s390: Avoid reloc overflows on undefined weak symbols

Replace relative long addressing instructions of weak symbols, which
will definitely resolve to zero, with either a load address of 0, a
NOP, or a trapping insn.

This prevents the PC32DBL relocation from overflowing in case the
binary will be loaded at 4GB or more.

bfd/ChangeLog:

	* bfd/elf64-s390.c (elf_s390_relocate_section): Replace
	instructions using undefined weak symbols with relative addressing
	to avoid relocation overflows.

ld/ChangeLog:
	* ld/testsuite/ld-s390/s390.exp:
	* ld/testsuite/ld-s390/8GB.ld: New test.
	* ld/testsuite/ld-s390/weakundef-1.dd: New test.
	* ld/testsuite/ld-s390/weakundef-1.s: New test.

bfd/elf64-s390.c

@@ -2475,6 +2475,60 @@ elf_s390_relocate_section (bfd *output_bfd,
 			    + h->plt.offset);
 	      goto do_relocation;
 	    }
+
+	  /* Replace relative long addressing instructions of weak
+	     symbols, which will definitely resolve to zero, with
+	     either a load address of 0, a NOP, or a trapping insn.
+	     This prevents the PC32DBL relocation from overflowing in
+	     case the binary will be loaded at 4GB or more.  */
+	  if (h != NULL
+	      && h->root.type == bfd_link_hash_undefweak
+	      && !h->root.linker_def
+	      && (bfd_link_executable (info)
+		  || ELF_ST_VISIBILITY (h->other) != STV_DEFAULT)
+	      && r_type == R_390_PC32DBL)
+	    {
+	      void *insn_start = contents + rel->r_offset - 2;
+	      uint16_t op = bfd_get_16 (input_bfd, insn_start) & 0xff0f;
+	      uint8_t reg = bfd_get_8 (input_bfd, insn_start + 1) & 0xf0;
+
+	      /* NOTE: The order of the if's is important!  */
+	      /* Replace load address relative long (larl) with load
+		 address (lay) */
+	      if (op == 0xc000)
+		{
+		  /* larl rX,<weak sym> -> lay rX,0(0)  */
+		  bfd_put_16 (output_bfd, 0xe300 | reg, insn_start);
+		  bfd_put_32 (output_bfd, 0x71, insn_start + 2);
+		  continue;
+		}
+	      /* Replace prefetch data relative long (pfdrl) with a NOP  */
+	      else if (op == 0xc602)
+		{
+		  /* Emit a 6-byte NOP: jgnop .  */
+		  bfd_put_16 (output_bfd, 0xc004, insn_start);
+		  bfd_put_32 (output_bfd, 0x0, insn_start + 2);
+		  continue;
+		}
+	      /* Replace the following instructions with a trap:
+		 - branch relative and save long (brasl)
+		 - load (logical) relative long (lrl, lgrl, lgfrl, llgfrl)
+		 - load (logical) halfword relative long (lhrl, lghrl, llhrl, llghrl)
+		 - store relative long (strl, stgrl)
+		 - store halfword relative long (sthrl)
+		 - execute relative long (exrl)
+		 - compare (logical) relative long (crl, clrl, cgrl, clgrl, cgfrl, clgfrl)
+		 - compare (logical) halfword relative long (chrl, cghrl, clhrl, clghrl)
+		 - branch relative on count high (brcth)  */
+	      else if (op == 0xc005 || (op & 0xff00) == 0xc400
+		       || (op & 0xff00) == 0xc600 || op == 0xcc06)
+		{
+		  /* Emit a 6-byte trap: jg .+2  */
+		  bfd_put_16 (output_bfd, 0xc0f4, insn_start);
+		  bfd_put_32 (output_bfd, 0x1, insn_start + 2);
+		  continue;
+		}
+	    }
 	  /* Fall through.  */
 
 	case R_390_8:

ld/testsuite/ld-s390/8GB.ld

@@ -0,0 +1 @@
+SECTIONS { . = 0x200000000; }

ld/testsuite/ld-s390/s390.exp

@@ -85,6 +85,9 @@ set s390xtests {
      "-m64" {pltoffset-1.s}
      {{objdump "-dzrj.text --stop-address=16" pltoffset-1.dd}}
      "pltoffset-1"}
+    {"WEAKUNDEF1: overflow test"
+     "-m elf64_s390 -dT 8GB.ld --no-error-rwx-segments" "" "-m64" {weakundef-1.s}
+     {{objdump "-dzrj.text" weakundef-1.dd}} "weakundef-1"}
 }
 
 if [istarget "s390-*-*"] {

ld/testsuite/ld-s390/weakundef-1.dd

@@ -0,0 +1,15 @@
+tmpdir/weakundef-1:     file format elf64-s390
+
+Disassembly of section .text:
+
+.* <foo>:
+.*:	c0 10 00 00 00 1e [	 ]*larl	%r1,20000003c <d>
+.*:	c0 10 00 00 00 1f [	 ]*larl	%r1,200000044 <wd>
+.*:	e3 10 00 00 00 71 [	 ]*lay	%r1,0
+.*:	c0 f4 00 00 00 01 [	 ]*jg	.*
+.*:	c0 f4 00 00 00 01 [	 ]*jg	.*
+.*:	c0 f4 00 00 00 01 [	 ]*jg	.*
+.*:	c0 f4 00 00 00 01 [	 ]*jg	.*
+.*:	c0 f4 00 00 00 01 [	 ]*jg	.*
+.*:	c0 f4 00 00 00 01 [	 ]*jg	.*
+.*:	c0 04 00 00 00 00 [	 ]*jgnop	.*

ld/testsuite/ld-s390/weakundef-1.s

@@ -0,0 +1,18 @@
+.text
+	.globl foo
+foo:
+	larl	%r1,d
+	larl	%r1,wd
+	larl	%r1,wu
+	brasl	%r1,wu
+	crl	%r1,wu
+	lrl	%r1,wu
+	strl	%r1,wu
+	exrl	%r1,wu
+	brcth	%r1,wu
+	pfdrl	%r1,wu
+	.weak	wd
+	.weak	wu
+.data
+d:	.quad 0x123
+wd:	.quad 0x123