Implement --just-symbols.

Differential Revision: https://reviews.llvm.org/D39348

llvm-svn: 326835

lld/ELF/Driver.cpp

@@ -1072,6 +1072,12 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   for (StringRef S : Config->Undefined)
     Symtab->fetchIfLazy<ELFT>(S);
 
+  // Handle the --just-symbols option. This may add absolute symbols
+  // to the symbol table.
+  for (auto *Arg : Args.filtered(OPT_just_symbols))
+    if (Optional<MemoryBufferRef> MB = readFile(Arg->getValue()))
+      readJustSymbolsFile<ELFT>(*MB);
+
   // If an entry symbol is in a static archive, pull out that file now
   // to complete the symbol table. After this, no new names except a
   // few linker-synthesized ones will be added to the symbol table.

lld/ELF/InputFiles.cpp

@@ -1175,6 +1175,42 @@ std::vector<StringRef> LazyObjFile::getSymbolNames() {
   }
 }
 
+// This is for --just-symbols.
+//
+// This option allows you to link your output against other existing
+// program, so that if you load both your program and the other program
+// into memory, your output can use program's symbols.
+//
+// What we are doing here is to read defined symbols from a given ELF
+// file and add them as absolute symbols.
+template <class ELFT> void elf::readJustSymbolsFile(MemoryBufferRef MB) {
+  typedef typename ELFT::Shdr Elf_Shdr;
+  typedef typename ELFT::Sym Elf_Sym;
+  typedef typename ELFT::SymRange Elf_Sym_Range;
+
+  StringRef ObjName = MB.getBufferIdentifier();
+  ELFFile<ELFT> Obj = check(ELFFile<ELFT>::create(MB.getBuffer()));
+  ArrayRef<Elf_Shdr> Sections = CHECK(Obj.sections(), ObjName);
+
+  for (const Elf_Shdr &Sec : Sections) {
+    if (Sec.sh_type != SHT_SYMTAB)
+      continue;
+
+    Elf_Sym_Range Syms = CHECK(Obj.symbols(&Sec), ObjName);
+    uint32_t FirstNonLocal = Sec.sh_info;
+    StringRef StringTable =
+        CHECK(Obj.getStringTableForSymtab(Sec, Sections), ObjName);
+
+    std::vector<std::pair<StringRef, uint64_t>> Ret;
+    for (const Elf_Sym &Sym : Syms.slice(FirstNonLocal))
+      if (Sym.st_shndx != SHN_UNDEF)
+        Symtab->addRegular(CHECK(Sym.getName(StringTable), ObjName),
+                           Sym.st_other, Sym.getType(), Sym.st_value,
+                           Sym.st_size, Sym.getBinding(), nullptr, nullptr);
+    return;
+  }
+}
+
 template void ArchiveFile::parse<ELF32LE>();
 template void ArchiveFile::parse<ELF32BE>();
 template void ArchiveFile::parse<ELF64LE>();
@@ -1204,3 +1240,8 @@ template class elf::SharedFile<ELF32LE>;
 template class elf::SharedFile<ELF32BE>;
 template class elf::SharedFile<ELF64LE>;
 template class elf::SharedFile<ELF64BE>;
+
+template void elf::readJustSymbolsFile<ELF32LE>(MemoryBufferRef);
+template void elf::readJustSymbolsFile<ELF32BE>(MemoryBufferRef);
+template void elf::readJustSymbolsFile<ELF64LE>(MemoryBufferRef);
+template void elf::readJustSymbolsFile<ELF64BE>(MemoryBufferRef);

lld/ELF/InputFiles.h

@@ -332,6 +332,9 @@ InputFile *createObjectFile(MemoryBufferRef MB, StringRef ArchiveName = "",
                             uint64_t OffsetInArchive = 0);
 InputFile *createSharedFile(MemoryBufferRef MB, StringRef DefaultSoName);
 
+// For --just-symbols
+template <class ELFT> void readJustSymbolsFile(MemoryBufferRef MB);
+
 extern std::vector<BinaryFile *> BinaryFiles;
 extern std::vector<BitcodeFile *> BitcodeFiles;
 extern std::vector<InputFile *> ObjectFiles;

lld/ELF/Options.td

@@ -180,6 +180,8 @@ defm image_base : Eq<"image-base">, HelpText<"Set the base address">;
 defm init: Eq<"init">, HelpText<"Specify an initializer function">,
   MetaVarName<"<symbol>">;
 
+defm just_symbols: Eq<"just-symbols">, HelpText<"Just link symbols">;
+
 defm library: Eq<"library">, HelpText<"Root name of library to use">,
   MetaVarName<"<libName>">;
 

lld/test/ELF/Inputs/just-symbols.s

@@ -0,0 +1,9 @@
+.globl foo, bar
+foo:
+  ret
+
+.section .data
+.type bar, @object
+.size bar, 40
+bar:
+  .zero 40

lld/test/ELF/just-symbols.s

@@ -0,0 +1,17 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %S/Inputs/just-symbols.s -o %t1
+# RUN: ld.lld %t1 -o %t1.exe -Ttext=0x10000
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t2
+# RUN: ld.lld %t2 -just-symbols=%t1.exe -o %t2.exe
+# RUN: llvm-readelf -symbols %t2.exe | FileCheck %s
+
+# CHECK: 0000000000011000    40 OBJECT  GLOBAL DEFAULT  ABS bar
+# CHECK: 0000000000010000     0 NOTYPE  GLOBAL DEFAULT  ABS foo
+
+.globl _start
+_start:
+  call foo
+  call bar
+  ret