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.
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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.texi), as well as the
beginnings of an internals manual (ldint.texi).
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.texi 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) 2012-2018 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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