[ELF2] - implemented default enterpteter for X86, X86_64, PPC and AArch64

[grimar]

Summary

Before this patch .interp section was never created (if -dynamic-linker was not specified) because there was no default dynamic linker assigned.

[hfinkel]

You're missing a test case.

Target.cpp

204 ↗

(On Diff #37082)

Please make this /lib64/ld64.so.1

[ruiu]

Target does not seems to be the right place to put OS-specific things. It is just architecture-specific (e.g. ELF on x86-64). I'd to write this in Driver.cpp.

[emaste]

As @ruiu says Linux-specific settings aren't appropriate there.
For reference on FreeBSD it should be /libexec/ld-elf.so.1.

[grimar]

Added test.
getDefaultInterp() is in Driver.cpp as was discussed via llvm-mails.

[ruiu]

ELF/Driver.cpp

70–72

The empty string is not a valid value for .interp, so let's error out.

error("--dynamic-linker is not specified");

216–217

I'd write

Config->DynamicLinker = getString(Args, OPT_dynamic_linker);
if (Config->DynamicLinker.empty())
  Config->DynamicLinker = getDefaultInterp();

so that we don't call the function if --dynamic-linker is givne.

[emaste]

ELF/Driver.cpp

60

This is not valid on non-Linux

[ruiu]

ELF/Driver.cpp

60

What's the value for FreeBSD?

We don't have to make this function complete from the beginning. I'm fine with this as long as we can add new entries to this function.

[grimar]

ELF/Driver.cpp

60

Yes, it is not valid on non-linux. But currently it looks there is no good way to find which OS we are targeting. So it is just basic implementation to support .interp section.

[• rafael]

What depends on this?

Both gcc and clang pass -dynamic_linker to ld.

Having ld itself guess the dynamic linker when not given that option seems like an historical artifact.

[ruiu]

Hm, that's an interesting point. Are you suggesting we always set the same value to .interp or make --dynamic_linker mandatory?

[grimar]

According to discussions in mail list.

ELF/Driver.cpp

if (S == "elf64ppc")
     return {ELF64BEKind, EM_PPC64};
   if (S == "elf_i386")
     return {ELF32LEKind, EM_386};
   if (S == "elf_x86_64")
     return {ELF64LEKind, EM_X86_64};
   error("Unknown emulation: " + S);
 }
 
+static StringRef getDefaultInterp() {
+  switch (Config->EMachine) {
+  case EM_386:
+    return "/lib/ld-linux-x86-64.so.2";
+  case EM_AARCH64:
+    return "/lib/ld-linux-aarch64.so.1";
+  case EM_PPC:
+    return "/lib/ld.so.1";
+  case EM_PPC64:
+    return "/lib64/ld64.so.1";
+  case EM_X86_64:
+    return "/lib64/ld-linux-x86-64.so.2";
+  }
+  // No interpreter specified.
+  // Assumes that it is taken from commandline.
+  return "";
+}
+
 // Opens and parses a file. Path has to be resolved already.
 // Newly created memory buffers are owned by this driver.
 void LinkerDriver::addFile(StringRef Path) {
   using namespace llvm::sys::fs;
   if (Config->Verbose)
     llvm::outs() << Path << "\n";
   auto MBOrErr = MemoryBuffer::getFile(Path);
   error(MBOrErr, "cannot open " + Path);
void LinkerDriver::createFiles(opt::InputArgList &Args) {
   Config->DiscardNone = Args.hasArg(OPT_discard_none);
   Config->EnableNewDtags = !Args.hasArg(OPT_disable_new_dtags);
   Config->ExportDynamic = Args.hasArg(OPT_export_dynamic);
   Config->NoInhibitExec = Args.hasArg(OPT_noinhibit_exec);
   Config->NoUndefined = Args.hasArg(OPT_no_undefined);
   Config->Shared = Args.hasArg(OPT_shared);
   Config->Verbose = Args.hasArg(OPT_verbose);
 
-  Config->DynamicLinker = getString(Args, OPT_dynamic_linker);
   Config->Entry = getString(Args, OPT_entry);
   Config->Fini = getString(Args, OPT_fini, "_fini");
   Config->Init = getString(Args, OPT_init, "_init");
   Config->OutputFile = getString(Args, OPT_o);
   Config->SoName = getString(Args, OPT_soname);
   Config->Sysroot = getString(Args, OPT_sysroot);
 
   for (auto *Arg : Args.filtered(OPT_z))
void LinkerDriver::createFiles(opt::InputArgList &Args) {
   if (Files.empty())
     error("no input files.");
 }
 
 template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   SymbolTable<ELFT> Symtab;
   Target.reset(createTarget());
 
+  Config->DynamicLinker =
+      getString(Args, OPT_dynamic_linker, getDefaultInterp());
+
   if (!Config->Shared) {
     // Add entry symbol.
     if (Config->Entry.empty())
       Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";
     Config->EntrySym = Symtab.addUndefined(Config->Entry);
 
     // In the assembly for 32 bit x86 the _GLOBAL_OFFSET_TABLE_ symbol
     // is magical and is used to produce a R_386_GOTPC relocation.

test/elf2/dynamic-reloc.s

 // RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 // RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/shared.s -o %t2.o
 // RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/dynamic-reloc.s -o %t3.o
 // RUN: ld.lld2 -shared %t2.o -o %t2.so
 // RUN: ld.lld2 %t.o %t3.o %t2.so -o %t
 // RUN: llvm-readobj -dynamic-table -r --expand-relocs -s %t | FileCheck %s
 // REQUIRES: x86
 
-// CHECK:      Index: 1
+// CHECK:      Index: 2
 // CHECK-NEXT: Name: .dynsym
 
 // CHECK:      Name: .rela.dyn
 // CHECK-NEXT: Type: SHT_RELA
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address: [[RELAADDR:.*]]
 // CHECK-NEXT: Offset:
 // CHECK-NEXT: Size: [[RELASIZE:.*]]
-// CHECK-NEXT: Link: 1
+// CHECK-NEXT: Link: 2
 // CHECK-NEXT: Info: 0
 // CHECK-NEXT: AddressAlignment: 8
 // CHECK-NEXT: EntrySize: 24
 
 // CHECK:      Name: .text
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC

test/elf2/interp.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=i686-unknown-linux %s -o %t86.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t64.o
+
+# RUN: ld.lld2 -shared %t86.o -o %t.so
+# RUN: ld.lld2 %t86.o %t.so -o %t
+# RUN: llvm-readobj --program-headers --dynamic-table -t -s -dyn-symbols -section-data -hash-table %t | FileCheck -check-prefix=CHECK_X86 %s
+
+# RUN: ld.lld2 -shared %t64.o -o %t.so
+# RUN: ld.lld2 %t64.o %t.so -o %t
+# RUN: llvm-readobj --program-headers --dynamic-table -t -s -dyn-symbols -section-data -hash-table %t | FileCheck -check-prefix=CHECK_X64 %s
+
+# RUN: ld.lld2 %t64.o %t.so -dynamic-linker /aaa/bbb -o %t
+# RUN: llvm-readobj --program-headers --dynamic-table -t -s -dyn-symbols -section-data -hash-table %t | FileCheck -check-prefix=CHECK_AAABBB %s
+
+# RUN: ld.lld2 %t64.o -o %t
+# RUN: llvm-readobj --program-headers --dynamic-table -t -s -dyn-symbols -section-data -hash-table %t | FileCheck -check-prefix=CHECK_NO %s
+
+# CHECK_X86:      Name: .interp
+# CHECK_X86-NEXT: Type: SHT_PROGBITS
+# CHECK_X86-NEXT: Flags [
+# CHECK_X86-NEXT:   SHF_ALLOC
+# CHECK_X86-NEXT: ]
+# CHECK_X86-NEXT: Address: 0x100D4
+# CHECK_X86-NEXT: Offset: 0xD4
+# CHECK_X86-NEXT: Size: 26
+# CHECK_X86-NEXT: Link: 0
+# CHECK_X86-NEXT: Info: 0
+# CHECK_X86-NEXT: AddressAlignment: 1
+# CHECK_X86-NEXT: EntrySize: 0
+# CHECK_X86-NEXT: SectionData (
+# CHECK_X86-NEXT:   0000: 2F6C6962 2F6C642D 6C696E75 782D7838  |/lib/ld-linux-x8|
+# CHECK_X86-NEXT:   0010: 362D3634 2E736F2E 3200               |6-64.so.2.|
+# CHECK_X86-NEXT: )
+
+# CHECK_X64:      Name: .interp
+# CHECK_X64-NEXT: Type: SHT_PROGBITS
+# CHECK_X64-NEXT: Flags [
+# CHECK_X64-NEXT:   SHF_ALLOC
+# CHECK_X64-NEXT: ]
+# CHECK_X64-NEXT: Address: 0x10158
+# CHECK_X64-NEXT: Offset: 0x158
+# CHECK_X64-NEXT: Size: 28
+# CHECK_X64-NEXT: Link: 0
+# CHECK_X64-NEXT: Info: 0
+# CHECK_X64-NEXT: AddressAlignment: 1
+# CHECK_X64-NEXT: EntrySize: 0
+# CHECK_X64-NEXT: SectionData (
+# CHECK_X64-NEXT:   0000: 2F6C6962 36342F6C 642D6C69 6E75782D  |/lib64/ld-linux-|
+# CHECK_X64-NEXT:   0010: 7838362D 36342E73 6F2E3200           |x86-64.so.2.|
+# CHECK_X64-NEXT: )
+
+# CHECK_AAABBB:      Name: .interp
+# CHECK_AAABBB:      SectionData (
+# CHECK_AAABBB-NEXT:   0000: 2F616161 2F626262 00                 |/aaa/bbb.|
+# CHECK_AAABBB-NEXT: )
+
+# CHECK_NO-NOT: Name: .interp
+
+.global _start
+_start:

test/elf2/relocation.s

 // CHECK-NEXT:  11017:   e8 04 00 00 00  callq  4
 
 .section .R_X86_64_64,"a",@progbits
 .global R_X86_64_64
 R_X86_64_64:
  .quad R_X86_64_64
 
 // CHECK:      Contents of section .R_X86_64_64:
-// CHECK-NEXT:   10158 58010100 00000000
+// CHECK-NEXT:   101ac ac010100 00000000
 
 .section .R_X86_64_GOTPCREL,"a",@progbits
 .global R_X86_64_GOTPCREL
 R_X86_64_GOTPCREL:
  .long zed@gotpcrel
 
 // 0x120A8 - 0x10160 = 8008
 // 8008 = 0x481f0000   in little endian
 // CHECK:      Contents of section .R_X86_64_GOTPCREL
-// CHECK-NEXT:   10160 481f0000
+// CHECK-NEXT:   101b4 f41e0000