S/390: Improve partial relro support for 64 bit

Currently on S/390 the .got.plt always comes first which prevents the
GNU_RELRO segment from being extended across the non-plt GOT entries.

Just swapping both unfortunately is not that simple since our ABI
requires the _GLOBAL_OFFSET_TABLE_ symbol to point to the very
beginning of the entire GOT. Of the 3 magic GOT entries the first is
accessed via got pointer while second and third are being accessed via
DT_PLTGOT.  In order to keep them together we make DT_PLTGOT to point
to the .got instead of .got.plt.  However, this violates an assumption
in the dynamic linker prelink undo code about the GOTPLT entries
starting at DT_PLTGOT + 3.  We got rid of this requirement with a
Glibc patch already in version 2.24:
https://sourceware.org/ml/libc-alpha/2016-06/msg01302.html

So the S/390 relro GOT layout will look like this with this patch:

      +----------------------------------+
      |got[0]: DYNAMIC                   |   <--- _GLOBAL_OFFSET_TABLE_ == DT_PLTGOT .got
      |got[1]: link_map parm             |
      |got[2]: &_dl_runtime_resolve      |
      +----------------------------------+
      |                                  |   non-plt GOT entries
      |                                  |
      |                                  |
      +----------------------------------+
      |                                  |   <--- .gotplt, PLT GOT entries
      |                                  |
      |                                  |
      |                                  |
      +----------------------------------+

The patch detects the current layout in size_dynamic_section in order
to deal also with linker scripts not generated by this ld version.
With partial relro enabled we pick a linker script where .got and
.got.plt are swapped which then triggers the rest of the logic.

ld/ChangeLog:

2018-07-18  Andreas Krebbel  <krebbel@linux.ibm.com>

	* emulparams/elf64_s390.sh: Define GENERATE_RELRO_SCRIPT and
	SEPARATE_GOTPLT.
	* testsuite/ld-s390/gotreloc_64-relro-1.dd: New test.
	* testsuite/ld-s390/gotreloc_64-norelro-1.dd: Renamed from ...
	* testsuite/ld-s390/gotreloc_64-1.dd: ... this.
	* testsuite/ld-s390/s390.exp: Split the GOT testcase into two.

bfd/ChangeLog:

2018-07-18  Andreas Krebbel  <krebbel@linux.ibm.com>

	* elf-s390-common.c (s390_gotplt_after_got_p): New function.
	(s390_got_pointer): New function.
	(s390_got_offset): New function.
	(s390_gotplt_offset): New function.
	* elf64-s390.c (allocate_dynrelocs): Adjust comment.
	(elf_s390_size_dynamic_sections): Move space for magic GOT entries
	from .got.plt to .got if necessary and pick the right location for
	_GLOBAL_OFFSET_TABLE_.
	(elf_s390_relocate_section): Use the wrapper functions from
	elf-s390-common.c to deal with both possible layouts (either .got
	or .got.plt first).
	(elf_s390_finish_dynamic_sections): Likewise.
	(elf_s390_finish_dynamic_symbol): Make the location of the GOT
	magic entries conditional.

1 files changed, 81 insertions, 0 deletions

diff --git a/bfd/elf-s390-common.c b/bfd/elf-s390-common.c
index b37cb25..032ba27 100644
--- a/bfd/elf-s390-common.c
+++ b/bfd/elf-s390-common.c
@@ -30,6 +30,87 @@ s390_is_ifunc_symbol_p (struct elf_link_hash_entry *h)
   return h->type == STT_GNU_IFUNC || eh->ifunc_resolver_address != 0;
 }
 
+/* Return true if .got.plt is supposed to be emitted after .got.  */
+
+static inline bfd_boolean
+s390_gotplt_after_got_p (struct bfd_link_info *info)
+{
+  struct elf_s390_link_hash_table *htab = elf_s390_hash_table (info);
+
+  if (!htab->elf.sgot || !htab->elf.sgotplt)
+    return TRUE;
+
+  if (htab->elf.sgot->output_section == htab->elf.sgotplt->output_section)
+    {
+      if (htab->elf.sgot->output_offset < htab->elf.sgotplt->output_offset)
+	return TRUE;
+    }
+  else
+    {
+      if (htab->elf.sgot->output_section->vma
+	  <= htab->elf.sgotplt->output_section->vma)
+	return TRUE;
+    }
+  return FALSE;
+}
+
+/* Return the value of the _GLOBAL_OFFSET_TABLE_ symbol.  */
+
+static inline bfd_vma
+s390_got_pointer (struct bfd_link_info *info)
+{
+  struct elf_s390_link_hash_table *htab = elf_s390_hash_table (info);
+  bfd_vma got_pointer;
+
+  BFD_ASSERT (htab && htab->elf.hgot);
+
+  got_pointer = (htab->elf.hgot->root.u.def.section->output_section->vma
+		 + htab->elf.hgot->root.u.def.section->output_offset);
+  /* Our ABI requires the GOT pointer to point at the very beginning
+     of the global offset table.  */
+  BFD_ASSERT (got_pointer
+	      <= (htab->elf.sgot->output_section->vma
+		  + htab->elf.sgot->output_offset));
+  BFD_ASSERT (got_pointer
+	      <= (htab->elf.sgotplt->output_section->vma
+		  + htab->elf.sgotplt->output_offset));
+
+  return got_pointer;
+}
+
+
+/* Return the offset of the .got versus _GLOBAL_OFFSET_TABLE_.  */
+
+static inline bfd_vma
+s390_got_offset (struct bfd_link_info *info)
+{
+  struct elf_s390_link_hash_table *htab = elf_s390_hash_table (info);
+
+  /* The absolute address of the .got in the target image.  */
+  bfd_vma got_address = (htab->elf.sgot->output_section->vma
+			 + htab->elf.sgot->output_offset);
+
+  /* GOT offset must not be negative.  */
+  BFD_ASSERT (s390_got_pointer (info) <= got_address);
+  return got_address - s390_got_pointer (info);
+}
+
+/* Return the offset of the .got.plt versus _GLOBAL_OFFSET_TABLE_.  */
+
+static inline bfd_vma
+s390_gotplt_offset (struct bfd_link_info *info)
+{
+  struct elf_s390_link_hash_table *htab = elf_s390_hash_table (info);
+
+  /* The absolute address of the .got.plt in the target image.  */
+  bfd_vma gotplt_address = (htab->elf.sgotplt->output_section->vma
+			    + htab->elf.sgotplt->output_offset);
+
+  /* GOT offset must not be negative.  */
+  BFD_ASSERT (s390_got_pointer (info) <= gotplt_address);
+  return gotplt_address - s390_got_pointer (info);
+}
+
 /* Create sections needed by STT_GNU_IFUNC symbol.  */
 
 static bfd_boolean