xnu-7195.101.1.tar.gz

[apple/xnu.git] / bsd / dev / arm / fasttrap_isa.c

/*
 * Copyright (c) 2007 Apple Inc. All rights reserved.
 */
/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License, Version 1.0 only
 * (the "License").  You may not use this file except in compliance
 * with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2005 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#include <sys/fasttrap_isa.h>
#include <sys/fasttrap_impl.h>
#include <sys/dtrace.h>
#include <sys/dtrace_impl.h>
#include <kern/task.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <mach/mach_vm.h>
#include <arm/proc_reg.h>
#include <arm/caches_internal.h>

#include <sys/dtrace_ptss.h>
#include <kern/debug.h>

#include <pexpert/pexpert.h>

extern dtrace_id_t dtrace_probeid_error;

/* Solaris proc_t is the struct. Darwin's proc_t is a pointer to it. */
#define proc_t struct proc /* Steer clear of the Darwin typedef for proc_t */

extern int dtrace_decode_arm(uint32_t instr);
extern int dtrace_decode_thumb(uint32_t instr);

/*
 * Lossless User-Land Tracing on ARM
 * ---------------------------------
 *
 * The details here will be fleshed out as more of this is implemented. The
 * basic design will be the same way as tracing works in x86.
 *
 * Some ARM specific issues:
 *
 * We need to patch differently for ARM instructions and Thumb instructions.
 * When we hit a probe, we check to see if the mode we're currently in is the
 * same as the mode we're patching for. If not, we remove the tracepoint and
 * abort. This ARM/Thumb information is pulled in from the arch specific
 * information in the fasttrap probe.
 *
 * On ARM, any instruction that uses registers can also use the pc as a
 * register. This presents problems during emulation because we have copied
 * the instruction and thus the pc can be different. Currently we've emulated
 * any instructions that use the pc if they can be used in a return probe.
 * Eventually we will want to support all instructions that use the pc, but
 * to do so requires disassembling the instruction and reconstituting it by
 * substituting a different register.
 *
 */

#define THUMB_INSTR(x) (*(uint16_t*) &(x))

#define SIGNEXTEND(x, v) ((((int) (x)) << (32-(v))) >> (32-(v)))
#define ALIGNADDR(x, v) (((x) >> (v)) << (v))
#define GETITSTATE(x) ((((x) >> 8) & 0xFC) | (((x) >> 25) & 0x3))
#define ISLASTINIT(x) (((x) & 0xF) == 8)

#define SET16(x, w) *((uint16_t*) (x)) = (w)
#define SET32(x, w) *((uint32_t*) (x)) = (w)

#define IS_ARM_NOP(x) ((x) == 0xE1A00000)
/* Marker for is-enabled probes */
#define IS_ARM_IS_ENABLED(x) ((x) == 0xE0200000)

#define IS_THUMB_NOP(x) ((x) == 0x46C0)
/* Marker for is-enabled probes */
#define IS_THUMB_IS_ENABLED(x) ((x) == 0x4040)

#define ARM_LDM_UF (1 << 23)
#define ARM_LDM_PF (1 << 24)
#define ARM_LDM_WF (1 << 21)

#define ARM_LDR_UF (1 << 23)
#define ARM_LDR_BF (1 << 22)

extern int dtrace_arm_condition_true(int cond, int cpsr);

int
fasttrap_tracepoint_init(proc_t *p, fasttrap_tracepoint_t *tp,
    user_addr_t pc, fasttrap_probe_type_t type)
{
#pragma unused(type)
        uint32_t instr;

        /*
         * Read the instruction at the given address out of the process's
         * address space. We don't have to worry about a debugger
         * changing this instruction before we overwrite it with our trap
         * instruction since P_PR_LOCK is set. Since instructions can span
         * pages, we potentially read the instruction in two parts. If the
         * second part fails, we just zero out that part of the instruction.
         */
        /*
         * APPLE NOTE: Of course, we do not have a P_PR_LOCK, so this is racey...
         */

        if (uread(p, &instr, 4, pc) != 0) {
                return -1;
        }

        /* We want &instr to always point to the saved instruction, so just copy the
         * whole thing When cast to a pointer to a uint16_t, that will give us a
         * pointer to the first two bytes, which is the thumb instruction.
         */
        tp->ftt_instr = instr;

        if (tp->ftt_fntype != FASTTRAP_FN_DONE_INIT) {
                switch (tp->ftt_fntype) {
                case FASTTRAP_FN_UNKNOWN:
                        /* Can't instrument without any information. We can add some heuristics later if necessary. */
                        return -1;

                case FASTTRAP_FN_USDT:
                        if (IS_ARM_NOP(instr) || IS_ARM_IS_ENABLED(instr)) {
                                tp->ftt_thumb = 0;
                        } else if (IS_THUMB_NOP(THUMB_INSTR(instr)) || IS_THUMB_IS_ENABLED(THUMB_INSTR(instr))) {
                                tp->ftt_thumb = 1;
                        } else {
                                /* Shouldn't reach here - this means we don't recognize
                                 * the instruction at one of the USDT probe locations
                                 */
                                return -1;
                        }
                        tp->ftt_fntype = FASTTRAP_FN_DONE_INIT;
                        break;

                case FASTTRAP_FN_ARM:
                        tp->ftt_thumb = 0;
                        tp->ftt_fntype = FASTTRAP_FN_DONE_INIT;
                        break;

                case FASTTRAP_FN_THUMB:
                        tp->ftt_thumb = 1;
                        tp->ftt_fntype = FASTTRAP_FN_DONE_INIT;
                        break;

                default:
                        return -1;
                }
        }

        if (tp->ftt_thumb) {
                tp->ftt_type = dtrace_decode_thumb(instr);
        } else {
                tp->ftt_type = dtrace_decode_arm(instr);
        }

        if (tp->ftt_type == FASTTRAP_T_INV) {
                /* This is an instruction we either don't recognize or can't instrument */
                printf("dtrace: fasttrap: Unrecognized instruction: %08x at %08x\n",
                    (tp->ftt_thumb && dtrace_instr_size(tp->ftt_instr, tp->ftt_thumb) == 2) ? tp->ftt_instr1 : instr, pc);
                return -1;
        }

        return 0;
}

int
fasttrap_tracepoint_install(proc_t *p, fasttrap_tracepoint_t *tp)
{
        /* The thumb patch is a 2 byte instruction regardless of the size of the original instruction */
        uint32_t instr;
        int size = tp->ftt_thumb ? 2 : 4;

        if (tp->ftt_thumb) {
                *((uint16_t*) &instr) = FASTTRAP_THUMB_INSTR;
        } else {
                instr = FASTTRAP_ARM_INSTR;
        }

        if (uwrite(p, &instr, size, tp->ftt_pc) != 0) {
                return -1;
        }

        tp->ftt_installed = 1;

        return 0;
}

int
fasttrap_tracepoint_remove(proc_t *p, fasttrap_tracepoint_t *tp)
{
        /* The thumb patch is a 2 byte instruction regardless of the size of the original instruction */
        uint32_t instr;
        int size = tp->ftt_thumb ? 2 : 4;

        /*
         * Distinguish between read or write failures and a changed
         * instruction.
         */
        if (uread(p, &instr, size, tp->ftt_pc) != 0) {
                goto end;
        }
        if (tp->ftt_thumb) {
                if (*((uint16_t*) &instr) != FASTTRAP_THUMB_INSTR) {
                        goto end;
                }
        } else {
                if (instr != FASTTRAP_ARM_INSTR) {
                        goto end;
                }
        }
        if (uwrite(p, &tp->ftt_instr, size, tp->ftt_pc) != 0) {
                return -1;
        }

end:
        tp->ftt_installed = 0;

        return 0;
}

static void
fasttrap_return_common(proc_t *p, arm_saved_state_t *regs, user_addr_t pc, user_addr_t new_pc)
{
        pid_t pid = p->p_pid;
        fasttrap_tracepoint_t *tp;
        fasttrap_bucket_t *bucket;
        fasttrap_id_t *id;
        lck_mtx_t *pid_mtx;
        int retire_tp = 1;

        pid_mtx = &cpu_core[CPU->cpu_id].cpuc_pid_lock;
        lck_mtx_lock(pid_mtx);
        bucket = &fasttrap_tpoints.fth_table[FASTTRAP_TPOINTS_INDEX(pid, pc)];

        for (tp = bucket->ftb_data; tp != NULL; tp = tp->ftt_next) {
                if (pid == tp->ftt_pid && pc == tp->ftt_pc &&
                    tp->ftt_proc->ftpc_acount != 0) {
                        break;
                }
        }

        /*
         * Don't sweat it if we can't find the tracepoint again; unlike
         * when we're in fasttrap_pid_probe(), finding the tracepoint here
         * is not essential to the correct execution of the process.
         */
        if (tp == NULL) {
                lck_mtx_unlock(pid_mtx);
                return;
        }

        for (id = tp->ftt_retids; id != NULL; id = id->fti_next) {
                fasttrap_probe_t *probe = id->fti_probe;
                /*
                 * If there's a branch that could act as a return site, we
                 * need to trace it, and check here if the program counter is
                 * external to the function.
                 */
                if (tp->ftt_type != FASTTRAP_T_LDM_PC &&
                    tp->ftt_type != FASTTRAP_T_POP_PC &&
                    new_pc - probe->ftp_faddr < probe->ftp_fsize) {
                        continue;
                }

                if (probe->ftp_prov->ftp_provider_type == DTFTP_PROVIDER_ONESHOT) {
                        if (os_atomic_xchg(&probe->ftp_triggered, 1, relaxed)) {
                                /* already triggered */
                                continue;
                        }
                }
                /*
                 * If we have at least one probe associated that
                 * is not a oneshot probe, don't remove the
                 * tracepoint
                 */
                else {
                        retire_tp = 0;
                }
#if defined(XNU_TARGET_OS_OSX)
                if (ISSET(current_proc()->p_lflag, P_LNOATTACH)) {
                        dtrace_probe(dtrace_probeid_error, 0 /* state */, id->fti_probe->ftp_id,
                            1 /* ndx */, -1 /* offset */, DTRACEFLT_UPRIV);
#else
                if (FALSE) {
#endif /* defined(XNU_TARGET_OS_OSX) */
                } else {
                        dtrace_probe(id->fti_probe->ftp_id,
                            pc - id->fti_probe->ftp_faddr,
                            regs->r[0], 0, 0, 0);
                }
        }
        if (retire_tp) {
                fasttrap_tracepoint_retire(p, tp);
        }

        lck_mtx_unlock(pid_mtx);
}

#if DEBUG
__dead2
#endif
static void
fasttrap_sigsegv(proc_t *p, uthread_t t, user_addr_t addr, arm_saved_state_t *regs)
{
        /* TODO: This function isn't implemented yet. In debug mode, panic the system to
         * find out why we're hitting this point. In other modes, kill the process.
         */
#if DEBUG
#pragma unused(p,t,addr,arm_saved_state)
        panic("fasttrap: sigsegv not yet implemented");
#else
#pragma unused(p,t,addr)
        /* Kill the process */
        regs->pc = 0;
#endif

#if 0
        proc_lock(p);

        /* Set fault address and mark signal */
        t->uu_code = addr;
        t->uu_siglist |= sigmask(SIGSEGV);

        /*
         * XXX These two line may be redundant; if not, then we need
         * XXX to potentially set the data address in the machine
         * XXX specific thread state structure to indicate the address.
         */
        t->uu_exception = KERN_INVALID_ADDRESS;         /* SIGSEGV */
        t->uu_subcode = 0;      /* XXX pad */

        proc_unlock(p);

        /* raise signal */
        signal_setast(t->uu_context.vc_thread);
#endif
}

static void
fasttrap_usdt_args(fasttrap_probe_t *probe, arm_saved_state_t *regs, int argc,
    uint32_t *argv)
{
        int i, x, cap = MIN(argc, probe->ftp_nargs);

        for (i = 0; i < cap; i++) {
                x = probe->ftp_argmap[i];

                if (x < 4) {
                        argv[i] = regs->r[x];
                } else {
                        fasttrap_fuword32_noerr(regs->sp + (x - 4) * sizeof(uint32_t), &argv[i]);
                }
        }

        for (; i < argc; i++) {
                argv[i] = 0;
        }
}

static void
set_thumb_flag(arm_saved_state_t *regs, user_addr_t pc)
{
        if (pc & 1) {
                regs->cpsr |= PSR_TF;
        } else {
                regs->cpsr &= ~PSR_TF;
        }
}

int
fasttrap_pid_probe(arm_saved_state_t *regs)
{
        proc_t *p = current_proc();
        user_addr_t new_pc = 0;
        fasttrap_bucket_t *bucket;
        lck_mtx_t *pid_mtx;
        fasttrap_tracepoint_t *tp, tp_local;
        pid_t pid;
        dtrace_icookie_t cookie;
        uint_t is_enabled = 0;
        int instr_size;
        int was_simulated = 1, retire_tp = 1;

        user_addr_t pc = regs->pc;

        uthread_t uthread = (uthread_t) get_bsdthread_info(current_thread());

        /*
         * It's possible that a user (in a veritable orgy of bad planning)
         * could redirect this thread's flow of control before it reached the
         * return probe fasttrap. In this case we need to kill the process
         * since it's in a unrecoverable state.
         */
        if (uthread->t_dtrace_step) {
                ASSERT(uthread->t_dtrace_on);
                fasttrap_sigtrap(p, uthread, pc);
                return 0;
        }

        /*
         * Clear all user tracing flags.
         */
        uthread->t_dtrace_ft = 0;
        uthread->t_dtrace_pc = 0;
        uthread->t_dtrace_npc = 0;
        uthread->t_dtrace_scrpc = 0;
        uthread->t_dtrace_astpc = 0;

        /*
         * Treat a child created by a call to vfork(2) as if it were its
         * parent. We know that there's only one thread of control in such a
         * process: this one.
         */
        if (p->p_lflag & P_LINVFORK) {
                proc_list_lock();
                while (p->p_lflag & P_LINVFORK) {
                        p = p->p_pptr;
                }
                proc_list_unlock();
        }

        pid = p->p_pid;
        pid_mtx = &cpu_core[CPU->cpu_id].cpuc_pid_lock;
        lck_mtx_lock(pid_mtx);
        bucket = &fasttrap_tpoints.fth_table[FASTTRAP_TPOINTS_INDEX(pid, pc)];

        /*
         * Lookup the tracepoint that the process just hit.
         */
        for (tp = bucket->ftb_data; tp != NULL; tp = tp->ftt_next) {
                if (pid == tp->ftt_pid && pc == tp->ftt_pc &&
                    tp->ftt_proc->ftpc_acount != 0) {
                        break;
                }
        }

        /*
         * If we couldn't find a matching tracepoint, either a tracepoint has
         * been inserted without using the pid<pid> ioctl interface (see
         * fasttrap_ioctl), or somehow we have mislaid this tracepoint.
         */
        if (tp == NULL) {
                lck_mtx_unlock(pid_mtx);
                return -1;
        }

        /* Default to always execute */
        int condition_code = 0xE;
        if (tp->ftt_thumb) {
                uint32_t itstate = GETITSTATE(regs->cpsr);
                if (itstate != 0) {
                        /* In IT block, make sure it's the last statement in the block */
                        if (ISLASTINIT(itstate)) {
                                condition_code = itstate >> 4;
                        } else {
                                printf("dtrace: fasttrap: Tried to trace instruction %08x at %08x but not at end of IT block\n",
                                    (tp->ftt_thumb && dtrace_instr_size(tp->ftt_instr, tp->ftt_thumb) == 2) ? tp->ftt_instr1 : tp->ftt_instr, pc);

                                fasttrap_tracepoint_remove(p, tp);
                                lck_mtx_unlock(pid_mtx);
                                return -1;
                        }
                }
        } else {
                condition_code = ARM_CONDCODE(tp->ftt_instr);
        }

        if (!tp->ftt_thumb != !(regs->cpsr & PSR_TF)) {
                /* The ARM/Thumb mode does not match what we expected for this probe.
                 * Remove this probe and bail.
                 */
                fasttrap_tracepoint_remove(p, tp);
                lck_mtx_unlock(pid_mtx);
                return -1;
        }

        if (tp->ftt_ids != NULL) {
                fasttrap_id_t *id;

                uint32_t s4;
                uint32_t *stack = (uint32_t *)regs->sp;

                /* First four parameters are passed in registers */
                fasttrap_fuword32_noerr((user_addr_t)(uint32_t)stack, &s4);

                for (id = tp->ftt_ids; id != NULL; id = id->fti_next) {
                        fasttrap_probe_t *probe = id->fti_probe;

#if defined(XNU_TARGET_OS_OSX)
                        if (ISSET(current_proc()->p_lflag, P_LNOATTACH)) {
                                dtrace_probe(dtrace_probeid_error, 0 /* state */, probe->ftp_id,
                                    1 /* ndx */, -1 /* offset */, DTRACEFLT_UPRIV);
#else
                        if (FALSE) {
#endif /* defined(XNU_TARGET_OS_OSX) */
                        } else {
                                if (probe->ftp_prov->ftp_provider_type == DTFTP_PROVIDER_ONESHOT) {
                                        if (os_atomic_xchg(&probe->ftp_triggered, 1, relaxed)) {
                                                /* already triggered */
                                                continue;
                                        }
                                }
                                /*
                                 * If we have at least probe associated that
                                 * is not a oneshot probe, don't remove the
                                 * tracepoint
                                 */
                                else {
                                        retire_tp = 0;
                                }
                                if (id->fti_ptype == DTFTP_ENTRY) {
                                        /*
                                         * We note that this was an entry
                                         * probe to help ustack() find the
                                         * first caller.
                                         */
                                        cookie = dtrace_interrupt_disable();
                                        DTRACE_CPUFLAG_SET(CPU_DTRACE_ENTRY);
                                        dtrace_probe(probe->ftp_id, regs->r[0], regs->r[1], regs->r[2], regs->r[3], s4);
                                        DTRACE_CPUFLAG_CLEAR(CPU_DTRACE_ENTRY);
                                        dtrace_interrupt_enable(cookie);
                                } else if (id->fti_ptype == DTFTP_IS_ENABLED) {
                                        /*
                                         * Note that in this case, we don't
                                         * call dtrace_probe() since it's only
                                         * an artificial probe meant to change
                                         * the flow of control so that it
                                         * encounters the true probe.
                                         */
                                        is_enabled = 1;
                                } else if (probe->ftp_argmap == NULL) {
                                        dtrace_probe(probe->ftp_id, regs->r[0], regs->r[1], regs->r[2], regs->r[3], s4);
                                } else {
                                        uint32_t t[5];

                                        fasttrap_usdt_args(probe, regs, 5, t);
                                        dtrace_probe(probe->ftp_id, t[0], t[1], t[2], t[3], t[4]);
                                }
                        }
                }
                if (retire_tp) {
                        fasttrap_tracepoint_retire(p, tp);
                }
        }
        /*
         * We're about to do a bunch of work so we cache a local copy of
         * the tracepoint to emulate the instruction, and then find the
         * tracepoint again later if we need to light up any return probes.
         */
        tp_local = *tp;
        lck_mtx_unlock(pid_mtx);
        tp = &tp_local;

        /*
         * If there's an is-enabled probe connected to this tracepoint it
         * means that there was a 'eor r0,r0,r0'
         * instruction that was placed there by DTrace when the binary was
         * linked. As this probe is, in fact, enabled, we need to stuff 1
         * into R0. Accordingly, we can bypass all the instruction
         * emulation logic since we know the inevitable result. It's possible
         * that a user could construct a scenario where the 'is-enabled'
         * probe was on some other instruction, but that would be a rather
         * exotic way to shoot oneself in the foot.
         */

        if (is_enabled) {
                regs->r[0] = 1;
                new_pc = regs->pc + (tp->ftt_thumb ? 2 : 4);
                goto done;
        }

        /* For USDT probes, bypass all the emulation logic for the nop instruction */
        if ((tp->ftt_thumb && IS_THUMB_NOP(THUMB_INSTR(tp->ftt_instr))) ||
            (!tp->ftt_thumb && IS_ARM_NOP(tp->ftt_instr))) {
                new_pc = regs->pc + (tp->ftt_thumb ? 2 : 4);
                goto done;
        }

        instr_size = dtrace_instr_size(tp->ftt_instr, tp->ftt_thumb);

        switch (tp->ftt_type) {
        case FASTTRAP_T_MOV_PC_REG:
        case FASTTRAP_T_CPY_PC:
        {
                if (!dtrace_arm_condition_true(condition_code, regs->cpsr)) {
                        new_pc = pc + instr_size;
                        break;
                }

                int rm;
                if (tp->ftt_thumb) {
                        rm = THUMB16_HRM(tp->ftt_instr1);
                } else {
                        rm = tp->ftt_instr & 0xF;
                }
                new_pc = regs->r[rm];

                /* This instruction does not change the Thumb state */

                break;
        }

        case FASTTRAP_T_STM_LR:
        case FASTTRAP_T_PUSH_LR:
        {
                /*
                 * This is a very common case, so we want to emulate this instruction if
                 * possible. However, on a push, it is possible that we might reach the end
                 * of a page and have to allocate a new page. Most of the time this will not
                 * happen, and we know that the push instruction can store at most 16 words,
                 * so check to see if we are far from the boundary, and if so, emulate. This
                 * can be made more aggressive by checking the actual number of words being
                 * pushed, but we won't do that for now.
                 *
                 * Some of the same issues that apply to POP_PC probably apply here also.
                 */

                int reglist;
                int ret;
                uintptr_t* base;

                if (!dtrace_arm_condition_true(condition_code, regs->cpsr)) {
                        new_pc = pc + instr_size;
                        break;
                }

                base = (uintptr_t*) regs->sp;
                if (((((uintptr_t) base) - 16 * 4) >> PAGE_SHIFT) != (((uintptr_t) base) >> PAGE_SHIFT)) {
                        /* Crosses the page boundary, go to emulation */
                        goto instr_emulate;
                }

                if (tp->ftt_thumb) {
                        if (instr_size == 4) {
                                /* We know we have to push lr, never push sp or pc */
                                reglist = tp->ftt_instr2 & 0x1FFF;
                        } else {
                                reglist = tp->ftt_instr1 & 0xFF;
                        }
                } else {
                        /* We know we have to push lr, never push sp or pc */
                        reglist = tp->ftt_instr & 0x1FFF;
                }

                /* Push the link register */
                base--;
                ret = fasttrap_suword32((uint32_t) base, regs->lr);
                if (ret == -1) {
                        fasttrap_sigsegv(p, uthread, (user_addr_t) base, regs);
                        new_pc = regs->pc;
                        break;
                }

                /* Start pushing from $r12 */
                int regmask = 1 << 12;
                int regnum = 12;

                while (regmask) {
                        if (reglist & regmask) {
                                base--;
                                ret = fasttrap_suword32((uint32_t) base, regs->r[regnum]);
                                if (ret == -1) {
                                        fasttrap_sigsegv(p, uthread, (user_addr_t) base, regs);
                                        new_pc = regs->pc;
                                        break;
                                }
                        }
                        regmask >>= 1;
                        regnum--;
                }

                regs->sp = (uintptr_t) base;

                new_pc = pc + instr_size;

                break;
        }


        case FASTTRAP_T_LDM_PC:
        case FASTTRAP_T_POP_PC:
        {
                /* TODO Two issues that will eventually need to be resolved:
                 *
                 * 1. Understand what the hardware does if we have to segfault (data abort) in
                 * the middle of a load multiple. We currently don't have a working segfault
                 * handler anyway, and with no swapfile we should never segfault on this load.
                 * If we do, we'll just kill the process by setting the pc to 0.
                 *
                 * 2. The emulation is no longer atomic. We currently only emulate pop for
                 * function epilogues, and so we should never have a race here because one
                 * thread should never be trying to manipulate another thread's stack frames.
                 * That is almost certainly a bug in the program.
                 *
                 * This will need to be fixed if we ever:
                 *   a. Ship dtrace externally, as this could be a potential attack vector
                 *   b. Support instruction level tracing, as we might then pop/ldm non epilogues.
                 *
                 */

                /* Assume ldmia! sp/pop ... pc */

                int regnum = 0, reglist;
                int ret;
                uintptr_t* base;

                if (!dtrace_arm_condition_true(condition_code, regs->cpsr)) {
                        new_pc = pc + instr_size;
                        break;
                }

                if (tp->ftt_thumb) {
                        if (instr_size == 4) {
                                /* We know we have to load the pc, don't do it twice */
                                reglist = tp->ftt_instr2 & 0x7FFF;
                        } else {
                                reglist = tp->ftt_instr1 & 0xFF;
                        }
                } else {
                        /* We know we have to load the pc, don't do it twice */
                        reglist = tp->ftt_instr & 0x7FFF;
                }

                base = (uintptr_t*) regs->sp;
                while (reglist) {
                        if (reglist & 1) {
                                ret = fasttrap_fuword32((uint32_t) base, &regs->r[regnum]);
                                if (ret == -1) {
                                        fasttrap_sigsegv(p, uthread, (user_addr_t) base, regs);
                                        new_pc = regs->pc;
                                        break;
                                }
                                base++;
                        }
                        reglist >>= 1;
                        regnum++;
                }

                ret = fasttrap_fuword32((uint32_t) base, &new_pc);
                if (ret == -1) {
                        fasttrap_sigsegv(p, uthread, (user_addr_t) base, regs);
                        new_pc = regs->pc;
                        break;
                }
                base++;

                regs->sp = (uintptr_t) base;

                set_thumb_flag(regs, new_pc);

                break;
        }

        case FASTTRAP_T_CB_N_Z:
        {
                /* Thumb mode instruction, and not permitted in IT block, so skip the condition code check */
                int rn = tp->ftt_instr1 & 0x7;
                int offset = (((tp->ftt_instr1 & 0x00F8) >> 2) | ((tp->ftt_instr1 & 0x0200) >> 3)) + 4;
                int nonzero = tp->ftt_instr1 & 0x0800;
                if (!nonzero != !(regs->r[rn] == 0)) {
                        new_pc = pc + offset;
                } else {
                        new_pc = pc + instr_size;
                }
                break;
        }

        case FASTTRAP_T_B_COND:
        {
                /* Use the condition code in the instruction and ignore the ITSTATE */

                int code, offset;
                if (tp->ftt_thumb) {
                        if (instr_size == 4) {
                                code = (tp->ftt_instr1 >> 6) & 0xF;
                                if (code == 14 || code == 15) {
                                        panic("fasttrap: Emulation of invalid branch");
                                }
                                int S = (tp->ftt_instr1 >> 10) & 1,
                                    J1 = (tp->ftt_instr2 >> 13) & 1,
                                    J2 = (tp->ftt_instr2 >> 11) & 1;
                                offset = 4 + SIGNEXTEND(
                                        (S << 20) | (J2 << 19) | (J1 << 18) |
                                        ((tp->ftt_instr1 & 0x003F) << 12) |
                                        ((tp->ftt_instr2 & 0x07FF) << 1),
                                        21);
                        } else {
                                code = (tp->ftt_instr1 >> 8) & 0xF;
                                if (code == 14 || code == 15) {
                                        panic("fasttrap: Emulation of invalid branch");
                                }
                                offset = 4 + (SIGNEXTEND(tp->ftt_instr1 & 0xFF, 8) << 1);
                        }
                } else {
                        code = ARM_CONDCODE(tp->ftt_instr);
                        if (code == 15) {
                                panic("fasttrap: Emulation of invalid branch");
                        }
                        offset = 8 + (SIGNEXTEND(tp->ftt_instr & 0x00FFFFFF, 24) << 2);
                }

                if (dtrace_arm_condition_true(code, regs->cpsr)) {
                        new_pc = pc + offset;
                } else {
                        new_pc = pc + instr_size;
                }

                break;
        }

        case FASTTRAP_T_B_UNCOND:
        {
                int offset;

                /* Unconditional branches can only be taken from Thumb mode */
                /* (This is different from an ARM branch with condition code "always") */
                ASSERT(tp->ftt_thumb == 1);

                if (!dtrace_arm_condition_true(condition_code, regs->cpsr)) {
                        new_pc = pc + instr_size;
                        break;
                }

                if (instr_size == 4) {
                        int S = (tp->ftt_instr1 >> 10) & 1,
                            J1 = (tp->ftt_instr2 >> 13) & 1,
                            J2 = (tp->ftt_instr2 >> 11) & 1;
                        int I1 = (J1 != S) ? 0 : 1, I2 = (J2 != S) ? 0 : 1;
                        offset = 4 + SIGNEXTEND(
                                (S << 24) | (I1 << 23) | (I2 << 22) |
                                ((tp->ftt_instr1 & 0x03FF) << 12) |
                                ((tp->ftt_instr2 & 0x07FF) << 1),
                                25);
                } else {
                        uint32_t instr1 = tp->ftt_instr1;
                        offset = 4 + (SIGNEXTEND(instr1 & 0x7FF, 11) << 1);
                }

                new_pc = pc + offset;

                break;
        }

        case FASTTRAP_T_BX_REG:
        {
                int reg;

                if (!dtrace_arm_condition_true(condition_code, regs->cpsr)) {
                        new_pc = pc + instr_size;
                        break;
                }

                if (tp->ftt_thumb) {
                        reg = THUMB16_HRM(tp->ftt_instr1);
                } else {
                        reg = ARM_RM(tp->ftt_instr);
                }
                new_pc = regs->r[reg];
                set_thumb_flag(regs, new_pc);

                break;
        }

        case FASTTRAP_T_LDR_PC_IMMED:
        case FASTTRAP_T_VLDR_PC_IMMED:
                /* Handle these instructions by replacing the PC in the instruction with another
                 * register. They are common, so we'd like to support them, and this way we do so
                 * without any risk of having to simulate a segfault.
                 */

                /* Fall through */

instr_emulate:
        case FASTTRAP_T_COMMON:
        {
                user_addr_t addr;
                uint8_t scratch[32];
                uint_t i = 0;
                fasttrap_instr_t emul_instr;
                emul_instr.instr32 = tp->ftt_instr;
                int emul_instr_size;

                /*
                 * Unfortunately sometimes when we emulate the instruction and have to replace the
                 * PC, there is no longer a thumb mode equivalent. We end up having to run the
                 * modified instruction in ARM mode. We use this variable to keep track of which
                 * mode we should emulate in. We still use the original variable to determine
                 * what mode to return to.
                 */
                uint8_t emul_thumb = tp->ftt_thumb;
                int save_reg = -1;
                uint32_t save_val = 0;

                /*
                 * Dealing with condition codes and emulation:
                 * We can't just uniformly do a condition code check here because not all instructions
                 * have condition codes. We currently do not support an instruction by instruction trace,
                 * so we can assume that either: 1. We are executing a Thumb instruction, in which case
                 * we either are not in an IT block and should execute always, or we are last in an IT
                 * block. Either way, the traced instruction will run correctly, and we won't have any
                 * problems when we return to the original code, because we will no longer be in the IT
                 * block. 2. We are executing an ARM instruction, in which case we are ok as long as
                 * we don't attempt to change the condition code.
                 */
                if (tp->ftt_type == FASTTRAP_T_LDR_PC_IMMED) {
                        /* We know we always have a free register (the one we plan to write the
                         * result value to!). So we'll replace the pc with that one.
                         */
                        int new_reg;
                        if (tp->ftt_thumb) {
                                /* Check to see if thumb or thumb2 */
                                if (instr_size == 2) {
                                        /*
                                         * Sadness. We need to emulate this instruction in ARM mode
                                         * because it has an 8 bit immediate offset. Instead of having
                                         * to deal with condition codes in the ARM instruction, we'll
                                         * just check the condition and abort if the condition is false.
                                         */
                                        if (!dtrace_arm_condition_true(condition_code, regs->cpsr)) {
                                                new_pc = pc + instr_size;
                                                break;
                                        }

                                        new_reg = (tp->ftt_instr1 >> 8) & 0x7;
                                        regs->r[new_reg] = ALIGNADDR(regs->pc + 4, 2);
                                        emul_thumb = 0;
                                        emul_instr.instr32 = 0xE5900000 | (new_reg << 16) | (new_reg << 12) | ((tp->ftt_instr1 & 0xFF) << 2);
                                } else {
                                        /* Thumb2. Just replace the register. */
                                        new_reg = (tp->ftt_instr2 >> 12) & 0xF;
                                        regs->r[new_reg] = ALIGNADDR(regs->pc + 4, 2);
                                        emul_instr.instr16.instr1 &= ~0x000F;
                                        emul_instr.instr16.instr1 |= new_reg;
                                }
                        } else {
                                /* ARM. Just replace the register. */
                                new_reg = (tp->ftt_instr >> 12) & 0xF;
                                regs->r[new_reg] = ALIGNADDR(regs->pc + 8, 2);
                                emul_instr.instr32 &= ~0x000F0000;
                                emul_instr.instr32 |= new_reg << 16;
                        }
                } else if (tp->ftt_type == FASTTRAP_T_VLDR_PC_IMMED) {
                        /* This instruction only uses one register, and if we're here, we know
                         * it must be the pc. So we'll just replace it with R0.
                         */
                        save_reg = 0;
                        save_val = regs->r[0];
                        regs->r[save_reg] = ALIGNADDR(regs->pc + (tp->ftt_thumb ? 4 : 8), 2);
                        if (tp->ftt_thumb) {
                                emul_instr.instr16.instr1 &= ~0x000F;
                        } else {
                                emul_instr.instr32 &= ~0x000F0000;
                        }
                }

                emul_instr_size = dtrace_instr_size(emul_instr.instr32, emul_thumb);

                /*
                 * At this point:
                 *   tp->ftt_thumb = thumb mode of original instruction
                 *   emul_thumb = thumb mode for emulation
                 *   emul_instr = instruction we are using to emulate original instruction
                 *   emul_instr_size = size of emulating instruction
                 */

                addr = uthread->t_dtrace_scratch->addr;

                if (addr == 0LL) {
                        fasttrap_sigtrap(p, uthread, pc);         // Should be killing target proc
                        new_pc = pc;
                        break;
                }

                uthread->t_dtrace_scrpc = addr;
                if (emul_thumb) {
                        /*
                         * No way to do an unconditional branch in Thumb mode, shove the address
                         * onto the user stack and go to the next location with a pop. This can
                         * segfault if this push happens to cross a stack page, but that's ok, since
                         * we are running in userland, and the kernel knows how to handle userland
                         * stack expansions correctly.
                         *
                         * Layout of scratch space for Thumb mode:
                         *   Emulated instruction
                         *   ldr save_reg, [pc, #16] (if necessary, restore any register we clobbered)
                         *   push { r0, r1 }
                         *   ldr r0, [pc, #4]
                         *   str r0, [sp, #4]
                         *   pop { r0, pc }
                         *   Location we should return to in original program
                         *   Saved value of clobbered register (if necessary)
                         */

                        bcopy(&emul_instr, &scratch[i], emul_instr_size); i += emul_instr_size;

                        if (save_reg != -1) {
                                uint16_t restore_inst = 0x4803;
                                restore_inst |= (save_reg & 0x7) << 8;
                                SET16(scratch + i, restore_inst); i += 2;               // ldr reg, [pc , #16]
                        }

                        SET16(scratch + i, 0xB403); i += 2;                             // push { r0, r1 }
                        SET16(scratch + i, 0x4801); i += 2;                             // ldr r0, [pc, #4]
                        SET16(scratch + i, 0x9001); i += 2;                             // str r0, [sp, #4]
                        SET16(scratch + i, 0xBD01); i += 2;                             // pop { r0, pc }

                        if (i % 4) {
                                SET16(scratch + i, 0); i += 2;                          // padding - saved 32 bit words must be aligned
                        }
                        SET32(scratch + i, pc + instr_size + (tp->ftt_thumb ? 1 : 0)); i += 4;          // Return address
                        if (save_reg != -1) {
                                SET32(scratch + i, save_val); i += 4;                   // saved value of clobbered register
                        }

                        uthread->t_dtrace_astpc = addr + i;
                        bcopy(&emul_instr, &scratch[i], emul_instr_size); i += emul_instr_size;
                        SET16(scratch + i, FASTTRAP_THUMB_RET_INSTR); i += 2;
                } else {
                        /*
                         * Layout of scratch space for ARM mode:
                         *   Emulated instruction
                         *   ldr save_reg, [pc, #12] (if necessary, restore any register we clobbered)
                         *   ldr pc, [pc, #4]
                         *   Location we should return to in original program
                         *   Saved value of clobbered register (if necessary)
                         */

                        bcopy(&emul_instr, &scratch[i], emul_instr_size); i += emul_instr_size;

                        if (save_reg != -1) {
                                uint32_t restore_inst = 0xE59F0004;
                                restore_inst |= save_reg << 12;
                                SET32(scratch + i, restore_inst); i += 4;               // ldr reg, [pc, #12]
                        }
                        SET32(scratch + i, 0xE51FF004); i += 4;                         // ldr pc, [pc, #4]

                        SET32(scratch + i, pc + instr_size + (tp->ftt_thumb ? 1 : 0)); i += 4;          // Return address
                        if (save_reg != -1) {
                                SET32(scratch + i, save_val); i += 4;                   // Saved value of clobbered register
                        }

                        uthread->t_dtrace_astpc = addr + i;
                        bcopy(&emul_instr, &scratch[i], emul_instr_size); i += emul_instr_size;
                        SET32(scratch + i, FASTTRAP_ARM_RET_INSTR); i += 4;
                }

                if (uwrite(p, scratch, i, uthread->t_dtrace_scratch->write_addr) != KERN_SUCCESS) {
                        fasttrap_sigtrap(p, uthread, pc);
                        new_pc = pc;
                        break;
                }

                if (tp->ftt_retids != NULL) {
                        uthread->t_dtrace_step = 1;
                        uthread->t_dtrace_ret = 1;
                        new_pc = uthread->t_dtrace_astpc + (emul_thumb ? 1 : 0);
                } else {
                        new_pc = uthread->t_dtrace_scrpc + (emul_thumb ? 1 : 0);
                }

                uthread->t_dtrace_pc = pc;
                uthread->t_dtrace_npc = pc + instr_size;
                uthread->t_dtrace_on = 1;
                was_simulated = 0;
                set_thumb_flag(regs, new_pc);
                break;
        }

        default:
                panic("fasttrap: mishandled an instruction");
        }

done:
        /*
         * APPLE NOTE:
         *
         * We're setting this earlier than Solaris does, to get a "correct"
         * ustack() output. In the Sun code,  a() -> b() -> c() -> d() is
         * reported at: d, b, a. The new way gives c, b, a, which is closer
         * to correct, as the return instruction has already exectued.
         */
        regs->pc = new_pc;

        /*
         * If there were no return probes when we first found the tracepoint,
         * we should feel no obligation to honor any return probes that were
         * subsequently enabled -- they'll just have to wait until the next
         * time around.
         */
        if (tp->ftt_retids != NULL) {
                /*
                 * We need to wait until the results of the instruction are
                 * apparent before invoking any return probes. If this
                 * instruction was emulated we can just call
                 * fasttrap_return_common(); if it needs to be executed, we
                 * need to wait until the user thread returns to the kernel.
                 */
                /*
                 * It used to be that only common instructions were simulated.
                 * For performance reasons, we now simulate some instructions
                 * when safe and go back to userland otherwise. The was_simulated
                 * flag means we don't need to go back to userland.
                 */
                if (was_simulated) {
                        fasttrap_return_common(p, regs, pc, new_pc);
                } else {
                        ASSERT(uthread->t_dtrace_ret != 0);
                        ASSERT(uthread->t_dtrace_pc == pc);
                        ASSERT(uthread->t_dtrace_scrpc != 0);
                        ASSERT(new_pc == uthread->t_dtrace_astpc);
                }
        }

        return 0;
}

int
fasttrap_return_probe(arm_saved_state_t *regs)
{
        proc_t *p = current_proc();
        uthread_t uthread = (uthread_t)get_bsdthread_info(current_thread());
        user_addr_t pc = uthread->t_dtrace_pc;
        user_addr_t npc = uthread->t_dtrace_npc;

        uthread->t_dtrace_pc = 0;
        uthread->t_dtrace_npc = 0;
        uthread->t_dtrace_scrpc = 0;
        uthread->t_dtrace_astpc = 0;

        /*
         * Treat a child created by a call to vfork(2) as if it were its
         * parent. We know that there's only one thread of control in such a
         * process: this one.
         */
        if (p->p_lflag & P_LINVFORK) {
                proc_list_lock();
                while (p->p_lflag & P_LINVFORK) {
                        p = p->p_pptr;
                }
                proc_list_unlock();
        }

        /*
         * We set rp->r_pc to the address of the traced instruction so
         * that it appears to dtrace_probe() that we're on the original
         * instruction, and so that the user can't easily detect our
         * complex web of lies. dtrace_return_probe() (our caller)
         * will correctly set %pc after we return.
         */
        regs->pc = pc;

        fasttrap_return_common(p, regs, pc, npc);

        return 0;
}

uint64_t
fasttrap_pid_getarg(void *arg, dtrace_id_t id, void *parg, int argno,
    int aframes)
{
#pragma unused(arg, id, parg, aframes)
        arm_saved_state_t* regs = find_user_regs(current_thread());

        /* First four arguments are in registers */
        if (argno < 4) {
                return regs->r[argno];
        }

        /* Look on the stack for the rest */
        uint32_t value;
        uint32_t* sp = (uint32_t*) regs->sp;
        DTRACE_CPUFLAG_SET(CPU_DTRACE_NOFAULT);
        value = dtrace_fuword32((user_addr_t) (sp + argno - 4));
        DTRACE_CPUFLAG_CLEAR(CPU_DTRACE_NOFAULT | CPU_DTRACE_BADADDR);

        return value;
}

uint64_t
fasttrap_usdt_getarg(void *arg, dtrace_id_t id, void *parg, int argno, int aframes)
{
#pragma unused(arg, id, parg, argno, aframes)
#if 0
        return fasttrap_anarg(ttolwp(curthread)->lwp_regs, 0, argno);
#endif

        return 0;
}