Skip to content

Use the .drectve section for exporting symbols from dlls on Windows #142568

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
wants to merge 3 commits into
base: master
Choose a base branch
from
Open

Use the .drectve section for exporting symbols from dlls on Windows #142568

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
94ec88c
Use the .drectve section for exporting symbols from dlls on Windows
bjorn3 Jun 16, 2025
5e2efb6
Revert some changes
bjorn3 Jun 16, 2025
c353c7d
Don't mark exported symbols as data
bjorn3 Jun 16, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
31 changes: 28 additions & 3 deletions compiler/rustc_codegen_ssa/src/back/link.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1976,9 +1976,10 @@ fn add_linked_symbol_object(
cmd: &mut dyn Linker,
sess: &Session,
tmpdir: &Path,
symbols: &[(String, SymbolExportKind)],
linked_symbols: &[(String, SymbolExportKind)],
exported_symbols: &[(String, SymbolExportKind)],
) {
if symbols.is_empty() {
if linked_symbols.is_empty() {
return;
}

Expand Down Expand Up @@ -2015,7 +2016,7 @@ fn add_linked_symbol_object(
None
};

for (sym, kind) in symbols.iter() {
for (sym, kind) in linked_symbols.iter() {
let symbol = file.add_symbol(object::write::Symbol {
name: sym.clone().into(),
value: 0,
Expand Down Expand Up @@ -2073,6 +2074,29 @@ fn add_linked_symbol_object(
}
}

if sess.target.is_like_msvc {
// Currently the compiler doesn't use `dllexport` (an LLVM attribute) to
// export symbols from a dynamic library. When building a dynamic library,
// however, we're going to want some symbols exported, so this adds a
// `.drectve` section which lists all the symbols using /EXPORT arguments.
//
// The linker will read these arguments from the `.drectve` section and
// export all the symbols from the dynamic library. Note that this is not
// as simple as just exporting all the symbols in the current crate (as
// specified by `codegen.reachable`) but rather we also need to possibly
// export the symbols of upstream crates. Upstream rlibs may be linked
// statically to this dynamic library, in which case they may continue to
// transitively be used and hence need their symbols exported.
let drectve = exported_symbols
.into_iter()
.map(|(sym, _kind)| format!(" /EXPORT:\"{sym}\""))
.collect::<Vec<_>>()
.join("");

let section = file.add_section(vec![], b".drectve".to_vec(), object::SectionKind::Linker);
file.append_section_data(section, drectve.as_bytes(), 1);
}

let path = tmpdir.join("symbols.o");
let result = std::fs::write(&path, file.write().unwrap());
if let Err(error) = result {
Expand Down Expand Up @@ -2248,6 +2272,7 @@ fn linker_with_args(
sess,
tmpdir,
&codegen_results.crate_info.linked_symbols[&crate_type],
&codegen_results.crate_info.exported_symbols[&crate_type],
);

// Sanitizer libraries.
Expand Down
128 changes: 87 additions & 41 deletions compiler/rustc_codegen_ssa/src/back/linker.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -337,7 +337,12 @@ pub(crate) trait Linker {
fn debuginfo(&mut self, strip: Strip, natvis_debugger_visualizers: &[PathBuf]);
fn no_crt_objects(&mut self);
fn no_default_libraries(&mut self);
fn export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType, symbols: &[String]);
fn export_symbols(
&mut self,
tmpdir: &Path,
crate_type: CrateType,
symbols: &[(String, SymbolExportKind)],
);
fn subsystem(&mut self, subsystem: &str);
fn linker_plugin_lto(&mut self);
fn add_eh_frame_header(&mut self) {}
Expand Down Expand Up @@ -770,7 +775,12 @@ impl<'a> Linker for GccLinker<'a> {
}
}

fn export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType, symbols: &[String]) {
fn export_symbols(
&mut self,
tmpdir: &Path,
crate_type: CrateType,
symbols: &[(String, SymbolExportKind)],
) {
// Symbol visibility in object files typically takes care of this.
if crate_type == CrateType::Executable {
let should_export_executable_symbols =
Expand Down Expand Up @@ -799,7 +809,7 @@ impl<'a> Linker for GccLinker<'a> {
// Write a plain, newline-separated list of symbols
let res: io::Result<()> = try {
let mut f = File::create_buffered(&path)?;
for sym in symbols {
for (sym, _) in symbols {
debug!(" _{sym}");
writeln!(f, "_{sym}")?;
}
Expand All @@ -814,7 +824,7 @@ impl<'a> Linker for GccLinker<'a> {
// .def file similar to MSVC one but without LIBRARY section
// because LD doesn't like when it's empty
writeln!(f, "EXPORTS")?;
for symbol in symbols {
for (symbol, _) in symbols {
debug!(" _{symbol}");
// Quote the name in case it's reserved by linker in some way
// (this accounts for names with dots in particular).
Expand All @@ -831,7 +841,7 @@ impl<'a> Linker for GccLinker<'a> {
writeln!(f, "{{")?;
if !symbols.is_empty() {
writeln!(f, " global:")?;
for sym in symbols {
for (sym, _) in symbols {
debug!(" {sym};");
writeln!(f, " {sym};")?;
}
Expand Down Expand Up @@ -1086,19 +1096,15 @@ impl<'a> Linker for MsvcLinker<'a> {
}
}

// Currently the compiler doesn't use `dllexport` (an LLVM attribute) to
// export symbols from a dynamic library. When building a dynamic library,
// however, we're going to want some symbols exported, so this function
// generates a DEF file which lists all the symbols.
//
// The linker will read this `*.def` file and export all the symbols from
// the dynamic library. Note that this is not as simple as just exporting
// all the symbols in the current crate (as specified by `codegen.reachable`)
// but rather we also need to possibly export the symbols of upstream
// crates. Upstream rlibs may be linked statically to this dynamic library,
// in which case they may continue to transitively be used and hence need
// their symbols exported.
fn export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType, symbols: &[String]) {
fn export_symbols(
&mut self,
tmpdir: &Path,
crate_type: CrateType,
_symbols: &[(String, SymbolExportKind)],
) {
// We already add /EXPORT arguments to the .drectve section of symbols.o. We generate
// a .DEF file here anyway as it might prevent auto-export of some symbols.

// Symbol visibility takes care of this typically
if crate_type == CrateType::Executable {
let should_export_executable_symbols =
Expand All @@ -1116,10 +1122,6 @@ impl<'a> Linker for MsvcLinker<'a> {
// straight to exports.
writeln!(f, "LIBRARY")?;
writeln!(f, "EXPORTS")?;
for symbol in symbols {
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I don't believe that we need the def file any more, we should be able to get away with passing /DLL to the linker.

Copy link
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

In the mingw docs I read something about the presence of a .DEF file disabling auto-export of certain symbols. I don't know if the same applies to MSVC, so I added it here just to be safe.

debug!(" _{symbol}");
writeln!(f, " {symbol}")?;
}
};
if let Err(error) = res {
self.sess.dcx().emit_fatal(errors::LibDefWriteFailure { error });
Expand Down Expand Up @@ -1259,14 +1261,19 @@ impl<'a> Linker for EmLinker<'a> {
self.cc_arg("-nodefaultlibs");
}

fn export_symbols(&mut self, _tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
fn export_symbols(
&mut self,
_tmpdir: &Path,
_crate_type: CrateType,
symbols: &[(String, SymbolExportKind)],
) {
debug!("EXPORTED SYMBOLS:");

self.cc_arg("-s");

let mut arg = OsString::from("EXPORTED_FUNCTIONS=");
let encoded = serde_json::to_string(
&symbols.iter().map(|sym| "_".to_owned() + sym).collect::<Vec<_>>(),
&symbols.iter().map(|(sym, _)| "_".to_owned() + sym).collect::<Vec<_>>(),
)
.unwrap();
debug!("{encoded}");
Expand Down Expand Up @@ -1428,8 +1435,13 @@ impl<'a> Linker for WasmLd<'a> {

fn no_default_libraries(&mut self) {}

fn export_symbols(&mut self, _tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
for sym in symbols {
fn export_symbols(
&mut self,
_tmpdir: &Path,
_crate_type: CrateType,
symbols: &[(String, SymbolExportKind)],
) {
for (sym, _) in symbols {
self.link_args(&["--export", sym]);
}

Expand Down Expand Up @@ -1563,7 +1575,7 @@ impl<'a> Linker for L4Bender<'a> {
self.cc_arg("-nostdlib");
}

fn export_symbols(&mut self, _: &Path, _: CrateType, _: &[String]) {
fn export_symbols(&mut self, _: &Path, _: CrateType, _: &[(String, SymbolExportKind)]) {
// ToDo, not implemented, copy from GCC
self.sess.dcx().emit_warn(errors::L4BenderExportingSymbolsUnimplemented);
}
Expand Down Expand Up @@ -1720,12 +1732,17 @@ impl<'a> Linker for AixLinker<'a> {

fn no_default_libraries(&mut self) {}

fn export_symbols(&mut self, tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
fn export_symbols(
&mut self,
tmpdir: &Path,
_crate_type: CrateType,
symbols: &[(String, SymbolExportKind)],
) {
let path = tmpdir.join("list.exp");
let res: io::Result<()> = try {
let mut f = File::create_buffered(&path)?;
// FIXME: use llvm-nm to generate export list.
for symbol in symbols {
for (symbol, _) in symbols {
debug!(" _{symbol}");
writeln!(f, " {symbol}")?;
}
Expand Down Expand Up @@ -1769,9 +1786,15 @@ fn for_each_exported_symbols_include_dep<'tcx>(
}
}

pub(crate) fn exported_symbols(tcx: TyCtxt<'_>, crate_type: CrateType) -> Vec<String> {
pub(crate) fn exported_symbols(
tcx: TyCtxt<'_>,
crate_type: CrateType,
) -> Vec<(String, SymbolExportKind)> {
if let Some(ref exports) = tcx.sess.target.override_export_symbols {
return exports.iter().map(ToString::to_string).collect();
return exports
.iter()
.map(|sym| (sym.to_string(), SymbolExportKind::Text /* FIXME */))
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Comment here to explain what needs fixing (and, maybe, why it's difficult)?

.collect();
}

if let CrateType::ProcMacro = crate_type {
Expand All @@ -1781,16 +1804,20 @@ pub(crate) fn exported_symbols(tcx: TyCtxt<'_>, crate_type: CrateType) -> Vec<St
}
}

fn exported_symbols_for_non_proc_macro(tcx: TyCtxt<'_>, crate_type: CrateType) -> Vec<String> {
fn exported_symbols_for_non_proc_macro(
tcx: TyCtxt<'_>,
crate_type: CrateType,
) -> Vec<(String, SymbolExportKind)> {
let mut symbols = Vec::new();
let export_threshold = symbol_export::crates_export_threshold(&[crate_type]);
for_each_exported_symbols_include_dep(tcx, crate_type, |symbol, info, cnum| {
// Do not export mangled symbols from cdylibs and don't attempt to export compiler-builtins
// from any cdylib. The latter doesn't work anyway as we use hidden visibility for
// compiler-builtins. Most linkers silently ignore it, but ld64 gives a warning.
if info.level.is_below_threshold(export_threshold) && !tcx.is_compiler_builtins(cnum) {
symbols.push(symbol_export::exporting_symbol_name_for_instance_in_crate(
tcx, symbol, cnum,
symbols.push((
symbol_export::exporting_symbol_name_for_instance_in_crate(tcx, symbol, cnum),
info.kind,
));
symbol_export::extend_exported_symbols(&mut symbols, tcx, symbol, cnum);
}
Expand All @@ -1799,7 +1826,7 @@ fn exported_symbols_for_non_proc_macro(tcx: TyCtxt<'_>, crate_type: CrateType) -
symbols
}

fn exported_symbols_for_proc_macro_crate(tcx: TyCtxt<'_>) -> Vec<String> {
fn exported_symbols_for_proc_macro_crate(tcx: TyCtxt<'_>) -> Vec<(String, SymbolExportKind)> {
// `exported_symbols` will be empty when !should_codegen.
if !tcx.sess.opts.output_types.should_codegen() {
return Vec::new();
Expand All @@ -1809,7 +1836,10 @@ fn exported_symbols_for_proc_macro_crate(tcx: TyCtxt<'_>) -> Vec<String> {
let proc_macro_decls_name = tcx.sess.generate_proc_macro_decls_symbol(stable_crate_id);
let metadata_symbol_name = exported_symbols::metadata_symbol_name(tcx);

vec![proc_macro_decls_name, metadata_symbol_name]
vec![
(proc_macro_decls_name, SymbolExportKind::Data),
(metadata_symbol_name, SymbolExportKind::Data),
]
}

pub(crate) fn linked_symbols(
Expand Down Expand Up @@ -1906,7 +1936,13 @@ impl<'a> Linker for PtxLinker<'a> {

fn ehcont_guard(&mut self) {}

fn export_symbols(&mut self, _tmpdir: &Path, _crate_type: CrateType, _symbols: &[String]) {}
fn export_symbols(
&mut self,
_tmpdir: &Path,
_crate_type: CrateType,
_symbols: &[(String, SymbolExportKind)],
) {
}

fn subsystem(&mut self, _subsystem: &str) {}

Expand Down Expand Up @@ -1975,10 +2011,15 @@ impl<'a> Linker for LlbcLinker<'a> {

fn ehcont_guard(&mut self) {}

fn export_symbols(&mut self, _tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
fn export_symbols(
&mut self,
_tmpdir: &Path,
_crate_type: CrateType,
symbols: &[(String, SymbolExportKind)],
) {
match _crate_type {
CrateType::Cdylib => {
for sym in symbols {
for (sym, _) in symbols {
self.link_args(&["--export-symbol", sym]);
}
}
Expand Down Expand Up @@ -2052,11 +2093,16 @@ impl<'a> Linker for BpfLinker<'a> {

fn ehcont_guard(&mut self) {}

fn export_symbols(&mut self, tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
fn export_symbols(
&mut self,
tmpdir: &Path,
_crate_type: CrateType,
symbols: &[(String, SymbolExportKind)],
) {
let path = tmpdir.join("symbols");
let res: io::Result<()> = try {
let mut f = File::create_buffered(&path)?;
for sym in symbols {
for (sym, _) in symbols {
writeln!(f, "{sym}")?;
}
};
Expand Down
4 changes: 2 additions & 2 deletions compiler/rustc_codegen_ssa/src/back/symbol_export.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -753,7 +753,7 @@ pub(crate) fn exporting_symbol_name_for_instance_in_crate<'tcx>(
/// Add it to the symbols list for all kernel functions, so that it is exported in the linked
/// object.
pub(crate) fn extend_exported_symbols<'tcx>(
symbols: &mut Vec<String>,
symbols: &mut Vec<(String, SymbolExportKind)>,
tcx: TyCtxt<'tcx>,
symbol: ExportedSymbol<'tcx>,
instantiating_crate: CrateNum,
Expand All @@ -767,7 +767,7 @@ pub(crate) fn extend_exported_symbols<'tcx>(
let undecorated = symbol_name_for_instance_in_crate(tcx, symbol, instantiating_crate);

// Add the symbol for the kernel descriptor (with .kd suffix)
symbols.push(format!("{undecorated}.kd"));
symbols.push((format!("{undecorated}.kd"), SymbolExportKind::Data));
}

fn maybe_emutls_symbol_name<'tcx>(
Expand Down
2 changes: 1 addition & 1 deletion compiler/rustc_codegen_ssa/src/lib.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -218,7 +218,7 @@ pub struct CrateInfo {
pub target_cpu: String,
pub target_features: Vec<String>,
pub crate_types: Vec<CrateType>,
pub exported_symbols: UnordMap<CrateType, Vec<String>>,
pub exported_symbols: UnordMap<CrateType, Vec<(String, SymbolExportKind)>>,
pub linked_symbols: FxIndexMap<CrateType, Vec<(String, SymbolExportKind)>>,
pub local_crate_name: Symbol,
pub compiler_builtins: Option<CrateNum>,
Expand Down
Loading