("+d") => {
            // copied from `riscv64-linux-gnu-gcc foo.c -c`, note though
            // that the `+d` target feature represents whether the double
            // float abi is enabled.
            let e_flags = elf::EF_RISCV_RVC | elf::EF_RISCV_FLOAT_ABI_DOUBLE;
            e_flags
        }
        _ => 0,
    };
    // adapted from LLVM's `MCELFObjectTargetWriter::getOSABI`
    let os_abi = match sess.target.options.os.as_ref() {
        "hermit" => elf::ELFOSABI_STANDALONE,
        "freebsd" => elf::ELFOSABI_FREEBSD,
        "solaris" => elf::ELFOSABI_SOLARIS,
        _ => elf::ELFOSABI_NONE,
    };
    let abi_version = 0;
    file.flags = FileFlags::Elf { os_abi, abi_version, e_flags };
    Some(file)
}

pub enum MetadataPosition {
    First,
    Last,
}

// For rlibs we "pack" rustc metadata into a dummy object file.
//
// Historically it was needed because rustc linked rlibs as whole-archive in some cases.
// In that case linkers try to include all files located in an archive, so if metadata is stored
// in an archive then it needs to be of a form that the linker is able to process.
// Now it's not clear whether metadata still needs to be wrapped into an object file or not.
//
// Note, though, that we don't actually want this metadata to show up in any
// final output of the compiler. Instead this is purely for rustc's own
// metadata tracking purposes.
//
// With the above in mind, each "flavor" of object format gets special
// handling here depending on the target:
//
// * MachO - macos-like targets will insert the metadata into a section that
//   is sort of fake dwarf debug info. Inspecting the source of the macos
//   linker this causes these sections to be skipped automatically because
//   it's not in an allowlist of otherwise well known dwarf section names to
//   go into the final artifact.
//
// * WebAssembly - we actually don't have any container format for this
//   target. WebAssembly doesn't support the `dylib` crate type anyway so
//   there's no need for us to support this at this time. Consequently the
//   metadata bytes are simply stored as-is into an rlib.
//
// * COFF - Windows-like targets create an object with a section that has
//   the `IMAGE_SCN_LNK_REMOVE` flag set which ensures that if the linker
//   ever sees the section it doesn't process it and it's removed.
//
// * ELF - All other targets are similar to Windows in that there's a
//   `SHF_EXCLUDE` flag we can set on sections in an object file to get
//   automatically removed from the final output.
pub fn create_rmeta_file(sess: &Session, metadata: &[u8]) -> (Vec<u8>, MetadataPosition) {
    let Some(mut file) = create_object_file(sess) else {
        // This is used to handle all "other" targets. This includes targets
        // in two categories:
        //
        // * Some targets don't have support in the `object` crate just yet
        //   to write an object file. These targets are likely to get filled
        //   out over time.
        //
        // * Targets like WebAssembly don't support dylibs, so the purpose
        //   of putting metadata in object files, to support linking rlibs
        //   into dylibs, is moot.
        //
        // In both of these cases it means that linking into dylibs will
        // not be supported by rustc. This doesn't matter for targets like
        // WebAssembly and for targets not supported by the `object` crate
        // yet it means that work will need to be done in the `object` crate
        // to add a case above.
        return (metadata.to_vec(), MetadataPosition::Last);
    };
    let section = file.add_section(
        file.segment_name(StandardSegment::Debug).to_vec(),
        b".rmeta".to_vec(),
        SectionKind::Debug,
    );
    match file.format() {
        BinaryFormat::Coff => {
            file.section_mut(section).flags =
                SectionFlags::Coff { characteristics: pe::IMAGE_SCN_LNK_REMOVE };
        }
        BinaryFormat::Elf => {
            file.section_mut(section).flags =
                SectionFlags::Elf { sh_flags: elf::SHF_EXCLUDE as u64 };
        }
        _ => {}
    };
    file.append_section_data(section, metadata, 1);
    (file.write().unwrap(), MetadataPosition::First)
}

// Historical note:
//
// When using link.exe it was seen that the section name `.note.rustc`
// was getting shortened to `.note.ru`, and according to the PE and COFF
// specification:
//
// > Executable images do not use a string table and do not support
// > section names longer than 8 characters
//
// https://docs.microsoft.com/en-us/windows/win32/debug/pe-format
//
// As a result, we choose a slightly shorter name! As to why
// `.note.rustc` works on MinGW, see
// https://github.com/llvm/llvm-project/blob/llvmorg-12.0.0/lld/COFF/Writer.cpp#L1190-L1197
pub fn create_compressed_metadata_file(
    sess: &Session,
    metadata: &EncodedMetadata,
    symbol_name: &str,
) -> Vec<u8> {
    let mut compressed = rustc_metadata::METADATA_HEADER.to_vec();
    FrameEncoder::new(&mut compressed).write_all(metadata.raw_data()).unwrap();
    let Some(mut file) = create_object_file(sess) else {
        return compressed.to_vec();
    };
    let section = file.add_section(
        file.segment_name(StandardSegment::Data).to_vec(),
        b".rustc".to_vec(),
        SectionKind::ReadOnlyData,
    );
    match file.format() {
        BinaryFormat::Elf => {
            // Explicitly set no flags to avoid SHF_ALLOC default for data section.
            file.section_mut(section).flags = SectionFlags::Elf { sh_flags: 0 };
        }
        _ => {}
    };
    let offset = file.append_section_data(section, &compressed, 1);

    // For MachO and probably PE this is necessary to prevent the linker from throwing away the
    // .rustc section. For ELF this isn't necessary, but it also doesn't harm.
    file.add_symbol(Symbol {
        name: symbol_name.as_bytes().to_vec(),
        value: offset,
        size: compressed.len() as u64,
        kind: SymbolKind::Data,
        scope: SymbolScope::Dynamic,
        weak: false,
        section: SymbolSection::Section(section),
        flags: SymbolFlags::None,
    });

    file.write().unwrap()
}