dynrelro section for read-only dynamic symbols copied into executable

Variables defined in shared libraries are copied into an executable's
.bss section when code in the executable is non-PIC and thus would
require dynamic text relocations to access the variable directly in
the shared library.  Recent x86 toolchains also copy variables into
the executable to gain a small speed improvement.

The problem is that if the variable was originally read-only, the copy
in .bss is writable, potentially opening a security hole.  This patch
cures that problem by putting the copy in a section that becomes
read-only after ld.so relocation, provided -z relro is in force.

The patch also fixes a microblaze linker segfault on attempting to
use dynamic bss variables.

bfd/
	PR ld/20995
	* elf-bfd.h (struct elf_link_hash_table): Add sdynrelro and
	sreldynrelro.
	(struct elf_backend_data): Add want_dynrelro.
	* elfxx-target.h (elf_backend_want_dynrelro): Define.
	(elfNN_bed): Update initializer.
	* elflink.c (_bfd_elf_create_dynamic_sections): Create
	sdynrelro and sreldynrelro sections.
	* elf32-arm.c (elf32_arm_adjust_dynamic_symbol): Place variables
	copied into the executable from read-only sections into sdynrelro.
	(elf32_arm_size_dynamic_sections): Handle sdynrelro.
	(elf32_arm_finish_dynamic_symbol): Select sreldynrelro for
	dynamic relocs in sdynrelro.
	(elf_backend_want_dynrelro): Define.
	* elf32-hppa.c (elf32_hppa_adjust_dynamic_symbol)
	(elf32_hppa_size_dynamic_sections, elf32_hppa_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf32-i386.c (elf_i386_adjust_dynamic_symbol)
	(elf_i386_size_dynamic_sections, elf_i386_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf32-metag.c (elf_metag_adjust_dynamic_symbol)
	(elf_metag_size_dynamic_sections, elf_metag_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf32-microblaze.c (microblaze_elf_adjust_dynamic_symbol)
	(microblaze_elf_size_dynamic_sections)
	(microblaze_elf_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf32-nios2.c (nios2_elf32_finish_dynamic_symbol)
	(nios2_elf32_adjust_dynamic_symbol)
	(nios2_elf32_size_dynamic_sections)
	(elf_backend_want_dynrelro): As above.
	* elf32-or1k.c (or1k_elf_finish_dynamic_symbol)
	(or1k_elf_adjust_dynamic_symbol, or1k_elf_size_dynamic_sections)
	(elf_backend_want_dynrelro): As above.
	* elf32-ppc.c (ppc_elf_adjust_dynamic_symbol)
	(ppc_elf_size_dynamic_sections, ppc_elf_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf32-s390.c (elf_s390_adjust_dynamic_symbol)
	(elf_s390_size_dynamic_sections, elf_s390_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf32-tic6x.c (elf32_tic6x_adjust_dynamic_symbol)
	(elf32_tic6x_size_dynamic_sections)
	(elf32_tic6x_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf32-tilepro.c (tilepro_elf_adjust_dynamic_symbol)
	(tilepro_elf_size_dynamic_sections)
	(tilepro_elf_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf64-ppc.c (ppc64_elf_adjust_dynamic_symbol)
	(ppc64_elf_size_dynamic_sections, ppc64_elf_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf64-s390.c (elf_s390_adjust_dynamic_symbol)
	(elf_s390_size_dynamic_sections, elf_s390_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elf64-x86-64.c (elf_x86_64_adjust_dynamic_symbol)
	(elf_x86_64_size_dynamic_sections)
	(elf_x86_64_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elfnn-aarch64.c (elfNN_aarch64_adjust_dynamic_symbol)
	(elfNN_aarch64_size_dynamic_sections)
	(elfNN_aarch64_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elfnn-riscv.c (riscv_elf_adjust_dynamic_symbol)
	(riscv_elf_size_dynamic_sections, riscv_elf_finish_dynamic_symbol)
	(elf_backend_want_dynrelro): As above.
	* elfxx-mips.c (_bfd_mips_elf_adjust_dynamic_symbol)
	(_bfd_mips_elf_size_dynamic_sections)
	(_bfd_mips_vxworks_finish_dynamic_symbol): As above.
	* elfxx-sparc.c (_bfd_sparc_elf_adjust_dynamic_symbol)
	(_bfd_sparc_elf_size_dynamic_sections)
	(_bfd_sparc_elf_finish_dynamic_symbol): As above.
	* elfxx-tilegx.c (tilegx_elf_adjust_dynamic_symbol)
	(tilegx_elf_size_dynamic_sections)
	(tilegx_elf_finish_dynamic_symbol): As above.
	* elf32-mips.c (elf_backend_want_dynrelro): Define.
	* elf64-mips.c (elf_backend_want_dynrelro): Define.
	* elf32-sparc.c (elf_backend_want_dynrelro): Define.
	* elf64-sparc.c (elf_backend_want_dynrelro): Define.
	* elf32-tilegx.c (elf_backend_want_dynrelro): Define.
	* elf64-tilegx.c (elf_backend_want_dynrelro): Define.
	* elf32-microblaze.c (microblaze_elf_adjust_dynamic_symbol): Tidy.
	(microblaze_elf_size_dynamic_sections): Handle sdynbss.
	* elf32-nios2.c (nios2_elf32_size_dynamic_sections): Make use
	of linker shortcuts to dynamic sections rather than comparing
	names.  Correctly set "got" flag.
ld/
	PR ld/20995
	* testsuite/ld-arm/farcall-mixed-app-v5.d: Update to suit changed
	stub hash table traversal caused by section id increment.  Accept
	the previous output too.
	* testsuite/ld-arm/farcall-mixed-app.d: Likewise.
	* testsuite/ld-arm/farcall-mixed-lib-v4t.d: Likewise.
	* testsuite/ld-arm/farcall-mixed-lib.d: Likewise.
	* testsuite/ld-elf/pr20995a.s, * testsuite/ld-elf/pr20995b.s,
	* testsuite/ld-elf/pr20995.r: New test.
	* testsuite/ld-elf/elf.exp: Run it.

1 files changed, 24 insertions, 8 deletions

diff --git a/bfd/elfxx-mips.c b/bfd/elfxx-mips.c
index 086d537..7c7cd38 100644
--- a/bfd/elfxx-mips.c
+++ b/bfd/elfxx-mips.c
@@ -9129,6 +9129,7 @@ _bfd_mips_elf_adjust_dynamic_symbol (struct bfd_link_info *info,
   bfd *dynobj;
   struct mips_elf_link_hash_entry *hmips;
   struct mips_elf_link_hash_table *htab;
+  asection *s, *srel;
 
   htab = mips_elf_hash_table (info);
   BFD_ASSERT (htab != NULL);
@@ -9371,10 +9372,20 @@ _bfd_mips_elf_adjust_dynamic_symbol (struct bfd_link_info *info,
      both the dynamic object and the regular object will refer to the
      same memory location for the variable.  */
 
+  if ((h->root.u.def.section->flags & SEC_READONLY) != 0)
+    {
+      s = htab->root.sdynrelro;
+      srel = htab->root.sreldynrelro;
+    }
+  else
+    {
+      s = htab->root.sdynbss;
+      srel = htab->root.srelbss;
+    }
   if ((h->root.u.def.section->flags & SEC_ALLOC) != 0)
     {
       if (htab->is_vxworks)
-	htab->root.srelbss->size += sizeof (Elf32_External_Rela);
+	srel->size += sizeof (Elf32_External_Rela);
       else
 	mips_elf_allocate_dynamic_relocations (dynobj, info, 1);
       h->needs_copy = 1;
@@ -9384,7 +9395,7 @@ _bfd_mips_elf_adjust_dynamic_symbol (struct bfd_link_info *info,
      dynamic will now refer to the local copy instead.  */
   hmips->possibly_dynamic_relocs = 0;
 
-  return _bfd_elf_adjust_dynamic_copy (info, h, htab->root.sdynbss);
+  return _bfd_elf_adjust_dynamic_copy (info, h, s);
 }
 
 /* This function is called after all the input files have been read,
@@ -9906,7 +9917,8 @@ _bfd_mips_elf_size_dynamic_sections (bfd *output_bfd,
 	       && s != htab->root.sgot
 	       && s != htab->root.sgotplt
 	       && s != htab->sstubs
-	       && s != htab->root.sdynbss)
+	       && s != htab->root.sdynbss
+	       && s != htab->root.sdynrelro)
 	{
 	  /* It's not one of our sections, so don't allocate space.  */
 	  continue;
@@ -11296,6 +11308,8 @@ _bfd_mips_vxworks_finish_dynamic_symbol (bfd *output_bfd,
   if (h->needs_copy)
     {
       Elf_Internal_Rela rel;
+      asection *srel;
+      bfd_byte *loc;
 
       BFD_ASSERT (h->dynindx != -1);
 
@@ -11304,11 +11318,13 @@ _bfd_mips_vxworks_finish_dynamic_symbol (bfd *output_bfd,
 		      + h->root.u.def.value);
       rel.r_info = ELF32_R_INFO (h->dynindx, R_MIPS_COPY);
       rel.r_addend = 0;
-      bfd_elf32_swap_reloca_out (output_bfd, &rel,
-				 htab->root.srelbss->contents
-				 + (htab->root.srelbss->reloc_count
-				    * sizeof (Elf32_External_Rela)));
-      ++htab->root.srelbss->reloc_count;
+      if ((h->root.u.def.section->flags & SEC_READONLY) != 0)
+	srel = htab->root.sreldynrelro;
+      else
+	srel = htab->root.srelbss;
+      loc = srel->contents + srel->reloc_count * sizeof (Elf32_External_Rela);
+      bfd_elf32_swap_reloca_out (output_bfd, &rel, loc);
+      ++srel->reloc_count;
     }
 
   /* If this is a mips16/microMIPS symbol, force the value to be even.  */