[LLD][COFF] add __{data,bss}_{start,end}__ symbols for Cygwin support

jeremyd2019 · jeremyd2019 · commit 7b71533512ed · 2025-04-17T11:48:00.000-07:00
Cygwin requires these symbols for its fork emulation to know what data
to copy into the child.  GNU ld defines these symbols for MinGW targets
also, so do the same here.

Signed-off-by: Jeremy Drake &lt;github@jdrake.com&gt;
diff --git a/lld/COFF/Driver.cpp b/lld/COFF/Driver.cpp
@@ -2039,6 +2039,7 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
     parseMerge(".ctors=.rdata");
     parseMerge(".dtors=.rdata");
     parseMerge(".CRT=.rdata");
+    parseMerge(".data_cygwin_nocopy=.data");
   }
 
   // Handle /section
@@ -2495,6 +2496,10 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
     if (config->mingw) {
       symtab.addAbsolute(symtab.mangle("__CTOR_LIST__"), 0);
       symtab.addAbsolute(symtab.mangle("__DTOR_LIST__"), 0);
+      symtab.addAbsolute("__data_start__", 0);
+      symtab.addAbsolute("__data_end__", 0);
+      symtab.addAbsolute("__bss_start__", 0);
+      symtab.addAbsolute("__bss_end__", 0);
     }
     if (config->debug || config->buildIDHash != BuildIDHash::None)
       if (symtab.findUnderscore("__buildid"))
diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
@@ -239,6 +239,7 @@ class Writer {
   void createRuntimePseudoRelocs();
   void createECChunks();
   void insertCtorDtorSymbols();
+  void insertBssDataStartEndSymbols();
   void markSymbolsWithRelocations(ObjFile *file, SymbolRVASet &usedSymbols);
   void createGuardCFTables();
   void markSymbolsForRVATable(ObjFile *file,
@@ -314,6 +315,7 @@ class Writer {
 
   OutputSection *textSec;
   OutputSection *hexpthkSec;
+  OutputSection *bssSec;
   OutputSection *rdataSec;
   OutputSection *buildidSec;
   OutputSection *dataSec;
@@ -1077,7 +1079,7 @@ void Writer::createSections() {
   textSec = createSection(".text", code | r | x);
   if (isArm64EC(ctx.config.machine))
     hexpthkSec = createSection(".hexpthk", code | r | x);
-  createSection(".bss", bss | r | w);
+  bssSec = createSection(".bss", bss | r | w);
   rdataSec = createSection(".rdata", data | r);
   buildidSec = createSection(".buildid", data | r);
   dataSec = createSection(".data", data | r | w);
@@ -1260,8 +1262,10 @@ void Writer::createMiscChunks() {
   if (config->autoImport)
     createRuntimePseudoRelocs();
 
-  if (config->mingw)
+  if (config->mingw) {
     insertCtorDtorSymbols();
+    insertBssDataStartEndSymbols();
+  }
 }
 
 // Create .idata section for the DLL-imported symbol table.
@@ -2369,6 +2373,31 @@ void Writer::insertCtorDtorSymbols() {
   }
 }
 
+// MinGW (really, Cygwin) specific.
+// The Cygwin startup code uses __data_start__ __data_end__ __bss_start__
+// and __bss_end__ to know what to copy during fork emulation.
+void Writer::insertBssDataStartEndSymbols() {
+  if (!dataSec->chunks.empty()) {
+    Symbol *dataStartSym = ctx.symtab.find("__data_start__");
+    Symbol *dataEndSym = ctx.symtab.find("__data_end__");
+    Chunk *endChunk = dataSec->chunks.back();
+    replaceSymbol<DefinedSynthetic>(dataStartSym, dataStartSym->getName(),
+                                    dataSec->chunks.front());
+    replaceSymbol<DefinedSynthetic>(dataEndSym, dataEndSym->getName(), endChunk,
+                                    endChunk->getSize());
+  }
+
+  if (!bssSec->chunks.empty()) {
+    Symbol *bssStartSym = ctx.symtab.find("__bss_start__");
+    Symbol *bssEndSym = ctx.symtab.find("__bss_end__");
+    Chunk *endChunk = bssSec->chunks.back();
+    replaceSymbol<DefinedSynthetic>(bssStartSym, bssStartSym->getName(),
+                                    bssSec->chunks.front());
+    replaceSymbol<DefinedSynthetic>(bssEndSym, bssEndSym->getName(), endChunk,
+                                    endChunk->getSize());
+  }
+}
+
 // Handles /section options to allow users to overwrite
 // section attributes.
 void Writer::setSectionPermissions() {
