[ELF] - Linkerscript: implement SIZEOF_HEADERS.

SIZEOF_HEADERS - Return the size in bytes of the output file’s headers.

It is is a feature used in FreeBsd script, for example.
There is a discussion on PR28688 page about it.

Differential revision: https://reviews.llvm.org/D23165

llvm-svn: 278204

lld/ELF/LinkerScript.cpp

@@ -238,7 +238,7 @@ template <class ELFT> void LinkerScript<ELFT>::assignAddresses() {
   }
 
   // Assign addresses as instructed by linker script SECTIONS sub-commands.
-  Dot = Out<ELFT>::ElfHeader->getSize() + Out<ELFT>::ProgramHeaders->getSize();
+  Dot = getSizeOfHeaders();
   uintX_t MinVA = std::numeric_limits<uintX_t>::max();
   uintX_t ThreadBssOffset = 0;
 
@@ -430,6 +430,11 @@ typename ELFT::uint LinkerScript<ELFT>::getOutputSectionSize(StringRef Name) {
   return 0;
 }
 
+template <class ELFT>
+typename ELFT::uint LinkerScript<ELFT>::getSizeOfHeaders() {
+  return Out<ELFT>::ElfHeader->getSize() + Out<ELFT>::ProgramHeaders->getSize();
+}
+
 // Returns indices of ELF headers containing specific section, identified
 // by Name. Each index is a zero based number of ELF header listed within
 // PHDRS {} script block.
@@ -915,6 +920,21 @@ static uint64_t getSectionSize(StringRef Name) {
   }
 }
 
+static uint64_t getSizeOfHeaders() {
+  switch (Config->EKind) {
+  case ELF32LEKind:
+    return Script<ELF32LE>::X->getSizeOfHeaders();
+  case ELF32BEKind:
+    return Script<ELF32BE>::X->getSizeOfHeaders();
+  case ELF64LEKind:
+    return Script<ELF64LE>::X->getSizeOfHeaders();
+  case ELF64BEKind:
+    return Script<ELF64BE>::X->getSizeOfHeaders();
+  default:
+    llvm_unreachable("unsupported target");
+  }
+}
+
 SymbolAssignment *ScriptParser::readAssignment(StringRef Name) {
   StringRef Op = next();
   assert(Op == "=" || Op == "+=");
@@ -1063,6 +1083,8 @@ Expr ScriptParser::readPrimary() {
     expect(")");
     return [=](uint64_t Dot) { return getSectionSize(Name); };
   }
+  if (Tok == "SIZEOF_HEADERS")
+    return [=](uint64_t Dot) { return getSizeOfHeaders(); };
 
   // Parse a symbol name or a number literal.
   uint64_t V = 0;

lld/ELF/LinkerScript.h

@@ -149,6 +149,7 @@ public:
   void addScriptedSymbols();
   bool hasPhdrsCommands();
   uintX_t getOutputSectionSize(StringRef Name);
+  uintX_t getSizeOfHeaders();
 
   std::vector<OutputSectionBase<ELFT> *> *OutputSections;
 

lld/test/ELF/linkerscript/linkerscript-sizeofheaders.s

@@ -0,0 +1,18 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo " SECTIONS {             \
+# RUN:          . = SIZEOF_HEADERS;  \
+# RUN:          _size = SIZEOF_HEADERS;  \
+# RUN:          .text : {*(.text.*)} \
+# RUN:          }" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -t %t1 | FileCheck %s
+
+#CHECK:      SYMBOL TABLE:
+#CHECK-NEXT:  0000000000000000 *UND* 00000000
+#CHECK-NEXT:  0000000000000120 .text 00000000 _start
+#CHECK-NEXT:  0000000000000120 *ABS* 00000000 _size
+
+.global _start
+_start:
+ nop