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C23 adds various <math.h> function families originally defined in TS
18661-4. Add the compoundn functions, which compute (1+X) to the
power Y for integer Y (and X at least -1). The integer exponent has
type long long int in C23; it was intmax_t in TS 18661-4, and as with
other interfaces changed after their initial appearance in the TS, I
don't think we need to support the original version of the interface.
Note that these functions are "compoundn" with a trailing "n", *not*
"compound" (CORE-MATH has the wrong name, for example).
As with pown, I strongly encourage searching for worst cases for ulps
error for these implementations (necessarily non-exhaustively, given
the size of the input space). I also expect a custom implementation
for a given format could be much faster as well as more accurate (I
haven't tested or benchmarked the CORE-MATH implementation for
binary32); this is one of the more complicated and less efficient
functions to implement in a type-generic way.
As with exp2m1 and exp10m1, this showed up places where the
powerpc64le IFUNC setup is not as self-contained as one might hope (in
this case, without the changes specific to powerpc64le, there were
undefined references to __GI___expf128).
Tested for x86_64 and x86, and with build-many-glibcs.py.
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The glibc-hwcaps subdirectories are extended by "z17". Libraries are loaded if
the z17 facility bits are active:
- Miscellaneous-instruction-extensions facility 4
- Vector-enhancements-facility 3
- Vector-Packed-Decimal-Enhancement Facility 3
- CPU: Concurrent-Functions Facility
tst-glibc-hwcaps.c is extended in order to test z17 via new marker6.
In case of running on a z17 with a kernel not recognizing z17 yet,
AT_PLATFORM will be z900 but vector-bit in AT_HWCAP is set. This situation
is now recognized and this testcase does not fail.
A fatal glibc error is dumped if glibc was build with architecture
level set for z17, but run on an older machine (See dl-hwcap-check.h).
Note, you might get an SIGILL before this check if you don't use:
configure --with-rtld-early-cflags=-march=<older-machine>
ld.so --list-diagnostics now also dumps information about s390.cpu_features.
Independent from z17, the s390x kernel won't introduce new HWCAP-Bits if there
is no special handling needed in kernel itself. For z17, we don't have new
HWCAP flags, but have to check the facility bits retrieved by
stfle-instruction.
Instead of storing all the stfle-bits (currently four 64bit values) in the
cpu_features struct, we now only store those bits, which are needed within
glibc itself. Note that we have this list twice, one with original values and
the other one which can be filtered with GLIBC_TUNABLES=glibc.cpu.hwcaps.
Those new fields are stored in so far reserved space in cpu_features struct.
Thus processes started in between the update of glibc package and we e.g. have
a new ld.so and an old libc.so, won't crash. The glibc internal ifunc-resolvers
would not select the best optimized variant.
The users of stfle-bits are also updated:
- parsing of GLIBC_TUNABLES=glibc.cpu.hwcaps
- glibc internal ifunc-resolvers
- __libc_ifunc_impl_list
- sysconf
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The BZ 32653 fix (12a497c716f0a06be5946cabb8c3ec22a079771e) kept the
stack pointer zeroing from make_main_stack_executable on
_dl_make_stack_executable. However, previously the 'stack_endp'
pointed to temporary variable created before the call of
_dl_map_object_from_fd; while now we use the __libc_stack_end
directly.
Since pthread_getattr_np relies on correct __libc_stack_end, if
_dl_make_stack_executable is called (for instance, when
glibc.rtld.execstack=2 is set) __libc_stack_end will be set to zero,
and the call will always fail.
The __libc_stack_end zero was used a mitigation hardening, but since
52a01100ad011293197637e42b5be1a479a2f4ae it is used solely on
pthread_getattr_np code. So there is no point in zeroing anymore.
Checked on x86_64-linux-gnu and i686-linux-gnu.
Reviewed-by: Sam James <sam@gentoo.org>
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The <termio.h> interface is absolutely ancient: it was obsoleted by
<termios.h> already in the first version of POSIX (1988) and thus
predates the very first version of Linux. Unfortunately, some constant
macros are used both by <termio.h> and <termios.h>; particularly
problematic is the baud rate constants since the termio interface
*requires* that the baud rate is set via an enumeration as part of
c_cflag.
In preparation of revamping the termios interface to support the
arbitrary baud rate capability that the Linux kernel has supported
since 2008, remove <termio.h> in the hope that no one still uses this
archaic interface.
Note that there is no actual code in glibc to support termio: it is
purely an unabstracted ioctl() interface.
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>
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Linux 6.12 adds AT_RENAME_* aliases for RENAME_* flags for renameat2,
and also AT_HANDLE_MNT_ID_UNIQUE. Add the first set of aliases to
stdio.h alongside the RENAME_* names, and AT_HANDLE_MNT_ID_UNIQUE to
bits/fcntl-linux.h.
Tested for x86_64.
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C2Y adds unsigned versions of the abs functions (see C2Y draft N3467 and
proposal N3349).
Tested for x86_64.
Signed-off-by: Lenard Mollenkopf <glibc@lenardmollenkopf.de>
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We mistakenly dropped the check in 27b96e069aad17cefea9437542180bff448ac3a0;
there's some other checks which we *can* drop, but let's worry about that
later.
Fixes the build on ppc64le where GCC is configured with --with-long-double-format=ieee.
Reviewed-by: Andreas Schwab <schwab@suse.de>
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Linux 6.14 has no new syscalls. Update the version number in
syscall-names.list to reflect that it is still current for 6.14.
Tested with build-many-glibcs.py.
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For all Linux distros with glibc 2.40 which I can find, GCC 14.2 is used
to compile glibc 2.40:
OS GCC URL
AOSC 14.2.0 https://aosc.io/
Arch Linux 14.2.0 https://archlinux.org/
ArchPOWER 14.2.0 https://archlinuxpower.org/
Artix 14.2.0 https://artixlinux.org/
Debian 14.2.0 https://www.debian.org/
Devuan 14.2.0 https://www.devuan.org/
Exherbo 14.2.0 https://www.exherbolinux.org/
Fedora 14.2.1 https://fedoraproject.org/
Gentoo 14.2.1 https://gentoo.org/
Kali Linux 14.2.0 https://www.kali.org/
KaOS 14.2.0 https://kaosx.us/
LiGurOS 14.2.0 https://liguros.gitlab.io/
Mageia 14.2.0 https://www.mageia.org/en/
Manjaro 14.2.0 https://manjaro.org/
NixOS 14.2.0 https://nixos.org/
openmamba 14.2.0 https://openmamba.org/
OpenMandriva 14.2.0 https://openmandriva.org/
openSUSE 14.2.0 https://www.opensuse.org/
Parabola 14.2.0 https://www.parabola.nu/
PLD Linux 14.2.0 https://pld-linux.org/
PureOS 14.2.0 https://pureos.net/
Raspbian 14.2.0 http://raspbian.org/
Slackware 14.2.0 http://www.slackware.com/
Solus 14.2.0 https://getsol.us/
T2 SDE 14.2.0 http://t2sde.org/
Ubuntu 14.2.0 https://www.ubuntu.com/
Wikidata 14.2.0 https://wikidata.org/
Support older versions of GCC to build glibc 2.42:
1. Need to work around bugs in older versions of GCC.
2. Can't use the new features in newer versions of GCC, which may be
required for new features, like _Float16 which requires GCC 12.1 or
above, in glibc,
The main benefit of supporting older versions of GCC is easier backport
of bug fixes to the older releases of glibc, which can be mitigated by
avoiding incompatible features in newer versions of GCC for critical bug
fixes. Require GCC 12.1 or newer to build. Remove GCC version check for
PowerPC and s390x.
TEST_CC and TEST_CXX can be used to test the glibc build with the older
versions of GCC.
For glibc developers who are using Linux OSes which don't come with GCC
12.1 or newer, they should build and install GCC 12.1 or newer to work
on glibc.
This fixes BZ #32539.
Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>
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When libgcc is built with pac-ret, it requires to autenticate the
unwinding frame based on CFI information. The _dl_tlsdesc_dynamic
uses a custom calling convention, where it is responsible to save
and restore all registers it might use (even volatile).
The pac-ret support added by 1be3d6eb823d8b952fa54b7bbc90cbecb8981380
was added only on the slow-path, but the fast path also adds DWARF
Register Rule Instruction (cfi_adjust_cfa_offset) since it requires
to save/restore some auxiliary register. It seems that this is not
fully supported neither by libgcc nor AArch64 ABI [1].
Instead, move paciasp/autiasp to function prologue/epilogue to be
used on both fast and slow paths.
I also corrected the _dl_tlsdesc_dynamic comment description, it was
copied from i386 implementation without any adjustment.
Checked on aarch64-linux-gnu with a toolchain built with
--enable-standard-branch-protection on a system with pac-ret
support.
[1] https://github.com/ARM-software/abi-aa/blob/main/aadwarf64/aadwarf64.rst#id1
Reviewed-by: Yury Khrustalev <yury.khrustalev@arm.com>
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C23 adds various <math.h> function families originally defined in TS
18661-4. Add the pown functions, which are like pow but with an
integer exponent. That exponent has type long long int in C23; it was
intmax_t in TS 18661-4, and as with other interfaces changed after
their initial appearance in the TS, I don't think we need to support
the original version of the interface. The test inputs are based on
the subset of test inputs for pow that use integer exponents that fit
in long long.
As the first such template implementation that saves and restores the
rounding mode internally (to avoid possible issues with directed
rounding and intermediate overflows or underflows in the wrong
rounding mode), support also needed to be added for using
SET_RESTORE_ROUND* in such template function implementations. This
required math-type-macros-float128.h to include <fenv_private.h>, so
it can tell whether SET_RESTORE_ROUNDF128 is defined. In turn, the
include order with <fenv_private.h> included before <math_private.h>
broke loongarch builds, showing up that
sysdeps/loongarch/math_private.h is really a fenv_private.h file
(maybe implemented internally before the consistent split of those
headers in 2018?) and needed to be renamed to fenv_private.h to avoid
errors with duplicate macro definitions if <math_private.h> is
included after <fenv_private.h>.
The underlying implementation uses __ieee754_pow functions (called
more than once in some cases, where the exponent does not fit in the
floating type). I expect a custom implementation for a given format,
that only handles integer exponents but handles larger exponents
directly, could be faster and more accurate in some cases.
I encourage searching for worst cases for ulps error for these
implementations (necessarily non-exhaustively, given the size of the
input space).
Tested for x86_64 and x86, and with build-many-glibcs.py.
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Using gcc -Wshift-overflow=2 -Wsystem-headers to compile a file
including <sys/mount.h> will cause a warning since 1 << 31 is undefined
behavior on platforms where int is 32-bits.
Signed-off-by: Collin Funk <collin.funk1@gmail.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
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Add function __inet_pton_chk which calls __chk_fail when the size of
argument dst is too small. inet_pton is redirected to __inet_pton_chk
or __inet_pton_warn when _FORTIFY_SOURCE is > 0.
Also add tests to debug/tst-fortify.c, update the abilist with
__inet_pton_chk and mention inet_pton fortification in maint.texi.
Co-authored-by: Frédéric Bérat <fberat@redhat.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>
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There are no new constants covered by tst-mman-consts.py,
tst-mount-consts.py or tst-sched-consts.py in Linux 6.13 that need any
header changes, so update the kernel version in those tests.
(tst-pidfd-consts.py will need updating separately along with adding
new constants to glibc.)
Tested with build-many-glibcs.py.
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The 7bb8045ec0 path made the '%n' fortify check ignore EMFILE errors
while trying to open /proc/self/maps, and this added a security
issue where EMFILE can be attacker-controlled thus making it
ineffective for some cases.
The EMFILE failure is reinstated but with a different error
message. Also, to improve the false positive of the hardening for
the cases where no new files can be opened, the
_dl_readonly_area now uses _dl_find_object to check if the
memory area is within a writable ELF segment. The procfs method is
still used as fallback.
Checked on x86_64-linux-gnu and i686-linux-gnu.
Reviewed-by: Arjun Shankar <arjun@redhat.com>
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- Create the __inet_ntop_chk routine that verifies that the builtin size
of the destination buffer is at least as big as the size given by the
user.
- Redirect calls from inet_ntop to __inet_ntop_chk or __inet_ntop_warn
- Update the abilist for this new routine
- Update the manual to mention the new fortification
Reviewed-by: Florian Weimer <fweimer@redhat.com>
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C23 adds various <math.h> function families originally defined in TS
18661-4. Add the powr functions, which are like pow, but with simpler
handling of special cases (based on exp(y*log(x)), so negative x and
0^0 are domain errors, powers of -0 are always +0 or +Inf never -0 or
-Inf, and 1^+-Inf and Inf^0 are also domain errors, while NaN^0 and
1^NaN are NaN). The test inputs are taken from those for pow, with
appropriate adjustments (including removing all tests that would be
domain errors from those in auto-libm-test-in and adding some more
such tests in libm-test-powr.inc).
The underlying implementation uses __ieee754_pow functions after
dealing with all special cases that need to be handled differently.
It might be a little faster (avoiding a wrapper and redundant checks
for special cases) to have an underlying implementation built
separately for both pow and powr with compile-time conditionals for
special-case handling, but I expect the benefit of that would be
limited given that both functions will end up needing to use the same
logic for computing pow outside of special cases.
My understanding is that powr(negative, qNaN) should raise "invalid":
that the rule on "invalid" for an argument outside the domain of the
function takes precedence over a quiet NaN argument producing a quiet
NaN result with no exceptions raised (for rootn it's explicit that the
0th root of qNaN raises "invalid"). I've raised this on the WG14
reflector to confirm the intent.
Tested for x86_64 and x86, and with build-many-glibcs.py.
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When an executable is invoked directly, we calculate $ORIGIN by calling
readlink on /proc/self/exe, which the Linux kernel resolves to the
target of any symlinks. However, if an executable is run through ld.so,
we cannot use /proc/self/exe and instead use the path given as an
argument. This leads to a different calculation of $ORIGIN, which is
most notable in that it causes ldd to behave differently (e.g., by not
finding a library) from directly running the program.
To make the behavior consistent, take advantage of the fact that the
kernel also resolves /proc/self/fd/ symlinks to the target of any
symlinks in the same manner, so once we have opened the main executable
in order to load it, replace the user-provided path with the result of
calling readlink("/proc/self/fd/N").
(On non-Linux platforms this resolution does not happen and so no
behavior change is needed.)
The __fd_to_filename requires _fitoa_word and _itoa_word, which for
32-bits pulls a lot of definitions from _itoa.c (due _ITOA_NEEDED
being defined). To simplify the build move the required function
to a new file, _fitoa_word.c.
Checked on x86_64-linux-gnu and i686-linux-gnu.
Co-authored-by: Geoffrey Thomas <geofft@ldpreload.com>
Reviewed-by: Geoffrey Thomas <geofft@ldpreload.com>
Tested-by: Geoffrey Thomas <geofft@ldpreload.com>
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Linux 6.13 adds four new syscalls. Update syscall-names.list and
regenerate the arch-syscall.h headers with build-many-glibcs.py
update-syscalls.
Tested with build-many-glibcs.py.
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The straightforward sched_getattr call serves as a test for
bug 32781, too.
Reviewed-by: Joseph Myers <josmyers@redhat.com>
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The GCC attribute expects an element count, not bytes.
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Current Bionic has this function, with enhanced error checking
(the undefined case terminates the process).
Reviewed-by: Joseph Myers <josmyers@redhat.com>
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We add sched_yield() API testing to the existing thread affinity
test case because it allows us to test sched_yield() operation
in the following scenarios:
* On a main thread.
* On multiple threads simultaneously.
* On every CPU the system reports simultaneously.
The ensures we exercise sched_yield() in as many scenarios as
we would exercise calls to the affinity functions.
Additionally, the test is improved by adding a semaphore to coordinate
all the threads running, so that an early starter thread won't consume
cpu resources that could be used to start the other threads.
Co-authored-by: DJ Delorie <dj@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
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C23 adds various <math.h> function families originally defined in TS
18661-4. Add the rsqrt functions (1/sqrt(x)). The test inputs are
taken from those for sqrt.
Tested for x86_64 and x86, and with build-many-glibcs.py.
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btrfs has a 65535 maximum link count. Include this value in pathconf to
give the real max link count for this filesystem.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Suggested-by: Stefan Liebler <stli@linux.ibm.com>
Reviewed-by: Stefan Liebler <stli@linux.ibm.com>
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powerpc was the only architecture with arch-specific hooks for
LD_SHOW_AUXV, and with the information moved to ld diagnostics there
is no need to keep the _dl_procinfo hook.
Checked with a build for all affected ABIs.
Reviewed-by: Peter Bergner <bergner@linux.ibm.com>
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The _dl_string_platform is moved to hwcapinfo.h, since it is only used
by hwcapinfo.c and test-get_hwcap internal test.
Checked on powerpc64le-linux-gnu.
Reviewed-by: Peter Bergner <bergner@linux.ibm.com>
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The ld.so diagnostics already prints AT_HWCAP values, but only in
hexadecimal. To avoid duplicating the strings, consolidate the
hwcap_names from cpu-features.h on a new file, dl-hwcap-info.h
(and it also improves the hwcap string description with more
values).
For future AT_HWCAP3/AT_HWCAP4 extensions, it is just a matter
to add them on dl-hwcap-info.c so both ld diagnostics and
tunable filtering will parse the new values.
Checked on powerpc64le-linux-gnu.
Reviewed-by: Peter Bergner <bergner@linux.ibm.com>
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Remove unused _dl_hwcap_string defines. As a result many dl-procinfo.h headers
can be removed. This also removes target specific _dl_procinfo implementations
which only printed HWCAP strings using dl_hwcap_string.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Remove LP64 and ILP32 ifdefs.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Cleanup pointer mangling.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Remove PTR_REG defines.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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This series removes various ILP32 defines that are now
no longer needed.
Remove PTR_ARG/SIZE_ARG.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Due to the extensible nature of the rseq area we can't explictly
initialize fields that are not part of the ABI yet. It was agreed with
upstream that all new fields will be documented as zero initialized by
userspace. Future kernels configured with CONFIG_DEBUG_RSEQ will
validate the content of all fields during registration.
Replace the explicit field initialization with a memset of the whole
rseq area which will cover fields as they are added to future kernels.
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>
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Test that when we return from a function that enabled GCS at runtime
we get SIGSEGV. Also test that ucontext contains GCS block with the
GCS pointer.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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These tests validate that GCS tunable works as expected depending
on the GCS markings in the test binaries.
Tests validate both static and dynamically linked binaries.
These new tests are AArch64 specific. Moreover, they are included only
if linker supports the "-z gcs=<value>" option. If built, these tests
will run on systems with and without HWCAP_GCS. In the latter case the
tests will be reported as UNSUPPORTED.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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The syscall pkey_alloc can return ENOSPC to indicate either that all
keys are in use or that the system runs in a mode in which memory
protection keys are disabled. In such case the test should not fail and
just return unsupported.
This matches the behaviour of the generic tst-pkey.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Florian Weimer <fweimer@redhat.com>
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Decorate BSS mappings with [anon: glibc: .bss <file>], for example
[anon: glibc: .bss /lib/libc.so.6]. The string ".bss" is already used
by bionic so use the same, but add the filename as well. If the name
would be longer than what the kernel allows, drop the directory part
of the path.
Refactor glibc.mem.decorate_maps check to a separate function and use
it to avoid assembling a name, which would not be used later.
Signed-off-by: Petr Malat <oss@malat.biz>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Linux 6.13 (662df3e5c3766) added a lightweight way to define guard areas
through madvise syscall. Instead of PROT_NONE the guard region through
mprotect, userland can madvise the same area with a special flag, and
the kernel ensures that accessing the area will trigger a SIGSEGV (as for
PROT_NONE mapping).
The madvise way has the advantage of less kernel memory consumption for
the process page-table (one less VMA per guard area), and slightly less
contention on kernel (also due to the fewer VMA areas being tracked).
The pthread_create allocates a new thread stack in two ways: if a guard
area is set (the default) it allocates the memory range required using
PROT_NONE and then mprotect the usable stack area. Otherwise, if a
guard page is not set it allocates the region with the required flags.
For the MADV_GUARD_INSTALL support, the stack area region is allocated
with required flags and then the guard region is installed. If the
kernel does not support it, the usual way is used instead (and
MADV_GUARD_INSTALL is disabled for future stack creations).
The stack allocation strategy is recorded on the pthread struct, and it
is used in case the guard region needs to be resized. To avoid needing
an extra field, the 'user_stack' is repurposed and renamed to 'stack_mode'.
This patch also adds a proper test for the pthread guard.
I checked on x86_64, aarch64, powerpc64le, and hppa with kernel 6.13.0-rc7.
Reviewed-by: DJ Delorie <dj@redhat.com>
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Use upper 32 bits of HWCAP.
Reviewed-by: Andreas K. Huettel <dilfridge@gentoo.org>
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Linux 6.13 was released with a change that overwrites those bytes.
This means that the check_unused subtest fails.
Update the manual accordingly.
Tested-by: Xi Ruoyao <xry111@xry111.site>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Unlike for BTI, the kernel does not process GCS properties so update
GL(dl_aarch64_gcs) before the GCS status is set.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Use the ARCH_SETUP_TLS hook to enable GCS in the static linked case.
The system call must be inlined and then GCS is enabled on a top
level stack frame that does not return and has no exception handlers
above it.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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This tunable controls Guarded Control Stack (GCS) for the process.
0 = disabled: do not enable GCS
1 = enforced: check markings and fail if any binary is not marked
2 = optional: check markings but keep GCS off if a binary is unmarked
3 = override: enable GCS, markings are ignored
By default it is 0, so GCS is disabled, value 1 will enable GCS.
The status is stored into GL(dl_aarch64_gcs) early and only applied
later, since enabling GCS is tricky: it must happen on a top level
stack frame. Using GL instead of GLRO because it may need updates
depending on loaded libraries that happen after readonly protection
is applied, however library marking based GCS setting is not yet
implemented.
Describe new tunable in the manual.
Co-authored-by: Yury Khrustalev <yury.khrustalev@arm.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Changed the makecontext logic: previously the first setcontext jumped
straight to the user callback function and the return address is set
to __startcontext. This does not work when GCS is enabled as the
integrity of the return address is protected, so instead the context
is setup such that setcontext jumps to __startcontext which calls the
user callback (passed in x20).
The map_shadow_stack syscall is used to allocate a suitably sized GCS
(which includes some reserved area to account for altstack signal
handlers and otherwise supports maximum number of 16 byte aligned
stack frames on the given stack) however the GCS is never freed as
the lifetime of ucontext and related stack is user managed.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
|
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Userspace ucontext needs to store GCSPR, it does not have to be
compatible with the kernel ucontext. For now we use the linux
struct gcs_context layout but only use the gcspr field from it.
Similar implementation to the longjmp code, supports switching GCS
if the target GCS is capped, and unwinding a continuous GCS to a
previous state.
Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
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