When dynamically linking, ifunc resolvers are called before TLS is
initialized, so they cannot be safely stack-protected.
We avoid disabling stack-protection on large numbers of files by
using __attribute__ ((__optimize__ ("-fno-stack-protector")))
to turn it off just for the resolvers themselves. (We provide
the attribute even when statically linking, because we will later
use it elsewhere too.)
For many years, the only effect of these macros has been to make
unistd.h declare getlogin_r. _POSIX_C_SOURCE >= 199506L also causes
this function to be declared. However, people who don't carefully
read all the headers might be confused into thinking they need to
define _REENTRANT for any threaded code (as was indeed the case a long
time ago).
Therefore, remove __USE_REENTRANT, and make _REENTRANT and _THREAD_SAFE
into synonyms for _POSIX_C_SOURCE=199506L. This will only affect
programs that don't select a higher conformance level some other way.
For instance, -std=c89 -D_REENTRANT will see a change in visible
declarations, but -std=c99 -D_POSIX_C_SOURCE=200809L -D_REENTRANT won't,
and -D_REENTRANT all by itself also won't, because _DEFAULT_SOURCE
implies _POSIX_C_SOURCE > 199506.
* include/features.h: Remove __USE_REENTRANT. Treat _REENTRANT
and _THREAD_SAFE the same as _POSIX_C_SOURCE=199506L, if a higher
POSIX conformance level has not been selected by other macros.
* NEWS, manual/creature.texi: Document this change.
* posix/unistd.h, posix/bits/unistd.h: Don't check __USE_REENTRANT.
* include/libc-symbols.h: Don't define _REENTRANT.
* scripts/check-installed-headers.sh: Don't undefine _REENTRANT.
This patch uses the libc_ifunc_hidden macro to create already existing ifunc functions
time and gettimeofday on intel. This way, the libc_hidden_def macro can be used
instead of the libc_ifunc_hidden_def one which was only used here. Thus the
macro is removed from libc-symbols.h.
On i386, the __GI_* symbols do not target the ifunc symbol and thus the
redirection construct has to be applied here.
ChangeLog:
* sysdeps/unix/sysv/linux/x86/gettimeofday.c (__gettimeofday):
Use libc_ifunc_hidden macro. Use libc_hidden_def instead of
libc_ifunc_hidden_def.
* sysdeps/unix/sysv/linux/x86/time.c (time): Likewise.
* sysdeps/unix/sysv/linux/i386/gettimeofday.c (__gettimeofday):
Redirect ifunced function in header for using it as type of ifunc'ed
function. Redefine libc_hidden_def to use fallback non ifunc'ed
function for __GI_* symbol.
* sysdeps/unix/sysv/linux/i386/time.c (time): Likewise.
* include/libc-symbols.h
(libc_ifunc_hidden_def, libc_ifunc_hidden_def1): Delete macro.
The current s390 ifunc resolver for vector optimized functions and the common
libc_ifunc macro in include/libc-symbols.h uses something like that to generate ifunc'ed functions:
extern void *__resolve___strlen(unsigned long int dl_hwcap) asm (strlen);
asm (".type strlen, %gnu_indirect_function");
This leads to false debug information:
objdump --dwarf=info libc.so:
...
<1><1e6424>: Abbrev Number: 43 (DW_TAG_subprogram)
<1e6425> DW_AT_external : 1
<1e6425> DW_AT_name : (indirect string, offset: 0x1146e): __resolve___strlen
<1e6429> DW_AT_decl_file : 1
<1e642a> DW_AT_decl_line : 23
<1e642b> DW_AT_linkage_name: (indirect string, offset: 0x1147a): strlen
<1e642f> DW_AT_prototyped : 1
<1e642f> DW_AT_type : <0x1e4ccd>
<1e6433> DW_AT_low_pc : 0x998e0
<1e643b> DW_AT_high_pc : 0x16
<1e6443> DW_AT_frame_base : 1 byte block: 9c (DW_OP_call_frame_cfa)
<1e6445> DW_AT_GNU_all_call_sites: 1
<1e6445> DW_AT_sibling : <0x1e6459>
<2><1e6449>: Abbrev Number: 44 (DW_TAG_formal_parameter)
<1e644a> DW_AT_name : (indirect string, offset: 0x1845): dl_hwcap
<1e644e> DW_AT_decl_file : 1
<1e644f> DW_AT_decl_line : 23
<1e6450> DW_AT_type : <0x1e4c8d>
<1e6454> DW_AT_location : 0x122115 (location list)
...
The debuginfo for the ifunc-resolver function contains the DW_AT_linkage_name
field, which names the real function name "strlen". If you perform an inferior
function call to strlen in lldb, then it fails due to something like that:
"error: no matching function for call to 'strlen'
candidate function not viable: no known conversion from 'const char [6]'
to 'unsigned long' for 1st argument"
The unsigned long is the dl_hwcap argument of the resolver function.
The strlen function itself has no debufinfo.
The s390 ifunc resolver for memset & co uses something like that:
asm (".globl FUNC"
".type FUNC, @gnu_indirect_function"
".set FUNC, __resolve_FUNC");
This way the debuginfo for the ifunc-resolver function does not conain the
DW_AT_linkage_name field and the real function has no debuginfo, too.
Using this strategy for the vector optimized functions leads to some troubles
for functions like strnlen. Here we have __strnlen and a weak alias strnlen.
The __strnlen function is the ifunc function, which is realized with the asm-
statement above. The weak_alias-macro can't be used here due to undefined symbol:
gcc ../sysdeps/s390/multiarch/strnlen.c -c ...
In file included from <command-line>:0:0:
../sysdeps/s390/multiarch/strnlen.c:28:24: error: ‘strnlen’ aliased to undefined symbol ‘__strnlen’
weak_alias (__strnlen, strnlen)
^
./../include/libc-symbols.h:111:26: note: in definition of macro ‘_weak_alias’
extern __typeof (name) aliasname __attribute__ ((weak, alias (#name)));
^
../sysdeps/s390/multiarch/strnlen.c:28:1: note: in expansion of macro ‘weak_alias’
weak_alias (__strnlen, strnlen)
^
make[2]: *** [build/string/strnlen.o] Error 1
As the __strnlen function is defined with asm-statements the function name
__strnlen isn't known by gcc. But the weak alias can also be done with an
asm statement to resolve this issue:
__asm__ (".weak strnlen\n\t"
".set strnlen,__strnlen\n");
In order to use the weak_alias macro, gcc needs to know the ifunc function. The
minimum gcc to build glibc is currently 4.7, which supports attribute((ifunc)).
See https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Function-Attributes.html.
It is only supported if gcc is configured with --enable-gnu-indirect-function
or gcc supports it by default for at least intel and s390x architecture.
This patch uses the old behaviour if gcc support is not available.
Usage of attribute ifunc is something like that:
__typeof (FUNC) FUNC __attribute__ ((ifunc ("__resolve_FUNC")));
Then gcc produces the same .globl, .type, .set assembler instructions like above.
And the debuginfo does not contain the DW_AT_linkage_name field and there is no
debuginfo for the real function, too.
But in order to get it work, there is also some extra work to do.
Currently, the glibc internal symbol on s390x e.g. __GI___strnlen is not the
ifunc symbol, but the fallback __strnlen_c symbol. Thus I have to omit the
libc_hidden_def macro in strnlen.c (here is the ifunc function __strnlen)
because it is already handled in strnlen-c.c (here is __strnlen_c).
Due to libc_hidden_proto (__strnlen) in string.h, compiling fails:
gcc ../sysdeps/s390/multiarch/strnlen.c -c ...
In file included from <command-line>:0:0:
../sysdeps/s390/multiarch/strnlen.c:53:24: error: ‘strnlen’ aliased to undefined symbol ‘__strnlen’
weak_alias (__strnlen, strnlen)
^
./../include/libc-symbols.h:111:26: note: in definition of macro ‘_weak_alias’
extern __typeof (name) aliasname __attribute__ ((weak, alias (#name)));
^
../sysdeps/s390/multiarch/strnlen.c:53:1: note: in expansion of macro ‘weak_alias’
weak_alias (__strnlen, strnlen)
^
make[2]: *** [build/string/strnlen.os] Error 1
I have to redirect the prototypes for __strnlen in string.h and create a copy
of the prototype for using as ifunc function:
__typeof (__redirect___strnlen) __strnlen __attribute__ ((ifunc ("__resolve_strnlen")));
weak_alias (__strnlen, strnlen)
This way there is no trouble with the internal __GI_* symbols.
Glibc builds fine with this construct and the debuginfo is "correct".
For functions without a __GI_* symbol like memccpy this redirection is not needed.
This patch adjusts the common libc_ifunc and libm_ifunc macro to use gcc
attribute ifunc. Due to this change, the macro users where the __GI_* symbol
does not target the ifunc symbol have to be prepared with the redirection
construct.
Furthermore a configure check to test gcc support is added. If it is not supported,
the old behaviour is used.
This patch also prepares the libc_ifunc macro to be useable in s390-ifunc-macro.
The s390 ifunc-resolver-functions do have an hwcaps parameter and not all
resolvers need the same initialization code. The next patch in this series
changes the s390 ifunc macros to use this common one.
ChangeLog:
* include/libc-symbols.h (__ifunc_resolver):
New macro is used by __ifunc* macros.
(__ifunc): New macro uses gcc attribute ifunc or inline assembly
depending on HAVE_GCC_IFUNC.
(libc_ifunc, libm_ifunc): Use __ifunc as base macro.
(libc_ifunc_redirected, libc_ifunc_hidden, libm_ifunc_init): New macro.
* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_finite.c:
Redirect ifunced function in header for using as type for ifunc function.
* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_finitef.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_isinf.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_isinff.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_isnan.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/memcmp.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/memcpy.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/memmove.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/mempcpy.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/memset.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/rawmemchr.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/strchr.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/strlen.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/strncmp.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/multiarch/strnlen.c: Likewise.
* sysdeps/powerpc/powerpc64/fpu/multiarch/s_finite.c: Likewise.
* sysdeps/powerpc/powerpc64/fpu/multiarch/s_finitef.c: Likewise.
* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isinf.c: Likewise.
* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isinff.c: Likewise.
* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isnan.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/memcmp.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/mempcpy.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/rawmemchr.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/stpncpy.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/strcat.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/strchr.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/strcmp.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/strcpy.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/strncmp.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/strncpy.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/strnlen.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/strrchr.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/strstr.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/wcschr.c: Likewise.
* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_isnanf.c:
Add libc_hidden_def() and use libc_ifunc_hidden() macro
instead of libc_ifunc() macro.
* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isnanf.c: Likewise.
* sysdeps/powerpc/powerpc64/multiarch/stpcpy.c: Likewise.
Existing interposed mallocs do not define the glibc-internal
fork callbacks (and they should not), so statically interposed
mallocs lead to link failures because the strong reference from
fork pulls in glibc's malloc, resulting in multiple definitions
of malloc-related symbols.
There are configure tests for assembler .weak support, and, as a
fallback, for .weakext support.
.weakext appears to be an ECOFF thing (although a few ELF targets
support it as well). .weak has been supported by the GNU assembler
for ELF targets since version 2.2, so given the requirement for ELF
the configure tests are obsolete; this patch removes them.
Tested for x86_64 (testsuite, and that installed shared libraries are
unchanged by the patch).
* configure.ac (libc_cv_asm_weak_directive): Remove configure
test.
(libc_cv_asm_weakext_directive): Likewise.
* configure: Regenerated.
* config.h.in (HAVE_ASM_WEAK_DIRECTIVE): Remove #undef.
(HAVE_ASM_WEAKEXT_DIRECTIVE): Likewise.
* include/libc-symbols.h
[!HAVE_ASM_WEAK_DIRECTIVE && !HAVE_ASM_WEAKEXT_DIRECTIVE]: Remove
#error.
[HAVE_ASM_WEAKEXT_DIRECTIVE]: Remove conditional code.
[!HAVE_ASM_WEAKEXT_DIRECTIVE]: Make code unconditional.
There is a configure test for the assembler .previous directive, and,
as a fallback, for .popsection.
glibc now only supports ELF. For ELF, the GNU assembler has supported
.previous since version 2.2 (support added by
Mon Jul 19 15:21:20 1993 Ken Raeburn (raeburn@rtl.cygnus.com)
* config/obj-elf.c (obj_elf_previous): New function.
(previous_section, previous_subsection): New vars.
(obj_elf_section): Save current place in case DWARF code wants us
to pop back to it. Handle unquoted section name as well as quoted
section name. Don't crash on invalid strings.
(obj_pseudo_table): Handle new pseudos "previous", "2byte", and
"4byte".
). Thus this configure test is obsolete, and this patch removes it
(and with it the fallback .popsection test).
Tested for x86_64 and x86 (testsuite, and that installed shared
libraries are unchanged by the patch).
* configure.ac (libc_cv_asm_previous_directive): Remove configure
test.
(libc_cv_asm_popsection_directive): Likewise.
* configure: Regenerated.
* config.h.in (HAVE_ASM_PREVIOUS_DIRECTIVE): Remove #undef.
(HAVE_ASM_POPSECTION_DIRECTIVE): Likewise.
* include/libc-symbols.h [HAVE_ASM_PREVIOUS_DIRECTIVE]
(__make_section_unallocated): Make definition unconditional.
[HAVE_ASM_POPSECTION_DIRECTIVE] (__make_section_unallocated):
Remove conditional definition.
[!HAVE_ASM_PREVIOUS_DIRECTIVE && !HAVE_ASM_POPSECTION_DIRECTIVE]
(__make_section_unallocated): Likewise.
With AIX port deprecated there is no need to check/define
HAVE_ASM_GLOBAL_DOT_NAME anymore since the current minimum binutils
supported (2.22) does not emit global symbol with dot.
This patch removes all the HAVE_ASM_GLOBAL_DOT_NAME definition and
checks for powerpc64 port.
Replace with IS_IN and IS_IN_LIB macros instead. This change results
in a change in generated code, because it fixes a subtle bug. The bug
was introduced when systemtap probes were added to lowlevellock.h,
which resulted in stap-probe.h being included in a number of places.
stap-probe.h always defines IN_LIB, which breaks a check in errno.h
and netdb.h since they rely on that macro to decide whether to
implement an internal version of a declaration or an external one.
The components that see a code change due to this are:
iconv_prog
libmemusage.so
libpcprofile.so
libSegFault.so
libutil.so.1
locale
localedef
nscd
All other built components (i.e. libc, libpthread, etc.) remain
unchanged by this on x86_64.
* elf/Makefile (CPPFLAGS-.os): Remove IN_LIB.
* elf/rtld-Rules (rtld-CPPFLAGS): Likewise.
* extra-lib.mk (CPPFLAGS-$(lib)): Likewise.
* include/libc-symbols.h (IS_IN_LIB): New macro.
* include/errno.h: Use IS_IN_LIB instead of IN_LIB.
* include/netdb.h: Likewise.
* include/stap-probe.h: Remove all uses of IN_LIB.
Define MODULE_NAME in the build command and define IN_MODULE using
MODULE_NAME. Verified that the generated code is unchanged on x86_64.
* Makeconfig (module-cppflags-real): Define MODULE_NAME
instead of IN_MODULE.
* include/libc-symbols.h (IN_MODULE): Define using
MODULE_NAME.
(PASTE_NAME, PASTE_NAME1): New macros.
* include/stap-probe.h (LIBC_PROBE_1): Use MODULE_NAME instead
of IN_LIB.
(STAP_PROBE_ASM): Likewise.
Remove libc-modules.h from the tree and auto-generate it from
soversions.i and the list of modules in the built-modules variable
defined in Makeconfig. Macros generated have increasing numbered
values, with built-modules having lower values starting from 1,
following which a separator value LIBS_BEGIN is added and then finally
the library names from soversions.i are appended to the list. This
allows us to conveniently differentiate between the versioned
libraries and other built modules, which is needed in errno.h and
netdb.h to decide whether to use an internal symbol or an external
one.
Verified that generated code remains unchanged on x86_64.
* Makeconfig (built-modules): List non-library modules to be
built.
(module-cppflags): Include libc-modules.h for
everything except shlib-versions.v.i.
(CPPFLAGS): Use it.
(before-compile): Add libc-modules.h.
($(common-objpfx)libc-modules.h,
$(common-objpfx)libc-modules.stmp): New targets.
(common-generated): Add libc-modules.h and libc-modules.stmp.
($(common-objpfx)Versions.v.i): Depend on libc-modules.h.
* include/libc-symbols.h: Don't include libc-modules.h.
* include/libc-modules.h: Remove file.
* scripts/gen-libc-modules.awk: New script to generate
libc-modules.h.
* sysdeps/unix/Makefile ($(common-objpfx)sysd-syscalls):
Depend on libc-modules.stmp.
The current scheme to identify which module a translation unit is
built in depends on defining multiple macros IS_IN_* and also defining
NOT_IN_libc if we're building a non-libc module. In addition, there
is an IN_LIB macro that does effectively the same thing, but for
different modules (notably the systemtap probes). This macro scheme
unifies both ideas to use just one macro IN_MODULE and assign it a
value depending on the module it is being built into. If the module
is not defined, it defaults to MODULE_libc.
Patches that follow will replace uses of IS_IN_* variables with the
IS_IN() macro. libc-symbols.h has been converted already to give an
example of how such a transition will look.
Verified that there are no relevant binary changes. One source change
that will crop up repeatedly is that of nscd_stat, since it uses the
build timestamp as a constant in its logic.
* Makeconfig (in-module): Get value of libof set for the
translation unit.
(CPPFLAGS): Use $(in-module).
* Makerules: Don't suffix routine names for nonlib.
* include/libc-modules.h: New file.
* include/libc-symbols.h: Include libc-modules.h
(IS_IN): New macro to replace IS_IN_* macros.
* elf/Makefile: Set libof-* for each routine.
* elf/rtld-Rules: Likewise.
* extra-modules.mk: Likewise.
* iconv/Makefile: Likewise.
* iconvdata/Makefile: Likewise.
* locale/Makefile: Likewise.
* malloc/Makefile: Likewise.
* nss/Makefile: Likewise.
* sysdeps/gnu/Makefile: Likewise.
* sysdeps/ieee754/ldbl-opt/Makefile: Likewise.
* sysdeps/unix/sysv/linux/Makefile: Likewise.
* sysdeps/s390/s390-64/Makefile: Likewise.
* nscd/Makefile: Set libof-* for each routine. Set CFLAGS and
CPPFLAGS for nscd instead of nonlib.
Completing the removal of the obsolete INTDEF / INTUSE mechanism, this
patch removes the final use - that for _dl_starting_up - replacing it
by rtld_hidden_def / rtld_hidden_proto. Having removed the last use,
the mechanism itself is also removed.
Tested for x86_64 that installed stripped shared libraries are
unchanged by the patch. (This is not much of a test since this
variable is only defined and used in the !HAVE_INLINED_SYSCALLS case.)
[BZ #14132]
* include/libc-symbols.h (INTUSE): Remove macro.
(INTDEF): Likewise.
(INTVARDEF): Likewise.
(_INTVARDEF): Likewise.
(INTDEF2): Likewise.
(INTVARDEF2): Likewise.
* elf/rtld.c [!HAVE_INLINED_SYSCALLS] (_dl_starting_up): Use
rtld_hidden_def instead of INTVARDEF.
* sysdeps/generic/ldsodefs.h [IS_IN_rtld]
(_dl_starting_up_internal): Remove declaration.
(_dl_starting_up): Use rtld_hidden_proto.
* elf/dl-init.c [!HAVE_INLINED_SYSCALLS] (_dl_starting_up): Remove
declaration.
[!HAVE_INLINED_SYSCALLS] (_dl_starting_up_internal): Likewise.
(_dl_init) [!HAVE_INLINED_SYSCALLS]: Don't use INTUSE with
_dl_starting_up.
* elf/dl-writev.h (_dl_writev): Likewise.
* sysdeps/powerpc/powerpc64/dl-machine.h [!HAVE_INLINED_SYSCALLS]
(DL_STARTING_UP_DEF): Use __GI__dl_starting_up instead of
_dl_starting_up_internal.
Fixes -Wundef warnings for MBSTATE_T and HAVE_STRING_H. The
HAVE_STRING_H bit is a merge from the gnulib fnmatch.c and the
MBSTATE_T fix has been posted to gnulib for inclusion.
Check wheter the compiler has the option -fno-tree-loop-distribute-patterns
to inhibit loop transformation to library calls and uses it on memset
and memmove default implementation to avoid recursive calls.
New configure option --enable-obsolete-rpc makes the deprecated RPC
headers and functions available at compile time as they were before
version 2.14. This option will be removed at some time in the future
after the TI-RPC library becomes fully sufficient for the needs of
existing applications.
We use a callback function into libc.so to get access to the data
structure with the information and have special versions of the test
macros which automatically use this function.
rather than r->r_brk.
2006-11-08 Jakub Jelinek <jakub@redhat.com>
* elf/dl-load.c (decompose_rpath): Return bool rather than void.
If l->l_name is on inhibit_rpath list, set sps->dirs to -1 and
return false, otherwise return true.
(cache_rpath): Return decompose_rpath return value.
2006-11-07 Jakub Jelinek <jakub@redhat.com>
* include/libc-symbols.h (declare_symbol): Rename to...
(declare_symbol_alias): ... this. Add ORIGINAL argument, imply
strong_alias (ORIGINAL, SYMBOL) in asm to make sure it preceedes
.size directive.
* sysdeps/gnu/errlist-compat.awk: Adjust for declare_symbol_alias
changes.
* sysdeps/gnu/siglist.c: Likewise.
of internal.
(__hidden_def1, __hidden_dot_def1): Remove.
(__hidden_def2, __hidden_def3): Remove.
(__hidden_ver1): New.
(hidden_ver, hidden_def, hidden_weak): Use it.
(hidden_data_ver, hidden_data_ver, hidden_data_weak): Use non-data
version of the macro.
* include/wchar.h (__wcscoll): Remove.
* wcsmbs/wcscoll.c: Define wcscoll directly instead of via __wcscoll.
* string/strcoll.c: Don't issue libc_hidden_def STRCOLL redefined.
* include/libc-symbols.h (stub_warning): Emit a marker section called
.gnu.glibc-stub.NAME.
* Makerules ($(objpfx)stubs): Depend on object files, not dep files.
Use objdump to collect those marker section names.
($(common-objpfx)shlib.lds): Discard .gnu.glibc-stub.* sections.
