/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 1990 The Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/cdefs.h>
#include "opt_capsicum.h"
#include "opt_kstack_pages.h"
#include "opt_ktrace.h"

#include <sys/param.h>
#include <sys/capsicum.h>
#include <sys/systm.h>
#include <sys/ktrace.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mutex.h>
#include <sys/priv.h>
#include <sys/proc.h>
#include <sys/smp.h>
#include <sys/sysproto.h>

#include <vm/vm.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <vm/vm_extern.h>

#include <machine/atomic.h>
#include <machine/cpu.h>
#include <machine/pcb.h>
#include <machine/pcb_ext.h>
#include <machine/proc.h>
#include <machine/sysarch.h>

#include <security/audit/audit.h>

#include <vm/vm_kern.h>         /* for kernel_map */

#define MAX_LD 8192
#define LD_PER_PAGE 512
#define NEW_MAX_LD(num)  rounddown2(num + LD_PER_PAGE, LD_PER_PAGE)
#define SIZE_FROM_LARGEST_LD(num) (NEW_MAX_LD(num) << 3)
#define NULL_LDT_BASE   ((caddr_t)NULL)

#ifdef SMP
static void set_user_ldt_rv(void *arg);
#endif
static int i386_set_ldt_data(struct thread *, int start, int num,
    union descriptor *descs);
static int i386_ldt_grow(struct thread *td, int len);

void
fill_based_sd(struct segment_descriptor *sdp, uint32_t base)
{

        sdp->sd_lobase = base & 0xffffff;
        sdp->sd_hibase = (base >> 24) & 0xff;
        sdp->sd_lolimit = 0xffff;       /* 4GB limit, wraps around */
        sdp->sd_hilimit = 0xf;
        sdp->sd_type = SDT_MEMRWA;
        sdp->sd_dpl = SEL_UPL;
        sdp->sd_p = 1;
        sdp->sd_xx = 0;
        sdp->sd_def32 = 1;
        sdp->sd_gran = 1;
}

/*
 * Construct special descriptors for "base" selectors.  Store them in
 * the PCB for later use by cpu_switch().  Store them in the GDT for
 * more immediate use.  The GDT entries are part of the current
 * context.  Callers must load related segment registers to complete
 * setting up the current context.
 */
void
set_fsbase(struct thread *td, uint32_t base)
{
        struct segment_descriptor sd;

        fill_based_sd(&sd, base);
        critical_enter();
        td->td_pcb->pcb_fsd = sd;
        if (td == curthread)
                PCPU_GET(fsgs_gdt)[0] = sd;
        critical_exit();
}

void
set_gsbase(struct thread *td, uint32_t base)
{
        struct segment_descriptor sd;

        fill_based_sd(&sd, base);
        critical_enter();
        td->td_pcb->pcb_gsd = sd;
        if (td == curthread)
                PCPU_GET(fsgs_gdt)[1] = sd;
        critical_exit();
}

#ifndef _SYS_SYSPROTO_H_
struct sysarch_args {
        int op;
        char *parms;
};
#endif

int
sysarch(struct thread *td, struct sysarch_args *uap)
{
        int error;
        union descriptor *lp;
        union {
                struct i386_ldt_args largs;
                struct i386_ioperm_args iargs;
                struct i386_get_xfpustate xfpu;
        } kargs;
        uint32_t base;
        struct segment_descriptor *sdp;

        AUDIT_ARG_CMD(uap->op);

#ifdef CAPABILITY_MODE
        /*
         * When adding new operations, add a new case statement here to
         * explicitly indicate whether or not the operation is safe to
         * perform in capability mode.
         */
        switch (uap->op) {
        case I386_GET_LDT:
        case I386_SET_LDT:
        case I386_GET_IOPERM:
        case I386_GET_FSBASE:
        case I386_SET_FSBASE:
        case I386_GET_GSBASE:
        case I386_SET_GSBASE:
        case I386_GET_XFPUSTATE:
                break;

        case I386_SET_IOPERM:
        default:
                if (CAP_TRACING(td))
                        ktrcapfail(CAPFAIL_SYSCALL, &uap->op);
                if (IN_CAPABILITY_MODE(td))
                        return (ECAPMODE);
                break;
        }
#endif

        switch (uap->op) {
        case I386_GET_IOPERM:
        case I386_SET_IOPERM:
                if ((error = copyin(uap->parms, &kargs.iargs,
                    sizeof(struct i386_ioperm_args))) != 0)
                        return (error);
                break;
        case I386_GET_LDT:
        case I386_SET_LDT:
                if ((error = copyin(uap->parms, &kargs.largs,
                    sizeof(struct i386_ldt_args))) != 0)
                        return (error);
                break;
        case I386_GET_XFPUSTATE:
                if ((error = copyin(uap->parms, &kargs.xfpu,
                    sizeof(struct i386_get_xfpustate))) != 0)
                        return (error);
                break;
        default:
                break;
        }

        switch (uap->op) {
        case I386_GET_LDT:
                error = i386_get_ldt(td, &kargs.largs);
                break;
        case I386_SET_LDT:
                if (kargs.largs.descs != NULL) {
                        if (kargs.largs.num > MAX_LD)
                                return (EINVAL);
                        lp = malloc(kargs.largs.num * sizeof(union descriptor),
                            M_TEMP, M_WAITOK);
                        error = copyin(kargs.largs.descs, lp,
                            kargs.largs.num * sizeof(union descriptor));
                        if (error == 0)
                                error = i386_set_ldt(td, &kargs.largs, lp);
                        free(lp, M_TEMP);
                } else {
                        error = i386_set_ldt(td, &kargs.largs, NULL);
                }
                break;
        case I386_GET_IOPERM:
                error = i386_get_ioperm(td, &kargs.iargs);
                if (error == 0)
                        error = copyout(&kargs.iargs, uap->parms,
                            sizeof(struct i386_ioperm_args));
                break;
        case I386_SET_IOPERM:
                error = i386_set_ioperm(td, &kargs.iargs);
                break;
        case I386_VM86:
                error = vm86_sysarch(td, uap->parms);
                break;
        case I386_GET_FSBASE:
                sdp = &td->td_pcb->pcb_fsd;
                base = sdp->sd_hibase << 24 | sdp->sd_lobase;
                error = copyout(&base, uap->parms, sizeof(base));
                break;
        case I386_SET_FSBASE:
                error = copyin(uap->parms, &base, sizeof(base));
                if (error == 0) {
                        /*
                         * Construct the special descriptor for fsbase
                         * and arrange for doreti to load its selector
                         * soon enough.
                         */
                        set_fsbase(td, base);
                        td->td_frame->tf_fs = GSEL(GUFS_SEL, SEL_UPL);
                }
                break;
        case I386_GET_GSBASE:
                sdp = &td->td_pcb->pcb_gsd;
                base = sdp->sd_hibase << 24 | sdp->sd_lobase;
                error = copyout(&base, uap->parms, sizeof(base));
                break;
        case I386_SET_GSBASE:
                error = copyin(uap->parms, &base, sizeof(base));
                if (error == 0) {
                        /*
                         * Construct the special descriptor for gsbase.
                         * The selector is loaded immediately, since we
                         * normally only reload %gs on context switches.
                         */
                        set_gsbase(td, base);
                        load_gs(GSEL(GUGS_SEL, SEL_UPL));
                }
                break;
        case I386_GET_XFPUSTATE:
                if (kargs.xfpu.len > cpu_max_ext_state_size -
                    sizeof(union savefpu))
                        return (EINVAL);
                npxgetregs(td);
                error = copyout((char *)(get_pcb_user_save_td(td) + 1),
                    kargs.xfpu.addr, kargs.xfpu.len);
                break;
        default:
                error = EINVAL;
                break;
        }
        return (error);
}

int
i386_extend_pcb(struct thread *td)
{
        int i, offset;
        u_long *addr;
        struct pcb_ext *ext;
        struct soft_segment_descriptor ssd = {
                0,                      /* segment base address (overwritten) */
                ctob(IOPAGES + 1) - 1,  /* length */
                SDT_SYS386TSS,          /* segment type */
                0,                      /* priority level */
                1,                      /* descriptor present */
                0, 0,
                0,                      /* default 32 size */
                0                       /* granularity */
        };

        ext = pmap_trm_alloc(ctob(IOPAGES + 1), M_WAITOK | M_ZERO);
        /* -16 is so we can convert a trapframe into vm86trapframe inplace */
        ext->ext_tss.tss_ss0 = GSEL(GDATA_SEL, SEL_KPL);
        /*
         * The last byte of the i/o map must be followed by an 0xff byte.
         * We arbitrarily allocate 16 bytes here, to keep the starting
         * address on a doubleword boundary.
         */
        offset = PAGE_SIZE - 16;
        ext->ext_tss.tss_ioopt = 
            (offset - ((unsigned)&ext->ext_tss - (unsigned)ext)) << 16;
        ext->ext_iomap = (caddr_t)ext + offset;
        ext->ext_vm86.vm86_intmap = (caddr_t)ext + offset - 32;

        addr = (u_long *)ext->ext_vm86.vm86_intmap;
        for (i = 0; i < (ctob(IOPAGES) + 32 + 16) / sizeof(u_long); i++)
                *addr++ = ~0;

        ssd.ssd_base = (unsigned)&ext->ext_tss;
        ssd.ssd_limit -= ((unsigned)&ext->ext_tss - (unsigned)ext);
        ssdtosd(&ssd, &ext->ext_tssd);

        KASSERT(td == curthread, ("giving TSS to !curthread"));
        KASSERT(td->td_pcb->pcb_ext == 0, ("already have a TSS!"));

        /* Switch to the new TSS. */
        critical_enter();
        ext->ext_tss.tss_esp0 = PCPU_GET(trampstk);
        td->td_pcb->pcb_ext = ext;
        PCPU_SET(private_tss, 1);
        *PCPU_GET(tss_gdt) = ext->ext_tssd;
        ltr(GSEL(GPROC0_SEL, SEL_KPL));
        critical_exit();

        return 0;
}

int
i386_set_ioperm(struct thread *td, struct i386_ioperm_args *uap)
{
        char *iomap;
        u_int i;
        int error;

        if ((error = priv_check(td, PRIV_IO)) != 0)
                return (error);
        if ((error = securelevel_gt(td->td_ucred, 0)) != 0)
                return (error);
        /*
         * XXX 
         * While this is restricted to root, we should probably figure out
         * whether any other driver is using this i/o address, as so not to
         * cause confusion.  This probably requires a global 'usage registry'.
         */

        if (td->td_pcb->pcb_ext == 0)
                if ((error = i386_extend_pcb(td)) != 0)
                        return (error);
        iomap = (char *)td->td_pcb->pcb_ext->ext_iomap;

        if (uap->start > uap->start + uap->length ||
            uap->start + uap->length > IOPAGES * PAGE_SIZE * NBBY)
                return (EINVAL);

        for (i = uap->start; i < uap->start + uap->length; i++) {
                if (uap->enable)
                        iomap[i >> 3] &= ~(1 << (i & 7));
                else
                        iomap[i >> 3] |= (1 << (i & 7));
        }
        return (error);
}

int
i386_get_ioperm(struct thread *td, struct i386_ioperm_args *uap)
{
        int i, state;
        char *iomap;

        if (uap->start >= IOPAGES * PAGE_SIZE * NBBY)
                return (EINVAL);

        if (td->td_pcb->pcb_ext == 0) {
                uap->length = 0;
                goto done;
        }

        iomap = (char *)td->td_pcb->pcb_ext->ext_iomap;

        i = uap->start;
        state = (iomap[i >> 3] >> (i & 7)) & 1;
        uap->enable = !state;
        uap->length = 1;

        for (i = uap->start + 1; i < IOPAGES * PAGE_SIZE * NBBY; i++) {
                if (state != ((iomap[i >> 3] >> (i & 7)) & 1))
                        break;
                uap->length++;
        }

done:
        return (0);
}

/*
 * Update the GDT entry pointing to the LDT to point to the LDT of the
 * current process. Manage dt_lock holding/unholding autonomously.
 */   
static void
set_user_ldt_locked(struct mdproc *mdp)
{
        struct proc_ldt *pldt;
        int gdt_idx;

        mtx_assert(&dt_lock, MA_OWNED);

        pldt = mdp->md_ldt;
        gdt_idx = GUSERLDT_SEL;
        gdt_idx += PCPU_GET(cpuid) * NGDT;      /* always 0 on UP */
        gdt[gdt_idx].sd = pldt->ldt_sd;
        lldt(GSEL(GUSERLDT_SEL, SEL_KPL));
        PCPU_SET(currentldt, GSEL(GUSERLDT_SEL, SEL_KPL));
}

void
set_user_ldt(struct mdproc *mdp)
{

        mtx_lock_spin(&dt_lock);
        set_user_ldt_locked(mdp);
        mtx_unlock_spin(&dt_lock);
}

#ifdef SMP
static void
set_user_ldt_rv(void *arg)
{
        struct proc *p;

        p = curproc;
        if (arg == p->p_vmspace)
                set_user_ldt(&p->p_md);
}
#endif

/*
 * dt_lock must be held. Returns with dt_lock held.
 */
struct proc_ldt *
user_ldt_alloc(struct mdproc *mdp, int len)
{
        struct proc_ldt *pldt, *new_ldt;

        mtx_assert(&dt_lock, MA_OWNED);
        mtx_unlock_spin(&dt_lock);
        new_ldt = malloc(sizeof(struct proc_ldt), M_SUBPROC, M_WAITOK);

        new_ldt->ldt_len = len = NEW_MAX_LD(len);
        new_ldt->ldt_base = pmap_trm_alloc(len * sizeof(union descriptor),
            M_WAITOK | M_ZERO);
        new_ldt->ldt_refcnt = 1;
        new_ldt->ldt_active = 0;

        mtx_lock_spin(&dt_lock);
        gdt_segs[GUSERLDT_SEL].ssd_base = (unsigned)new_ldt->ldt_base;
        gdt_segs[GUSERLDT_SEL].ssd_limit = len * sizeof(union descriptor) - 1;
        ssdtosd(&gdt_segs[GUSERLDT_SEL], &new_ldt->ldt_sd);

        if ((pldt = mdp->md_ldt) != NULL) {
                if (len > pldt->ldt_len)
                        len = pldt->ldt_len;
                bcopy(pldt->ldt_base, new_ldt->ldt_base,
                    len * sizeof(union descriptor));
        } else
                bcopy(ldt, new_ldt->ldt_base, sizeof(union descriptor) * NLDT);

        return (new_ldt);
}

/*
 * Must be called with dt_lock held.  Returns with dt_lock unheld.
 */
void
user_ldt_free(struct thread *td)
{
        struct mdproc *mdp;
        struct proc_ldt *pldt;

        mtx_assert(&dt_lock, MA_OWNED);
        mdp = &td->td_proc->p_md;
        if ((pldt = mdp->md_ldt) == NULL) {
                mtx_unlock_spin(&dt_lock);
                return;
        }

        if (td == curthread) {
                lldt(_default_ldt);
                PCPU_SET(currentldt, _default_ldt);
        }

        mdp->md_ldt = NULL;
        user_ldt_deref(pldt);
}

void
user_ldt_deref(struct proc_ldt *pldt)
{

        mtx_assert(&dt_lock, MA_OWNED);
        if (--pldt->ldt_refcnt == 0) {
                mtx_unlock_spin(&dt_lock);
                pmap_trm_free(pldt->ldt_base, pldt->ldt_len *
                    sizeof(union descriptor));
                free(pldt, M_SUBPROC);
        } else
                mtx_unlock_spin(&dt_lock);
}

/*
 * Note for the authors of compat layers (linux, etc): copyout() in
 * the function below is not a problem since it presents data in
 * arch-specific format (i.e. i386-specific in this case), not in
 * the OS-specific one.
 */
int
i386_get_ldt(struct thread *td, struct i386_ldt_args *uap)
{
        struct proc_ldt *pldt;
        char *data;
        u_int nldt, num;
        int error;

#ifdef DEBUG
        printf("i386_get_ldt: start=%u num=%u descs=%p\n",
            uap->start, uap->num, (void *)uap->descs);
#endif

        num = min(uap->num, MAX_LD);
        data = malloc(num * sizeof(union descriptor), M_TEMP, M_WAITOK);
        mtx_lock_spin(&dt_lock);
        pldt = td->td_proc->p_md.md_ldt;
        nldt = pldt != NULL ? pldt->ldt_len : NLDT;
        if (uap->start >= nldt) {
                num = 0;
        } else {
                num = min(num, nldt - uap->start);
                bcopy(pldt != NULL ?
                    &((union descriptor *)(pldt->ldt_base))[uap->start] :
                    &ldt[uap->start], data, num * sizeof(union descriptor));
        }
        mtx_unlock_spin(&dt_lock);
        error = copyout(data, uap->descs, num * sizeof(union descriptor));
        if (error == 0)
                td->td_retval[0] = num;
        free(data, M_TEMP);
        return (error);
}

int
i386_set_ldt(struct thread *td, struct i386_ldt_args *uap,
    union descriptor *descs)
{
        struct mdproc *mdp;
        struct proc_ldt *pldt;
        union descriptor *dp;
        u_int largest_ld, i;
        int error;

#ifdef DEBUG
        printf("i386_set_ldt: start=%u num=%u descs=%p\n",
            uap->start, uap->num, (void *)uap->descs);
#endif
        error = 0;
        mdp = &td->td_proc->p_md;

        if (descs == NULL) {
                /* Free descriptors */
                if (uap->start == 0 && uap->num == 0) {
                        /*
                         * Treat this as a special case, so userland needn't
                         * know magic number NLDT.
                         */
                        uap->start = NLDT;
                        uap->num = MAX_LD - NLDT;
                }
                mtx_lock_spin(&dt_lock);
                if ((pldt = mdp->md_ldt) == NULL ||
                    uap->start >= pldt->ldt_len) {
                        mtx_unlock_spin(&dt_lock);
                        return (0);
                }
                largest_ld = uap->start + uap->num;
                if (largest_ld > pldt->ldt_len)
                        largest_ld = pldt->ldt_len;
                for (i = uap->start; i < largest_ld; i++)
                        atomic_store_rel_64(&((uint64_t *)(pldt->ldt_base))[i],
                            0);
                mtx_unlock_spin(&dt_lock);
                return (0);
        }

        if (uap->start != LDT_AUTO_ALLOC || uap->num != 1) {
                /* verify range of descriptors to modify */
                largest_ld = uap->start + uap->num;
                if (uap->start >= MAX_LD || largest_ld > MAX_LD)
                        return (EINVAL);
        }

        /* Check descriptors for access violations */
        for (i = 0; i < uap->num; i++) {
                dp = &descs[i];

                switch (dp->sd.sd_type) {
                case SDT_SYSNULL:       /* system null */ 
                        dp->sd.sd_p = 0;
                        break;
                case SDT_SYS286TSS: /* system 286 TSS available */
                case SDT_SYSLDT:    /* system local descriptor table */
                case SDT_SYS286BSY: /* system 286 TSS busy */
                case SDT_SYSTASKGT: /* system task gate */
                case SDT_SYS286IGT: /* system 286 interrupt gate */
                case SDT_SYS286TGT: /* system 286 trap gate */
                case SDT_SYSNULL2:  /* undefined by Intel */ 
                case SDT_SYS386TSS: /* system 386 TSS available */
                case SDT_SYSNULL3:  /* undefined by Intel */
                case SDT_SYS386BSY: /* system 386 TSS busy */
                case SDT_SYSNULL4:  /* undefined by Intel */ 
                case SDT_SYS386IGT: /* system 386 interrupt gate */
                case SDT_SYS386TGT: /* system 386 trap gate */
                case SDT_SYS286CGT: /* system 286 call gate */ 
                case SDT_SYS386CGT: /* system 386 call gate */
                        return (EACCES);

                /* memory segment types */
                case SDT_MEMEC:   /* memory execute only conforming */
                case SDT_MEMEAC:  /* memory execute only accessed conforming */
                case SDT_MEMERC:  /* memory execute read conforming */
                case SDT_MEMERAC: /* memory execute read accessed conforming */
                         /* Must be "present" if executable and conforming. */
                        if (dp->sd.sd_p == 0)
                                return (EACCES);
                        break;
                case SDT_MEMRO:   /* memory read only */
                case SDT_MEMROA:  /* memory read only accessed */
                case SDT_MEMRW:   /* memory read write */
                case SDT_MEMRWA:  /* memory read write accessed */
                case SDT_MEMROD:  /* memory read only expand dwn limit */
                case SDT_MEMRODA: /* memory read only expand dwn lim accessed */
                case SDT_MEMRWD:  /* memory read write expand dwn limit */  
                case SDT_MEMRWDA: /* memory read write expand dwn lim acessed */
                case SDT_MEME:    /* memory execute only */ 
                case SDT_MEMEA:   /* memory execute only accessed */
                case SDT_MEMER:   /* memory execute read */
                case SDT_MEMERA:  /* memory execute read accessed */
                        break;
                default:
                        return (EINVAL);
                }

                /* Only user (ring-3) descriptors may be present. */
                if (dp->sd.sd_p != 0 && dp->sd.sd_dpl != SEL_UPL)
                        return (EACCES);
        }

        if (uap->start == LDT_AUTO_ALLOC && uap->num == 1) {
                /* Allocate a free slot */
                mtx_lock_spin(&dt_lock);
                if ((pldt = mdp->md_ldt) == NULL) {
                        if ((error = i386_ldt_grow(td, NLDT + 1))) {
                                mtx_unlock_spin(&dt_lock);
                                return (error);
                        }
                        pldt = mdp->md_ldt;
                }
again:
                /*
                 * start scanning a bit up to leave room for NVidia and
                 * Wine, which still user the "Blat" method of allocation.
                 */
                dp = &((union descriptor *)(pldt->ldt_base))[NLDT];
                for (i = NLDT; i < pldt->ldt_len; ++i) {
                        if (dp->sd.sd_type == SDT_SYSNULL)
                                break;
                        dp++;
                }
                if (i >= pldt->ldt_len) {
                        if ((error = i386_ldt_grow(td, pldt->ldt_len+1))) {
                                mtx_unlock_spin(&dt_lock);
                                return (error);
                        }
                        goto again;
                }
                uap->start = i;
                error = i386_set_ldt_data(td, i, 1, descs);
                mtx_unlock_spin(&dt_lock);
        } else {
                largest_ld = uap->start + uap->num;
                mtx_lock_spin(&dt_lock);
                if (!(error = i386_ldt_grow(td, largest_ld))) {
                        error = i386_set_ldt_data(td, uap->start, uap->num,
                            descs);
                }
                mtx_unlock_spin(&dt_lock);
        }
        if (error == 0)
                td->td_retval[0] = uap->start;
        return (error);
}

static int
i386_set_ldt_data(struct thread *td, int start, int num,
    union descriptor *descs)
{
        struct mdproc *mdp;
        struct proc_ldt *pldt;
        uint64_t *dst, *src;
        int i;

        mtx_assert(&dt_lock, MA_OWNED);

        mdp = &td->td_proc->p_md;
        pldt = mdp->md_ldt;
        dst = (uint64_t *)(pldt->ldt_base);
        src = (uint64_t *)descs;

        /*
         * Atomic(9) is used only to get 64bit atomic store with
         * cmpxchg8b when available.  There is no op without release
         * semantic.
         */
        for (i = 0; i < num; i++)
                atomic_store_rel_64(&dst[start + i], src[i]);
        return (0);
}

static int
i386_ldt_grow(struct thread *td, int len) 
{
        struct mdproc *mdp;
        struct proc_ldt *new_ldt, *pldt;
        caddr_t old_ldt_base;
        int old_ldt_len;

        mtx_assert(&dt_lock, MA_OWNED);

        if (len > MAX_LD)
                return (ENOMEM);
        if (len < NLDT + 1)
                len = NLDT + 1;

        mdp = &td->td_proc->p_md;
        old_ldt_base = NULL_LDT_BASE;
        old_ldt_len = 0;

        /* Allocate a user ldt. */
        if ((pldt = mdp->md_ldt) == NULL || len > pldt->ldt_len) {
                new_ldt = user_ldt_alloc(mdp, len);
                if (new_ldt == NULL)
                        return (ENOMEM);
                pldt = mdp->md_ldt;

                if (pldt != NULL) {
                        if (new_ldt->ldt_len <= pldt->ldt_len) {
                                /*
                                 * We just lost the race for allocation, so
                                 * free the new object and return.
                                 */
                                mtx_unlock_spin(&dt_lock);
                                pmap_trm_free(new_ldt->ldt_base,
                                   new_ldt->ldt_len * sizeof(union descriptor));
                                free(new_ldt, M_SUBPROC);
                                mtx_lock_spin(&dt_lock);
                                return (0);
                        }

                        /*
                         * We have to substitute the current LDT entry for
                         * curproc with the new one since its size grew.
                         */
                        old_ldt_base = pldt->ldt_base;
                        old_ldt_len = pldt->ldt_len;
                        pldt->ldt_sd = new_ldt->ldt_sd;
                        pldt->ldt_base = new_ldt->ldt_base;
                        pldt->ldt_len = new_ldt->ldt_len;
                } else
                        mdp->md_ldt = pldt = new_ldt;
#ifdef SMP
                /*
                 * Signal other cpus to reload ldt.  We need to unlock dt_lock
                 * here because other CPU will contest on it since their
                 * curthreads won't hold the lock and will block when trying
                 * to acquire it.
                 */
                mtx_unlock_spin(&dt_lock);
                smp_rendezvous(NULL, set_user_ldt_rv, NULL,
                    td->td_proc->p_vmspace);
#else
                set_user_ldt_locked(&td->td_proc->p_md);
                mtx_unlock_spin(&dt_lock);
#endif
                if (old_ldt_base != NULL_LDT_BASE) {
                        pmap_trm_free(old_ldt_base, old_ldt_len *
                            sizeof(union descriptor));
                        free(new_ldt, M_SUBPROC);
                }
                mtx_lock_spin(&dt_lock);
        }
        return (0);
}