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.

4 files changed, 23 insertions, 3 deletions

diff --git a/ld/emulparams/elf64_s390.sh b/ld/emulparams/elf64_s390.sh
index 5309774..8d02042 100644
--- a/ld/emulparams/elf64_s390.sh
+++ b/ld/emulparams/elf64_s390.sh
@@ -11,9 +11,12 @@ NOP=0x07070707
 TEMPLATE_NAME=elf32
 GENERATE_SHLIB_SCRIPT=yes
 GENERATE_PIE_SCRIPT=yes
+GENERATE_RELRO_SCRIPT=yes
 NO_SMALL_DATA=yes
 EXTRA_EM_FILE=s390
 IREL_IN_PLT=
+SEPARATE_GOTPLT=0
+test -z "$RELRO" && unset SEPARATE_GOTPLT
 
 # Treat a host that matches the target with the possible exception of "x"
 # in the name as if it were native.
diff --git a/ld/testsuite/ld-s390/gotreloc_64-1.dd b/ld/testsuite/ld-s390/gotreloc_64-norelro-1.dd
index 8c8c619..8c8c619 100644
--- a/ld/testsuite/ld-s390/gotreloc_64-1.dd
+++ b/ld/testsuite/ld-s390/gotreloc_64-norelro-1.dd
diff --git a/ld/testsuite/ld-s390/gotreloc_64-relro-1.dd b/ld/testsuite/ld-s390/gotreloc_64-relro-1.dd
new file mode 100644
index 0000000..36f5001
--- /dev/null
+++ b/ld/testsuite/ld-s390/gotreloc_64-relro-1.dd
@@ -0,0 +1,12 @@
+tmpdir/gotreloc_64-1:     file format elf64-s390
+
+Disassembly of section .text:
+
+.* <foo>:
+.*:	c0 10 00 00 00 0e [	 ]*larl	%r1,.* <bar>
+.*:	c0 10 00 00 00 0b [	 ]*larl	%r1,.* <bar>
+.*:	c4 1d 00 00 0f 1a [	 ]*lrl	%r1,.* <_GLOBAL_OFFSET_TABLE_\+0x18>
+.*:	58 10 c0 18 [	 ]*l	%r1,24\(%r12\)
+.*:	e3 10 c0 18 00 58 [	 ]*ly	%r1,24\(%r12\)
+.* <bar>:
+.*:	00 00 01 23       	.long	0x00000123
diff --git a/ld/testsuite/ld-s390/s390.exp b/ld/testsuite/ld-s390/s390.exp
index e391841..1934d92 100644
--- a/ld/testsuite/ld-s390/s390.exp
+++ b/ld/testsuite/ld-s390/s390.exp
@@ -70,10 +70,15 @@ set s390xtests {
      {{readelf -WSsrl tlsbin_64.rd} {objdump -dzrj.text tlsbin_64.dd}
       {objdump -sj.got tlsbin_64.sd} {objdump -sj.tdata tlsbin_64.td}}
      "tlsbin_64"}
-    {"GOT: symbol address load from got to larl"
-     "-shared -melf64_s390 --hash-style=sysv --version-script=gotreloc-1.ver" ""
+    {"GOT: norelro symbol address load from got to larl"
+     "-shared -melf64_s390 -z norelro --hash-style=sysv --version-script=gotreloc-1.ver" ""
+     "-m64" {gotreloc-1.s}
+     {{objdump -dzrj.text gotreloc_64-norelro-1.dd}}
+     "gotreloc_64-1"}
+    {"GOT: relro symbol address load from got to larl"
+     "-shared -melf64_s390 -z relro --hash-style=sysv --version-script=gotreloc-1.ver" ""
      "-m64" {gotreloc-1.s}
-     {{objdump -dzrj.text gotreloc_64-1.dd}}
+     {{objdump -dzrj.text gotreloc_64-relro-1.dd}}
      "gotreloc_64-1"}
     {"PLT: offset test"
      "-shared -m elf64_s390 -dT pltoffset-1.ld" ""