dynrelro section for read-only dynamic symbols copied into executable

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.