Make TLS work for PIE executables on x86-64.

[ed]

Summary

Caution! http://i2.kym-cdn.com/photos/images/facebook/000/234/739/fa5.jpg

While trying to get PIE work on CloudABI for x86-64, I noticed that even
though GNU ld would generate functional binaries, LLD would not. It
turns out that we generate relocations for referencing TLS objects
inside of the text segment, which shouldn't happen.

This change extends the isRelRelative() function to list some additional
relocation types that should be treated as relative. This makes my C
library unit testing binary work on x86-64.

More details: https://llvm.org/bugs/show_bug.cgi?id=27174

[grimar]

I think this need split into 2: for x86_64 and aarch64 separatelly.
And also this needs a testcase.

[grimar]

ELF/Target.cpp

811

Change itself looks ok for me.

[ruiu]

The code looks good, so please add a test case.

[ed]

Remove the aarch64 changes. Also add a test case.

I've noticed that my change does make one of the existing tests fail.
I'm not entirely sure, but my suspicion is that this test is in fact
invalid. GNU ld seems to reject object files containing these
relocations when creating a shared object. Maybe we should extend LLD to
throw an error message as well?

[ruiu]

It seems that this change affects how the dynamic relocations are created for TLS variables. Rafael is rewriting that part of the code, so you want to wait for him to finish it instead of trying to understand the current code (which is Writer::scanRelocs).

[ed]

This is interesting. Together with http://reviews.llvm.org/D18739, test/ELF/tls-initial-exec-local.s passes again.

If you want, I can put it back in the source tree. I still guess the test looks a bit shady for using static TLS in a shared object.

[ed]

Put tls-initial-exec-local.s back in now that the GOT works properly.

[ed]

Okay. Now that my other change has landed, I think we can go ahead with this change as well. Any thoughts?

[ruiu]

Yes, LGTM. Thank you for all these patches!

ELF/Target.cpp

 bool X86_64TargetInfo::needsPltImpl(uint32_t Type) const {
   return Type == R_X86_64_PLT32;
 }
 
 bool X86_64TargetInfo::isRelRelative(uint32_t Type) const {
   switch (Type) {
   default:
     return false;
+  case R_X86_64_GOTTPOFF:
+  case R_X86_64_TPOFF32:
   case R_X86_64_DTPOFF32:
   case R_X86_64_DTPOFF64:
   case R_X86_64_PC8:
   case R_X86_64_PC16:
   case R_X86_64_PC32:
   case R_X86_64_PC64:
   case R_X86_64_PLT32:
     return true;

test/ELF/tls-initial-exec-local.s

-// REQUIRES: x86
-// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
-// RUN: ld.lld -shared %t.o -o %t
-// RUN: llvm-readobj -r -s %t | FileCheck %s
-// RUN: llvm-objdump -d %t | FileCheck --check-prefix=DISASM %s
-
-// CHECK:      Name: .got
-// CHECK-NEXT: Type: SHT_PROGBITS
-// CHECK-NEXT: Flags [
-// CHECK-NEXT:   SHF_ALLOC (0x2)
-// CHECK-NEXT:   SHF_WRITE (0x1)
-// CHECK-NEXT: ]
-// CHECK-NEXT: Address: 0x2090
-
-// CHECK:      Relocations [
-// CHECK-NEXT:   Section ({{.*}}) .rela.dyn {
-// CHECK-NEXT:     0x2090 R_X86_64_TPOFF64 - 0x0
-// CHECK-NEXT:     0x2098 R_X86_64_TPOFF64 - 0x4
-// CHECK-NEXT:   }
-// CHECK-NEXT: ]
-
-// 0x1007 + 4233 = 0x2090
-// 0x100e + 4234 = 0x2098
-// DISASM:      Disassembly of section .text:
-// DISASM-NEXT: .text:
-// DISASM-NEXT:  1000: {{.*}} addq      4233(%rip), %rax
-// DISASM-NEXT:  1007: {{.*}} addq      4234(%rip), %rax
-
-        addq    foo@GOTTPOFF(%rip), %rax
-        addq    bar@GOTTPOFF(%rip), %rax
-
-        .section        .tbss,"awT",@nobits
-foo:
-        .long 0
-bar:
-        .long 0

test/ELF/x86-64-tls-pie.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-cloudabi %s -o %t1.o
+# RUN: ld.lld -pie %t1.o -o %t
+# RUN: llvm-readobj -r %t | FileCheck %s
+
+# Bug 27174: R_X86_64_TPOFF32 and R_X86_64_GOTTPOFF relocations should
+# be eliminated when building a PIE executable, as the static TLS layout
+# is fixed.
+#
+# CHECK:      Relocations [
+# CHECK-NEXT: ]
+
+  .globl  _start
+_start:
+  movq  %fs:0, %rax
+  movl  $3, i@TPOFF(%rax)
+
+  movq  %fs:0, %rdx
+  movq  i@GOTTPOFF(%rip), %rcx
+  movl  $3, (%rdx,%rcx)
+
+  .section  .tbss.i,"awT",@nobits
+  .globl  i
+i:
+  .long 0
+  .size i, 4