[lld] [LLD][COFF] add __{data,bss}_{start,end}__ symbols for Cygwin support (PR #136180)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-coff

Author: None (jeremyd2019)

<details>
<summary>Changes</summary>

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.

/cc @<!-- -->mstorsjo 

---
Full diff: https://github.com/llvm/llvm-project/pull/136180.diff


2 Files Affected:

- (modified) lld/COFF/Driver.cpp (+5) 
- (modified) lld/COFF/Writer.cpp (+31-2) 


``````````diff
diff --git a/lld/COFF/Driver.cpp b/lld/COFF/Driver.cpp
index 7aa13bdce488e..72e25634c19d8 100644
--- 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
index 6ed1f884a9636..32a480e3126e2 100644
--- 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() {

``````````

</details>


https://github.com/llvm/llvm-project/pull/136180