[ELF] Allow references to reserved symbols in linker scripts

This requires collectign all symbols referenced in the linker script
and adding them to symbol table as undefined symbol.

Differential Revision: https://reviews.llvm.org/D31147

llvm-svn: 298577

3 files changed, 15 insertions, 4 deletions

diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index 87a54304669..09c232a846c 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -921,6 +921,11 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   if (ErrorCount)
     return;
 
+  // Some symbols (such as __ehdr_start) are defined lazily only when there
+  // are undefined symbols for them, so we add these to trigger that logic.
+  for (StringRef Sym : Script->Opt.UndefinedSymbols)
+    Symtab.addUndefined(Sym);
+
   for (auto *Arg : Args.filtered(OPT_wrap))
     Symtab.wrap(Arg->getValue());
 
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index f98e689f0b9..adb7ce862bb 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -999,8 +999,8 @@ ExprValue LinkerScript::getSymbolValue(const Twine &Loc, StringRef S) {
   if (SymbolBody *B = findSymbol(S)) {
     if (auto *D = dyn_cast<DefinedRegular>(B))
       return {D->Section, D->Value};
-    auto *C = cast<DefinedCommon>(B);
-    return {InX::Common, C->Offset};
+    if (auto *C = dyn_cast<DefinedCommon>(B))
+      return {InX::Common, C->Offset};
   }
   error(Loc + ": symbol not found: " + S);
   return 0;
@@ -1867,8 +1867,11 @@ Expr ScriptParser::readPrimary() {
     return [=] { return V; };
 
   // Tok is a symbol name.
-  if (Tok != "." && !isValidCIdentifier(Tok))
-    setError("malformed number: " + Tok);
+  if (Tok != ".") {
+    if (!isValidCIdentifier(Tok))
+      setError("malformed number: " + Tok);
+    Script->Opt.UndefinedSymbols.push_back(Tok);
+  }
   return [=] { return Script->getSymbolValue(Location, Tok); };
 }
 
diff --git a/lld/ELF/LinkerScript.h b/lld/ELF/LinkerScript.h
index 00e4f9ec96e..6dc70518656 100644
--- a/lld/ELF/LinkerScript.h
+++ b/lld/ELF/LinkerScript.h
@@ -217,6 +217,9 @@ struct ScriptConfiguration {
 
   // A map from memory region name to a memory region descriptor.
   llvm::DenseMap<llvm::StringRef, MemoryRegion> MemoryRegions;
+
+  // A list of undefined symbols referenced by the script.
+  std::vector<llvm::StringRef> UndefinedSymbols;
 };
 
 class LinkerScript {