qemu-e2k/linux-user/aarch64/signal.c
Richard Henderson 78fd56ba13 linux-user/aarch64: Implement SME signal handling
Set the SM bit in the SVE record on signal delivery, create the ZA record.
Restore SM and ZA state according to the records present on return.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220708151540.18136-41-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
2022-07-11 13:43:52 +01:00

757 lines
22 KiB
C

/*
* Emulation of Linux signals
*
* Copyright (c) 2003 Fabrice Bellard
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, see <http://www.gnu.org/licenses/>.
*/
#include "qemu/osdep.h"
#include "qemu.h"
#include "user-internals.h"
#include "signal-common.h"
#include "linux-user/trace.h"
struct target_sigcontext {
uint64_t fault_address;
/* AArch64 registers */
uint64_t regs[31];
uint64_t sp;
uint64_t pc;
uint64_t pstate;
/* 4K reserved for FP/SIMD state and future expansion */
char __reserved[4096] __attribute__((__aligned__(16)));
};
struct target_ucontext {
abi_ulong tuc_flags;
abi_ulong tuc_link;
target_stack_t tuc_stack;
target_sigset_t tuc_sigmask;
/* glibc uses a 1024-bit sigset_t */
char __unused[1024 / 8 - sizeof(target_sigset_t)];
/* last for future expansion */
struct target_sigcontext tuc_mcontext;
};
/*
* Header to be used at the beginning of structures extending the user
* context. Such structures must be placed after the rt_sigframe on the stack
* and be 16-byte aligned. The last structure must be a dummy one with the
* magic and size set to 0.
*/
struct target_aarch64_ctx {
uint32_t magic;
uint32_t size;
};
#define TARGET_FPSIMD_MAGIC 0x46508001
struct target_fpsimd_context {
struct target_aarch64_ctx head;
uint32_t fpsr;
uint32_t fpcr;
uint64_t vregs[32 * 2]; /* really uint128_t vregs[32] */
};
#define TARGET_EXTRA_MAGIC 0x45585401
struct target_extra_context {
struct target_aarch64_ctx head;
uint64_t datap; /* 16-byte aligned pointer to extra space cast to __u64 */
uint32_t size; /* size in bytes of the extra space */
uint32_t reserved[3];
};
#define TARGET_SVE_MAGIC 0x53564501
struct target_sve_context {
struct target_aarch64_ctx head;
uint16_t vl;
uint16_t flags;
uint16_t reserved[2];
/* The actual SVE data immediately follows. It is laid out
* according to TARGET_SVE_SIG_{Z,P}REG_OFFSET, based off of
* the original struct pointer.
*/
};
#define TARGET_SVE_VQ_BYTES 16
#define TARGET_SVE_SIG_ZREG_SIZE(VQ) ((VQ) * TARGET_SVE_VQ_BYTES)
#define TARGET_SVE_SIG_PREG_SIZE(VQ) ((VQ) * (TARGET_SVE_VQ_BYTES / 8))
#define TARGET_SVE_SIG_REGS_OFFSET \
QEMU_ALIGN_UP(sizeof(struct target_sve_context), TARGET_SVE_VQ_BYTES)
#define TARGET_SVE_SIG_ZREG_OFFSET(VQ, N) \
(TARGET_SVE_SIG_REGS_OFFSET + TARGET_SVE_SIG_ZREG_SIZE(VQ) * (N))
#define TARGET_SVE_SIG_PREG_OFFSET(VQ, N) \
(TARGET_SVE_SIG_ZREG_OFFSET(VQ, 32) + TARGET_SVE_SIG_PREG_SIZE(VQ) * (N))
#define TARGET_SVE_SIG_FFR_OFFSET(VQ) \
(TARGET_SVE_SIG_PREG_OFFSET(VQ, 16))
#define TARGET_SVE_SIG_CONTEXT_SIZE(VQ) \
(TARGET_SVE_SIG_PREG_OFFSET(VQ, 17))
#define TARGET_SVE_SIG_FLAG_SM 1
#define TARGET_ZA_MAGIC 0x54366345
struct target_za_context {
struct target_aarch64_ctx head;
uint16_t vl;
uint16_t reserved[3];
/* The actual ZA data immediately follows. */
};
#define TARGET_ZA_SIG_REGS_OFFSET \
QEMU_ALIGN_UP(sizeof(struct target_za_context), TARGET_SVE_VQ_BYTES)
#define TARGET_ZA_SIG_ZAV_OFFSET(VQ, N) \
(TARGET_ZA_SIG_REGS_OFFSET + (VQ) * TARGET_SVE_VQ_BYTES * (N))
#define TARGET_ZA_SIG_CONTEXT_SIZE(VQ) \
TARGET_ZA_SIG_ZAV_OFFSET(VQ, VQ * TARGET_SVE_VQ_BYTES)
struct target_rt_sigframe {
struct target_siginfo info;
struct target_ucontext uc;
};
struct target_rt_frame_record {
uint64_t fp;
uint64_t lr;
};
static void target_setup_general_frame(struct target_rt_sigframe *sf,
CPUARMState *env, target_sigset_t *set)
{
int i;
__put_user(0, &sf->uc.tuc_flags);
__put_user(0, &sf->uc.tuc_link);
target_save_altstack(&sf->uc.tuc_stack, env);
for (i = 0; i < 31; i++) {
__put_user(env->xregs[i], &sf->uc.tuc_mcontext.regs[i]);
}
__put_user(env->xregs[31], &sf->uc.tuc_mcontext.sp);
__put_user(env->pc, &sf->uc.tuc_mcontext.pc);
__put_user(pstate_read(env), &sf->uc.tuc_mcontext.pstate);
__put_user(env->exception.vaddress, &sf->uc.tuc_mcontext.fault_address);
for (i = 0; i < TARGET_NSIG_WORDS; i++) {
__put_user(set->sig[i], &sf->uc.tuc_sigmask.sig[i]);
}
}
static void target_setup_fpsimd_record(struct target_fpsimd_context *fpsimd,
CPUARMState *env)
{
int i;
__put_user(TARGET_FPSIMD_MAGIC, &fpsimd->head.magic);
__put_user(sizeof(struct target_fpsimd_context), &fpsimd->head.size);
__put_user(vfp_get_fpsr(env), &fpsimd->fpsr);
__put_user(vfp_get_fpcr(env), &fpsimd->fpcr);
for (i = 0; i < 32; i++) {
uint64_t *q = aa64_vfp_qreg(env, i);
#if TARGET_BIG_ENDIAN
__put_user(q[0], &fpsimd->vregs[i * 2 + 1]);
__put_user(q[1], &fpsimd->vregs[i * 2]);
#else
__put_user(q[0], &fpsimd->vregs[i * 2]);
__put_user(q[1], &fpsimd->vregs[i * 2 + 1]);
#endif
}
}
static void target_setup_extra_record(struct target_extra_context *extra,
uint64_t datap, uint32_t extra_size)
{
__put_user(TARGET_EXTRA_MAGIC, &extra->head.magic);
__put_user(sizeof(struct target_extra_context), &extra->head.size);
__put_user(datap, &extra->datap);
__put_user(extra_size, &extra->size);
}
static void target_setup_end_record(struct target_aarch64_ctx *end)
{
__put_user(0, &end->magic);
__put_user(0, &end->size);
}
static void target_setup_sve_record(struct target_sve_context *sve,
CPUARMState *env, int size)
{
int i, j, vq = sve_vq(env);
memset(sve, 0, sizeof(*sve));
__put_user(TARGET_SVE_MAGIC, &sve->head.magic);
__put_user(size, &sve->head.size);
__put_user(vq * TARGET_SVE_VQ_BYTES, &sve->vl);
if (FIELD_EX64(env->svcr, SVCR, SM)) {
__put_user(TARGET_SVE_SIG_FLAG_SM, &sve->flags);
}
/* Note that SVE regs are stored as a byte stream, with each byte element
* at a subsequent address. This corresponds to a little-endian store
* of our 64-bit hunks.
*/
for (i = 0; i < 32; ++i) {
uint64_t *z = (void *)sve + TARGET_SVE_SIG_ZREG_OFFSET(vq, i);
for (j = 0; j < vq * 2; ++j) {
__put_user_e(env->vfp.zregs[i].d[j], z + j, le);
}
}
for (i = 0; i <= 16; ++i) {
uint16_t *p = (void *)sve + TARGET_SVE_SIG_PREG_OFFSET(vq, i);
for (j = 0; j < vq; ++j) {
uint64_t r = env->vfp.pregs[i].p[j >> 2];
__put_user_e(r >> ((j & 3) * 16), p + j, le);
}
}
}
static void target_setup_za_record(struct target_za_context *za,
CPUARMState *env, int size)
{
int vq = sme_vq(env);
int vl = vq * TARGET_SVE_VQ_BYTES;
int i, j;
memset(za, 0, sizeof(*za));
__put_user(TARGET_ZA_MAGIC, &za->head.magic);
__put_user(size, &za->head.size);
__put_user(vl, &za->vl);
if (size == TARGET_ZA_SIG_CONTEXT_SIZE(0)) {
return;
}
assert(size == TARGET_ZA_SIG_CONTEXT_SIZE(vq));
/*
* Note that ZA vectors are stored as a byte stream,
* with each byte element at a subsequent address.
*/
for (i = 0; i < vl; ++i) {
uint64_t *z = (void *)za + TARGET_ZA_SIG_ZAV_OFFSET(vq, i);
for (j = 0; j < vq * 2; ++j) {
__put_user_e(env->zarray[i].d[j], z + j, le);
}
}
}
static void target_restore_general_frame(CPUARMState *env,
struct target_rt_sigframe *sf)
{
sigset_t set;
uint64_t pstate;
int i;
target_to_host_sigset(&set, &sf->uc.tuc_sigmask);
set_sigmask(&set);
for (i = 0; i < 31; i++) {
__get_user(env->xregs[i], &sf->uc.tuc_mcontext.regs[i]);
}
__get_user(env->xregs[31], &sf->uc.tuc_mcontext.sp);
__get_user(env->pc, &sf->uc.tuc_mcontext.pc);
__get_user(pstate, &sf->uc.tuc_mcontext.pstate);
pstate_write(env, pstate);
}
static void target_restore_fpsimd_record(CPUARMState *env,
struct target_fpsimd_context *fpsimd)
{
uint32_t fpsr, fpcr;
int i;
__get_user(fpsr, &fpsimd->fpsr);
vfp_set_fpsr(env, fpsr);
__get_user(fpcr, &fpsimd->fpcr);
vfp_set_fpcr(env, fpcr);
for (i = 0; i < 32; i++) {
uint64_t *q = aa64_vfp_qreg(env, i);
#if TARGET_BIG_ENDIAN
__get_user(q[0], &fpsimd->vregs[i * 2 + 1]);
__get_user(q[1], &fpsimd->vregs[i * 2]);
#else
__get_user(q[0], &fpsimd->vregs[i * 2]);
__get_user(q[1], &fpsimd->vregs[i * 2 + 1]);
#endif
}
}
static bool target_restore_sve_record(CPUARMState *env,
struct target_sve_context *sve,
int size, int *svcr)
{
int i, j, vl, vq, flags;
bool sm;
__get_user(vl, &sve->vl);
__get_user(flags, &sve->flags);
sm = flags & TARGET_SVE_SIG_FLAG_SM;
/* The cpu must support Streaming or Non-streaming SVE. */
if (sm
? !cpu_isar_feature(aa64_sme, env_archcpu(env))
: !cpu_isar_feature(aa64_sve, env_archcpu(env))) {
return false;
}
/*
* Note that we cannot use sve_vq() because that depends on the
* current setting of PSTATE.SM, not the state to be restored.
*/
vq = sve_vqm1_for_el_sm(env, 0, sm) + 1;
/* Reject mismatched VL. */
if (vl != vq * TARGET_SVE_VQ_BYTES) {
return false;
}
/* Accept empty record -- used to clear PSTATE.SM. */
if (size <= sizeof(*sve)) {
return true;
}
/* Reject non-empty but incomplete record. */
if (size < TARGET_SVE_SIG_CONTEXT_SIZE(vq)) {
return false;
}
*svcr = FIELD_DP64(*svcr, SVCR, SM, sm);
/*
* Note that SVE regs are stored as a byte stream, with each byte element
* at a subsequent address. This corresponds to a little-endian load
* of our 64-bit hunks.
*/
for (i = 0; i < 32; ++i) {
uint64_t *z = (void *)sve + TARGET_SVE_SIG_ZREG_OFFSET(vq, i);
for (j = 0; j < vq * 2; ++j) {
__get_user_e(env->vfp.zregs[i].d[j], z + j, le);
}
}
for (i = 0; i <= 16; ++i) {
uint16_t *p = (void *)sve + TARGET_SVE_SIG_PREG_OFFSET(vq, i);
for (j = 0; j < vq; ++j) {
uint16_t r;
__get_user_e(r, p + j, le);
if (j & 3) {
env->vfp.pregs[i].p[j >> 2] |= (uint64_t)r << ((j & 3) * 16);
} else {
env->vfp.pregs[i].p[j >> 2] = r;
}
}
}
return true;
}
static bool target_restore_za_record(CPUARMState *env,
struct target_za_context *za,
int size, int *svcr)
{
int i, j, vl, vq;
if (!cpu_isar_feature(aa64_sme, env_archcpu(env))) {
return false;
}
__get_user(vl, &za->vl);
vq = sme_vq(env);
/* Reject mismatched VL. */
if (vl != vq * TARGET_SVE_VQ_BYTES) {
return false;
}
/* Accept empty record -- used to clear PSTATE.ZA. */
if (size <= TARGET_ZA_SIG_CONTEXT_SIZE(0)) {
return true;
}
/* Reject non-empty but incomplete record. */
if (size < TARGET_ZA_SIG_CONTEXT_SIZE(vq)) {
return false;
}
*svcr = FIELD_DP64(*svcr, SVCR, ZA, 1);
for (i = 0; i < vl; ++i) {
uint64_t *z = (void *)za + TARGET_ZA_SIG_ZAV_OFFSET(vq, i);
for (j = 0; j < vq * 2; ++j) {
__get_user_e(env->zarray[i].d[j], z + j, le);
}
}
return true;
}
static int target_restore_sigframe(CPUARMState *env,
struct target_rt_sigframe *sf)
{
struct target_aarch64_ctx *ctx, *extra = NULL;
struct target_fpsimd_context *fpsimd = NULL;
struct target_sve_context *sve = NULL;
struct target_za_context *za = NULL;
uint64_t extra_datap = 0;
bool used_extra = false;
int sve_size = 0;
int za_size = 0;
int svcr = 0;
target_restore_general_frame(env, sf);
ctx = (struct target_aarch64_ctx *)sf->uc.tuc_mcontext.__reserved;
while (ctx) {
uint32_t magic, size, extra_size;
__get_user(magic, &ctx->magic);
__get_user(size, &ctx->size);
switch (magic) {
case 0:
if (size != 0) {
goto err;
}
if (used_extra) {
ctx = NULL;
} else {
ctx = extra;
used_extra = true;
}
continue;
case TARGET_FPSIMD_MAGIC:
if (fpsimd || size != sizeof(struct target_fpsimd_context)) {
goto err;
}
fpsimd = (struct target_fpsimd_context *)ctx;
break;
case TARGET_SVE_MAGIC:
if (sve || size < sizeof(struct target_sve_context)) {
goto err;
}
sve = (struct target_sve_context *)ctx;
sve_size = size;
break;
case TARGET_ZA_MAGIC:
if (za || size < sizeof(struct target_za_context)) {
goto err;
}
za = (struct target_za_context *)ctx;
za_size = size;
break;
case TARGET_EXTRA_MAGIC:
if (extra || size != sizeof(struct target_extra_context)) {
goto err;
}
__get_user(extra_datap,
&((struct target_extra_context *)ctx)->datap);
__get_user(extra_size,
&((struct target_extra_context *)ctx)->size);
extra = lock_user(VERIFY_READ, extra_datap, extra_size, 0);
if (!extra) {
return 1;
}
break;
default:
/* Unknown record -- we certainly didn't generate it.
* Did we in fact get out of sync?
*/
goto err;
}
ctx = (void *)ctx + size;
}
/* Require FPSIMD always. */
if (fpsimd) {
target_restore_fpsimd_record(env, fpsimd);
} else {
goto err;
}
/* SVE data, if present, overwrites FPSIMD data. */
if (sve && !target_restore_sve_record(env, sve, sve_size, &svcr)) {
goto err;
}
if (za && !target_restore_za_record(env, za, za_size, &svcr)) {
goto err;
}
if (env->svcr != svcr) {
env->svcr = svcr;
arm_rebuild_hflags(env);
}
unlock_user(extra, extra_datap, 0);
return 0;
err:
unlock_user(extra, extra_datap, 0);
return 1;
}
static abi_ulong get_sigframe(struct target_sigaction *ka,
CPUARMState *env, int size)
{
abi_ulong sp;
sp = target_sigsp(get_sp_from_cpustate(env), ka);
sp = (sp - size) & ~15;
return sp;
}
typedef struct {
int total_size;
int extra_base;
int extra_size;
int std_end_ofs;
int extra_ofs;
int extra_end_ofs;
} target_sigframe_layout;
static int alloc_sigframe_space(int this_size, target_sigframe_layout *l)
{
/* Make sure there will always be space for the end marker. */
const int std_size = sizeof(struct target_rt_sigframe)
- sizeof(struct target_aarch64_ctx);
int this_loc = l->total_size;
if (l->extra_base) {
/* Once we have begun an extra space, all allocations go there. */
l->extra_size += this_size;
} else if (this_size + this_loc > std_size) {
/* This allocation does not fit in the standard space. */
/* Allocate the extra record. */
l->extra_ofs = this_loc;
l->total_size += sizeof(struct target_extra_context);
/* Allocate the standard end record. */
l->std_end_ofs = l->total_size;
l->total_size += sizeof(struct target_aarch64_ctx);
/* Allocate the requested record. */
l->extra_base = this_loc = l->total_size;
l->extra_size = this_size;
}
l->total_size += this_size;
return this_loc;
}
static void target_setup_frame(int usig, struct target_sigaction *ka,
target_siginfo_t *info, target_sigset_t *set,
CPUARMState *env)
{
target_sigframe_layout layout = {
/* Begin with the size pointing to the reserved space. */
.total_size = offsetof(struct target_rt_sigframe,
uc.tuc_mcontext.__reserved),
};
int fpsimd_ofs, fr_ofs, sve_ofs = 0, za_ofs = 0;
int sve_size = 0, za_size = 0;
struct target_rt_sigframe *frame;
struct target_rt_frame_record *fr;
abi_ulong frame_addr, return_addr;
/* FPSIMD record is always in the standard space. */
fpsimd_ofs = alloc_sigframe_space(sizeof(struct target_fpsimd_context),
&layout);
/* SVE state needs saving only if it exists. */
if (cpu_isar_feature(aa64_sve, env_archcpu(env)) ||
cpu_isar_feature(aa64_sme, env_archcpu(env))) {
sve_size = QEMU_ALIGN_UP(TARGET_SVE_SIG_CONTEXT_SIZE(sve_vq(env)), 16);
sve_ofs = alloc_sigframe_space(sve_size, &layout);
}
if (cpu_isar_feature(aa64_sme, env_archcpu(env))) {
/* ZA state needs saving only if it is enabled. */
if (FIELD_EX64(env->svcr, SVCR, ZA)) {
za_size = TARGET_ZA_SIG_CONTEXT_SIZE(sme_vq(env));
} else {
za_size = TARGET_ZA_SIG_CONTEXT_SIZE(0);
}
za_ofs = alloc_sigframe_space(za_size, &layout);
}
if (layout.extra_ofs) {
/* Reserve space for the extra end marker. The standard end marker
* will have been allocated when we allocated the extra record.
*/
layout.extra_end_ofs
= alloc_sigframe_space(sizeof(struct target_aarch64_ctx), &layout);
} else {
/* Reserve space for the standard end marker.
* Do not use alloc_sigframe_space because we cheat
* std_size therein to reserve space for this.
*/
layout.std_end_ofs = layout.total_size;
layout.total_size += sizeof(struct target_aarch64_ctx);
}
/* We must always provide at least the standard 4K reserved space,
* even if we don't use all of it (this is part of the ABI)
*/
layout.total_size = MAX(layout.total_size,
sizeof(struct target_rt_sigframe));
/*
* Reserve space for the standard frame unwind pair: fp, lr.
* Despite the name this is not a "real" record within the frame.
*/
fr_ofs = layout.total_size;
layout.total_size += sizeof(struct target_rt_frame_record);
frame_addr = get_sigframe(ka, env, layout.total_size);
trace_user_setup_frame(env, frame_addr);
frame = lock_user(VERIFY_WRITE, frame_addr, layout.total_size, 0);
if (!frame) {
goto give_sigsegv;
}
target_setup_general_frame(frame, env, set);
target_setup_fpsimd_record((void *)frame + fpsimd_ofs, env);
target_setup_end_record((void *)frame + layout.std_end_ofs);
if (layout.extra_ofs) {
target_setup_extra_record((void *)frame + layout.extra_ofs,
frame_addr + layout.extra_base,
layout.extra_size);
target_setup_end_record((void *)frame + layout.extra_end_ofs);
}
if (sve_ofs) {
target_setup_sve_record((void *)frame + sve_ofs, env, sve_size);
}
if (za_ofs) {
target_setup_za_record((void *)frame + za_ofs, env, za_size);
}
/* Set up the stack frame for unwinding. */
fr = (void *)frame + fr_ofs;
__put_user(env->xregs[29], &fr->fp);
__put_user(env->xregs[30], &fr->lr);
if (ka->sa_flags & TARGET_SA_RESTORER) {
return_addr = ka->sa_restorer;
} else {
return_addr = default_rt_sigreturn;
}
env->xregs[0] = usig;
env->xregs[29] = frame_addr + fr_ofs;
env->xregs[30] = return_addr;
env->xregs[31] = frame_addr;
env->pc = ka->_sa_handler;
/* Invoke the signal handler as if by indirect call. */
if (cpu_isar_feature(aa64_bti, env_archcpu(env))) {
env->btype = 2;
}
/*
* Invoke the signal handler with both SM and ZA disabled.
* When clearing SM, ResetSVEState, per SMSTOP.
*/
if (FIELD_EX64(env->svcr, SVCR, SM)) {
arm_reset_sve_state(env);
}
if (env->svcr) {
env->svcr = 0;
arm_rebuild_hflags(env);
}
if (info) {
tswap_siginfo(&frame->info, info);
env->xregs[1] = frame_addr + offsetof(struct target_rt_sigframe, info);
env->xregs[2] = frame_addr + offsetof(struct target_rt_sigframe, uc);
}
unlock_user(frame, frame_addr, layout.total_size);
return;
give_sigsegv:
unlock_user(frame, frame_addr, layout.total_size);
force_sigsegv(usig);
}
void setup_rt_frame(int sig, struct target_sigaction *ka,
target_siginfo_t *info, target_sigset_t *set,
CPUARMState *env)
{
target_setup_frame(sig, ka, info, set, env);
}
void setup_frame(int sig, struct target_sigaction *ka,
target_sigset_t *set, CPUARMState *env)
{
target_setup_frame(sig, ka, 0, set, env);
}
long do_rt_sigreturn(CPUARMState *env)
{
struct target_rt_sigframe *frame = NULL;
abi_ulong frame_addr = env->xregs[31];
trace_user_do_rt_sigreturn(env, frame_addr);
if (frame_addr & 15) {
goto badframe;
}
if (!lock_user_struct(VERIFY_READ, frame, frame_addr, 1)) {
goto badframe;
}
if (target_restore_sigframe(env, frame)) {
goto badframe;
}
target_restore_altstack(&frame->uc.tuc_stack, env);
unlock_user_struct(frame, frame_addr, 0);
return -QEMU_ESIGRETURN;
badframe:
unlock_user_struct(frame, frame_addr, 0);
force_sig(TARGET_SIGSEGV);
return -QEMU_ESIGRETURN;
}
long do_sigreturn(CPUARMState *env)
{
return do_rt_sigreturn(env);
}
void setup_sigtramp(abi_ulong sigtramp_page)
{
uint32_t *tramp = lock_user(VERIFY_WRITE, sigtramp_page, 8, 0);
assert(tramp != NULL);
/*
* mov x8,#__NR_rt_sigreturn; svc #0
* Since these are instructions they need to be put as little-endian
* regardless of target default or current CPU endianness.
*/
__put_user_e(0xd2801168, &tramp[0], le);
__put_user_e(0xd4000001, &tramp[1], le);
default_rt_sigreturn = sigtramp_page;
unlock_user(tramp, sigtramp_page, 8);
}