[ELF] Shared libraries should have entry point

[phosek]

Summary

Shared libraries should have entry set following the same rules as for regular binaries. The only difference is that in case the default entry point (_start or __start) isn't found (unless it was set explicitly), we shouldn't give a warning as in case of regular binaries.

[phosek]

Ideally, Config->Entry would have 3 values: unset, set to default, set explicitly, but I don't know if there's a better way to achieve this than what I did in this patch.

[ruiu]

LGTM

Yeah, the entry point handling is becoming a bit messy because we have so many different ways to specify entry points. I'll try to simplify it.

lld/trunk/ELF/Config.h

struct Configuration {
   bool DisableVerify;
   bool EhFrameHdr;
   bool EnableNewDtags;
   bool ExportDynamic;
   bool FatalWarnings;
   bool GcSections;
   bool GdbIndex;
   bool GnuHash = false;
+  bool HasEntry = false;
   bool ICF;
   bool Mips64EL = false;
   bool MipsN32Abi = false;
   bool NoGnuUnique;
   bool NoUndefinedVersion;
   bool Nostdlib;
   bool OFormatBinary;
   bool OMagic;

lld/trunk/ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->Bsymbolic = Args.hasArg(OPT_Bsymbolic);
   Config->BsymbolicFunctions = Args.hasArg(OPT_Bsymbolic_functions);
   Config->Demangle = getArg(Args, OPT_demangle, OPT_no_demangle, true);
   Config->DisableVerify = Args.hasArg(OPT_disable_verify);
   Config->EhFrameHdr = Args.hasArg(OPT_eh_frame_hdr);
   Config->EnableNewDtags = !Args.hasArg(OPT_disable_new_dtags);
   Config->ExportDynamic = Args.hasArg(OPT_export_dynamic);
   Config->FatalWarnings = Args.hasArg(OPT_fatal_warnings);
+  Config->HasEntry = Args.hasArg(OPT_entry);
   Config->GcSections = getArg(Args, OPT_gc_sections, OPT_no_gc_sections, false);
   Config->GdbIndex = Args.hasArg(OPT_gdb_index);
   Config->ICF = Args.hasArg(OPT_icf);
   Config->NoGnuUnique = Args.hasArg(OPT_no_gnu_unique);
   Config->NoUndefinedVersion = Args.hasArg(OPT_no_undefined_version);
   Config->Nostdlib = Args.hasArg(OPT_nostdlib);
   Config->OMagic = Args.hasArg(OPT_omagic);
   Config->Pie = getArg(Args, OPT_pie, OPT_nopie, false);
template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
 
   // Add the start symbol.
   // It initializes either Config->Entry or Config->EntryAddr.
   // Note that AMDGPU binaries have no entries.
   if (!Config->Entry.empty()) {
     // It is either "-e <addr>" or "-e <symbol>".
     if (!Config->Entry.getAsInteger(0, Config->EntryAddr))
       Config->Entry = "";
-  } else if (!Config->Shared && !Config->Relocatable &&
-             Config->EMachine != EM_AMDGPU) {
+  } else if (!Config->Relocatable && Config->EMachine != EM_AMDGPU) {
     // -e was not specified. Use the default start symbol name
     // if it is resolvable.
     Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";
   }
 
   // If an object file defining the entry symbol is in an archive file,
   // extract the file now.
   if (Symtab.find(Config->Entry))

lld/trunk/ELF/Writer.cpp

template <class ELFT> typename ELFT::uint Writer<ELFT>::getEntryAddr() {
   // Case 1, 2 or 3
   if (Config->Entry.empty())
     return Config->EntryAddr;
   if (SymbolBody *B = Symtab<ELFT>::X->find(Config->Entry))
     return B->getVA<ELFT>();
 
   // Case 4
   if (OutputSectionBase *Sec = findSection(".text")) {
+    if (!Config->Shared || Config->HasEntry)
       warn("cannot find entry symbol " + Config->Entry + "; defaulting to 0x" +
            utohexstr(Sec->Addr));
     return Sec->Addr;
   }
 
   // Case 5
+  if (!Config->Shared || Config->HasEntry)
     warn("cannot find entry symbol " + Config->Entry +
          "; not setting start address");
   return 0;
 }
 
 template <class ELFT> static uint8_t getELFEncoding() {
   if (ELFT::TargetEndianness == llvm::support::little)
     return ELFDATA2LSB;
   return ELFDATA2MSB;
 }

lld/trunk/test/ELF/gc-sections-keep-shared-start.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+
+# RUN: ld.lld -shared --gc-sections -o %t1 %t
+# RUN: llvm-readobj --elf-output-style=GNU --file-headers --symbols %t1
+#   | FileCheck %s
+# CHECK: Entry point address:               0x1000
+# CHECK: 0000000000001000     0 FUNC    LOCAL  HIDDEN     4 _start
+# CHECK: 0000000000001006     0 FUNC    LOCAL  HIDDEN     4 internal
+# CHECK: 0000000000001005     0 FUNC    GLOBAL DEFAULT    4 foobar
+
+.section .text.start,"ax"
+.globl _start
+.type _start,%function
+.hidden _start
+_start:
+  jmp internal
+
+.section .text.foobar,"ax"
+.globl foobar
+.type foobar,%function
+foobar:
+  ret
+
+.section .text.internal,"ax"
+.globl internal
+.hidden internal
+.type internal,%function
+internal:
+  ret