[LLD][ELF][AArch64] Set _GLOBAL_OFFSET_TABLE_ at the start of .got

[zatrazz]

Summary

The commit 18aa0be36ed9 changed the default GotBaseSymInGotPlt to true
for AArch64. This is different than binutils, where
_GLOBAL_OFFSET_TABLE_ points at the start or .got instead of .got.plt.

It seems to not intefere with current relocations used by LLVM. However
as indicated by PR#40357 [1] gcc generates R_AARCH64_LD64_GOTPAGE_LO15
for -pie (in fact it also generated the relocation for -fpic in some
cases).

This change is requires to correctly handle R_AARCH64_LD64_GOTPAGE_LO15
by lld from objects generated by gcc.

[1] https://bugs.llvm.org/show_bug.cgi?id=40357

[MaskRay]

Confirmed.

aarch64-linux-gnu-gcc -nostdlib -shared a.c -fpic -o a.so
clang -target aarch64-linux-gnu -fuse-ld=lld -nostdlib -shared a.c -fpic -o a.so

The previous one does use .got for _GLOBAL_OFFSET_TABLE_. Thanks for spotting this issue.

// binutils elfnn-aarch64.c uses .got
  s = bfd_make_section_anyway_with_flags (abfd, ".got", flags);
  if (s == NULL
      || !bfd_set_section_alignment (s, bed->s->log_file_align))
    return FALSE;
  htab->sgot = s;
  htab->sgot->size += GOT_ENTRY_SIZE;

  if (bed->want_got_sym)
    {
      /* Define the symbol _GLOBAL_OFFSET_TABLE_ at the start of the .got
         (or .got.plt) section.  We don't do this in the linker script
         because we don't want to define the symbol if we are not creating
         a global offset table.  */
      h = _bfd_elf_define_linkage_sym (abfd, info, s,
                                       "_GLOBAL_OFFSET_TABLE_");
      elf_hash_table (info)->hgot = h;
      if (h == NULL)
        return FALSE;
    }

lld/ELF/Arch/AArch64.cpp

 //===- AArch64.cpp --------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
AArch64::AArch64() {
   pltRel = R_AARCH64_JUMP_SLOT;
   symbolicRel = R_AARCH64_ABS64;
   tlsDescRel = R_AARCH64_TLSDESC;
   tlsGotRel = R_AARCH64_TLS_TPREL64;
   pltHeaderSize = 32;
   pltEntrySize = 16;
   ipltEntrySize = 16;
   defaultMaxPageSize = 65536;
+  gotBaseSymInGotPlt = false;
 
   // Align to the 2 MiB page size (known as a superpage or huge page).
   // FreeBSD automatically promotes 2 MiB-aligned allocations.
   defaultImageBase = 0x200000;
 
   needsThunks = true;
 }
 

lld/test/ELF/global-offset-table-position-aarch64.s

 // CHECK: Name: _GLOBAL_OFFSET_TABLE_ (11)
 // CHECK-NEXT:     Value:
 // CHECK-NEXT:     Size: 0
 // CHECK-NEXT:     Binding: Local (0x0)
 // CHECK-NEXT:     Type: None (0x0)
 // CHECK-NEXT:     Other [ (0x2)
 // CHECK-NEXT:       STV_HIDDEN (0x2)
 // CHECK-NEXT:     ]
-// CHECK-NEXT:     Section: .got.plt
+// CHECK-NEXT:     Section: .got