[PATCH] D31147: [ELF] Allow references to reserved symbols in linker scripts

[Petr Hosek via Phabricator via llvm-commits]

phosek created this revision.
phosek added a project: lld.

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


Repository:
  rL LLVM

https://reviews.llvm.org/D31147

Files:
  ELF/Driver.cpp
  ELF/LinkerScript.cpp
  ELF/LinkerScript.h
  test/ELF/linkerscript/symbol-reserved.s


Index: test/ELF/linkerscript/symbol-reserved.s
===================================================================
--- /dev/null
+++ test/ELF/linkerscript/symbol-reserved.s
@@ -0,0 +1,16 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "PROVIDE_HIDDEN(newsym = __ehdr_start + 5);" > %t.script
+# RUN: ld.lld -o %t1 %t.script %t
+# RUN: llvm-objdump -t %t1 | FileCheck %s
+
+# CHECK: 0000000000200005 .text 00000000 .hidden newsym
+
+# RUN: ld.lld -o %t1.so %t.script %t -shared
+# RUN: llvm-objdump -t %t1.so | FileCheck --check-prefix=SHARED %s
+
+# SHARED: 0000000000000005 .dynsym 00000000 .hidden newsym
+
+.global _start
+_start:
+  lea newsym(%rip),%rax
Index: ELF/LinkerScript.h
===================================================================
--- ELF/LinkerScript.h
+++ ELF/LinkerScript.h
@@ -217,6 +217,8 @@
 
   // A map from memory region name to a memory region descriptor.
   llvm::DenseMap<llvm::StringRef, MemoryRegion> MemoryRegions;
+
+  std::vector<llvm::StringRef> Undefined;
 };
 
 extern ScriptConfiguration *ScriptConfig;
@@ -297,6 +299,8 @@
   void addSymbol(SymbolAssignment *Cmd);
   void processCommands(OutputSectionFactory &Factory);
 
+  void addUndefined();
+
   ExprValue getSymbolValue(const Twine &Loc, StringRef S) override;
   bool isDefined(StringRef S) override;
 };
Index: ELF/LinkerScript.cpp
===================================================================
--- ELF/LinkerScript.cpp
+++ ELF/LinkerScript.cpp
@@ -392,6 +392,13 @@
 }
 
 template <class ELFT>
+void LinkerScript<ELFT>::addUndefined() {
+  for (const llvm::StringRef &Name : Opt.Undefined) {
+    Symtab<ELFT>::X->addUndefined(Name);
+  }
+}
+
+template <class ELFT>
 void LinkerScript<ELFT>::processCommands(OutputSectionFactory &Factory) {
   // A symbol can be assigned before any section is mentioned in the linker
   // script. In an DSO, the symbol values are addresses, so the only important
@@ -998,8 +1005,8 @@
   if (SymbolBody *B = Symtab<ELFT>::X->find(S)) {
     if (auto *D = dyn_cast<DefinedRegular>(B))
       return {D->Section, D->Value};
-    auto *C = cast<DefinedCommon>(B);
-    return {In<ELFT>::Common, C->Offset};
+    if (auto *C = dyn_cast<DefinedCommon>(B))
+      return {In<ELFT>::Common, C->Offset};
   }
   error(Loc + ": symbol not found: " + S);
   return 0;
@@ -1871,8 +1878,11 @@
     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);
+    Opt.Undefined.push_back(Tok);
+  }
   return [=] { return ScriptBase->getSymbolValue(Location, Tok); };
 }
 
Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -878,6 +878,8 @@
   if (ErrorCount)
     return;
 
+  Script<ELFT>::X->addUndefined();
+
   for (auto *Arg : Args.filtered(OPT_wrap))
     Symtab.wrap(Arg->getValue());
 


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