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.
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README for LD
This is the GNU linker. It is distributed with other "binary
utilities" which should be in ../binutils. See ../binutils/README for
more general notes, including where to send bug reports.
There are many features of the linker:
* The linker uses a Binary File Descriptor library (../bfd)
that it uses to read and write object files. This helps
insulate the linker itself from the format of object files.
* The linker supports a number of different object file
formats. It can even handle multiple formats at once:
Read two input formats and write a third.
* The linker can be configured for cross-linking.
* The linker supports a control language.
* There is a user manual (ld.texinfo), as well as the
beginnings of an internals manual (ldint.texinfo).
Installation
============
See ../binutils/README.
If you want to make a cross-linker, you may want to specify
a different search path of -lfoo libraries than the default.
You can do this by setting the LIB_PATH variable in ./Makefile
or using the --with-lib-path configure switch.
To build just the linker, make the target all-ld from the top level
directory (one directory above this one).
Porting to a new target
=======================
See the ldint.texinfo manual.
Reporting bugs etc
===========================
See ../binutils/README.
Known problems
==============
The Solaris linker normally exports all dynamic symbols from an
executable. The GNU linker does not do this by default. This is
because the GNU linker tries to present the same interface for all
similar targets (in this case, all native ELF targets). This does not
matter for normal programs, but it can make a difference for programs
which try to dlopen an executable, such as PERL or Tcl. You can make
the GNU linker export all dynamic symbols with the -E or
--export-dynamic command line option.
HP/UX 9.01 has a shell bug that causes the linker scripts to be
generated incorrectly. The symptom of this appears to be "fatal error
- scanner input buffer overflow" error messages. There are various
workarounds to this:
* Build and install bash, and build with "make SHELL=bash".
* Update to a version of HP/UX with a working shell (e.g., 9.05).
* Replace "(. ${srcdir}/scripttempl/${SCRIPT_NAME}.sc)" in
genscripts.sh with "sh ${srcdir}..." (no parens) and make sure the
emulparams script used exports any shell variables it sets.
Copyright (C) 2012-2016 Free Software Foundation, Inc.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.
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