ia32/os/sysi86.c

	sysi86.c revision ae115bc77f6fcde83175c75b4206dc2e50747966
/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (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 2007 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

/*  Copyright (c) 1990, 1991 UNIX System Laboratories, Inc. */
/*  Copyright (c) 1984, 1986, 1987, 1988, 1989, 1990 AT&T   */
/*    All Rights Reserved   */

/*  Copyright (c) 1987, 1988 Microsoft Corporation  */
/*    All Rights Reserved   */

#pragma ident   "%Z%%M% %I% %E% SMI"

#include <sys/param.h>
#include <sys/types.h>
#include <sys/sysmacros.h>
#include <sys/systm.h>
#include <sys/signal.h>
#include <sys/errno.h>
#include <sys/fault.h>
#include <sys/syscall.h>
#include <sys/cpuvar.h>
#include <sys/sysi86.h>
#include <sys/psw.h>
#include <sys/cred.h>
#include <sys/policy.h>
#include <sys/thread.h>
#include <sys/debug.h>
#include <sys/ontrap.h>
#include <sys/privregs.h>
#include <sys/x86_archext.h>
#include <sys/vmem.h>
#include <sys/kmem.h>
#include <sys/mman.h>
#include <sys/archsystm.h>
#include <vm/hat.h>
#include <vm/as.h>
#include <vm/seg.h>
#include <vm/seg_kmem.h>
#include <vm/faultcode.h>
#include <sys/fp.h>
#include <sys/cmn_err.h>
#include <sys/segments.h>
#include <sys/clock.h>

static void setup_ldt(proc_t *pp);
static void *ldt_map(proc_t *pp, uint_t seli);
static void ldt_free(proc_t *pp);

/*
 * sysi86 System Call
 */

/* ARGSUSED */
int
sysi86(short cmd, uintptr_t arg1, uintptr_t arg2, uintptr_t arg3)
{
    struct ssd ssd;
    int error = 0;
    int c;
    proc_t *pp = curproc;

    switch (cmd) {

    /*
     * The SI86V86 subsystem call of the SYSI86 system call
     * supports only one subcode -- V86SC_IOPL.
     */
    case SI86V86:
        if (arg1 == V86SC_IOPL) {
            struct regs *rp = lwptoregs(ttolwp(curthread));
            greg_t oldpl = rp->r_ps & PS_IOPL;
            greg_t newpl = arg2 & PS_IOPL;

            /*
             * Must be privileged to run this system call
             * if giving more io privilege.
             */
            if (newpl > oldpl && (error =
                secpolicy_sys_config(CRED(), B_FALSE)) != 0)
                return (set_errno(error));
            rp->r_ps ^= oldpl ^ newpl;
        } else
            error = EINVAL;
        break;

    /*
     * Set a segment descriptor
     */
    case SI86DSCR:
        /*
         * There are considerable problems here manipulating
         * resources shared by many running lwps.  Get everyone
         * into a safe state before changing the LDT.
         */
        if (curthread != pp->p_agenttp && !holdlwps(SHOLDFORK1)) {
            error = EINTR;
            break;
        }

        if (get_udatamodel() == DATAMODEL_LP64) {
            error = EINVAL;
            break;
        }

        if (copyin((caddr_t)arg1, &ssd, sizeof (ssd)) < 0) {
            error = EFAULT;
            break;
        }

        error = setdscr(&ssd);

        mutex_enter(&pp->p_lock);
        if (curthread != pp->p_agenttp)
            continuelwps(pp);
        mutex_exit(&pp->p_lock);
        break;

    case SI86FPHW:
        c = fp_kind & 0xff;
        if (suword32((void *)arg1, c) == -1)
            error = EFAULT;
        break;

    case SI86FPSTART:
        /*
         * arg1 is the address of _fp_hw
         * arg2 is the desired x87 FCW value
         * arg3 is the desired SSE MXCSR value
         * a return value of one means SSE hardware, else none.
         */
        c = fp_kind & 0xff;
        if (suword32((void *)arg1, c) == -1) {
            error = EFAULT;
            break;
        }
        fpsetcw((uint16_t)arg2, (uint32_t)arg3);
        return (fp_kind == __FP_SSE ? 1 : 0);

    /* real time clock management commands */

    case WTODC:
        if ((error = secpolicy_settime(CRED())) == 0) {
            timestruc_t ts;
            mutex_enter(&tod_lock);
            gethrestime(&ts);
            tod_set(ts);
            mutex_exit(&tod_lock);
        }
        break;

/* Give some timezone playing room */
#define ONEWEEK (7 * 24 * 60 * 60)

    case SGMTL:
        /*
         * Called from 32 bit land, negative values
         * are not sign extended, so we do that here
         * by casting it to an int and back.  We also
         * clamp the value to within reason and detect
         * when a 64 bit call overflows an int.
         */
        if ((error = secpolicy_settime(CRED())) == 0) {
            int newlag = (int)arg1;

#ifdef _SYSCALL32_IMPL
            if (get_udatamodel() == DATAMODEL_NATIVE &&
                (long)newlag != (long)arg1) {
                error = EOVERFLOW;
            } else
#endif
            if (newlag >= -ONEWEEK && newlag <= ONEWEEK)
                sgmtl(newlag);
            else
                error = EOVERFLOW;
        }
        break;

    case GGMTL:
        if (get_udatamodel() == DATAMODEL_NATIVE) {
            if (sulword((void *)arg1, ggmtl()) == -1)
                error = EFAULT;
#ifdef _SYSCALL32_IMPL
        } else {
            time_t gmtl;

            if ((gmtl = ggmtl()) > INT32_MAX) {
                /*
                 * Since gmt_lag can at most be
                 * +/- 12 hours, something is
                 * *seriously* messed up here.
                 */
                error = EOVERFLOW;
            } else if (suword32((void *)arg1, (int32_t)gmtl) == -1)
                error = EFAULT;
#endif
        }
        break;

    case RTCSYNC:
        if ((error = secpolicy_settime(CRED())) == 0)
            rtcsync();
        break;

    /* END OF real time clock management commands */

    default:
        error = EINVAL;
        break;
    }
    return (error == 0 ? 0 : set_errno(error));
}

void
usd_to_ssd(user_desc_t *usd, struct ssd *ssd, selector_t sel)
{
    ssd->bo = USEGD_GETBASE(usd);
    ssd->ls = USEGD_GETLIMIT(usd);
    ssd->sel = sel;

    /*
     * set type, dpl and present bits.
     */
    ssd->acc1 = usd->usd_type;
    ssd->acc1 |= usd->usd_dpl << 5;
    ssd->acc1 |= usd->usd_p << (5 + 2);

    /*
     * set avl, DB and granularity bits.
     */
    ssd->acc2 = usd->usd_avl;

#if defined(__amd64)
    ssd->acc2 |= usd->usd_long << 1;
#else
    ssd->acc2 |= usd->usd_reserved << 1;
#endif

    ssd->acc2 |= usd->usd_def32 << (1 + 1);
    ssd->acc2 |= usd->usd_gran << (1 + 1 + 1);
}

static void
ssd_to_usd(struct ssd *ssd, user_desc_t *usd)
{

    USEGD_SETBASE(usd, ssd->bo);
    USEGD_SETLIMIT(usd, ssd->ls);

    /*
     * set type, dpl and present bits.
     */
    usd->usd_type = ssd->acc1;
    usd->usd_dpl = ssd->acc1 >> 5;
    usd->usd_p = ssd->acc1 >> (5 + 2);

    ASSERT(usd->usd_type >= SDT_MEMRO);
    ASSERT(usd->usd_dpl == SEL_UPL);

    /*
     * set avl, DB and granularity bits.
     */
    usd->usd_avl = ssd->acc2;

#if defined(__amd64)
    usd->usd_long = ssd->acc2 >> 1;
#else
    usd->usd_reserved = ssd->acc2 >> 1;
#endif

    usd->usd_def32 = ssd->acc2 >> (1 + 1);
    usd->usd_gran = ssd->acc2 >> (1 + 1 + 1);
}

static void
ssd_to_sgd(struct ssd *ssd, gate_desc_t *sgd)
{

    sgd->sgd_looffset = ssd->bo;
    sgd->sgd_hioffset = ssd->bo >> 16;

    sgd->sgd_selector = ssd->ls;
    /*
     * set type, dpl and present bits.
     */
    sgd->sgd_type = ssd->acc1;
    sgd->sgd_dpl = ssd->acc1 >> 5;
    sgd->sgd_p = ssd->acc1 >> 7;
    ASSERT(sgd->sgd_type == SDT_SYSCGT);
    ASSERT(sgd->sgd_dpl == SEL_UPL);

#if defined(__i386) /* reserved, ignored in amd64 */
    sgd->sgd_stkcpy = 0;
#endif
}

/*
 * Load LDT register with the current process's LDT.
 */
void
ldt_load(void)
{
    /*
     */
    *((system_desc_t *)&CPU->cpu_gdt[GDT_LDT]) = curproc->p_ldt_desc;
    wr_ldtr(ULDT_SEL);
}

/*
 * Store a NULL selector in the LDTR. All subsequent illegal references to
 * the LDT will result in a #gp.
 */
void
ldt_unload(void)
{
    CPU->cpu_gdt[GDT_LDT] = zero_udesc;
    wr_ldtr(0);
}

/*ARGSUSED*/
static void
ldt_savectx(proc_t *p)
{
    ASSERT(p->p_ldt != NULL);
    ASSERT(p == curproc);

#if defined(__amd64)
    /*
     * The 64-bit kernel must be sure to clear any stale ldt
     * selectors when context switching away from a process that
     * has a private ldt. Consider the following example:
     *
     *  Wine creats a ldt descriptor and points a segment register
     *  to it.
     *
     *  We then context switch away from wine lwp to kernel
     *  thread and hit breakpoint in kernel with kmdb
     *
     *  When we continue and resume from kmdb we will #gp
     *  fault since kmdb will have saved the stale ldt selector
     *  from wine and will try to restore it but we are no longer in
     *  the context of the wine process and do not have our
     *  ldtr register pointing to the private ldt.
     */
    clr_ldt_sregs();
#endif

    ldt_unload();
    cpu_fast_syscall_enable(NULL);
}

static void
ldt_restorectx(proc_t *p)
{
    ASSERT(p->p_ldt != NULL);
    ASSERT(p == curproc);

    ldt_load();
    cpu_fast_syscall_disable(NULL);
}

/*
 * When a process with a private LDT execs, fast syscalls must be enabled for
 * the new process image.
 */
/* ARGSUSED */
static void
ldt_freectx(proc_t *p, int isexec)
{
    ASSERT(p->p_ldt);

    if (isexec) {
        kpreempt_disable();
        cpu_fast_syscall_enable(NULL);
        kpreempt_enable();
    }

    /*
     * ldt_free() will free the memory used by the private LDT, reset the
     * process's descriptor, and re-program the LDTR.
     */
    ldt_free(p);
}

/*
 * Install ctx op that ensures syscall/sysenter are disabled.
 * See comments below.
 *
 * When a thread with a private LDT forks, the new process
 * must have the LDT context ops installed.
 */
/* ARGSUSED */
static void
ldt_installctx(proc_t *p, proc_t *cp)
{
    proc_t      *targ = p;
    kthread_t   *t;

    /*
     * If this is a fork, operate on the child process.
     */
    if (cp != NULL) {
        targ = cp;
        ldt_dup(p, cp);
    }

    /*
     * The process context ops expect the target process as their argument.
     */
    ASSERT(removepctx(targ, targ, ldt_savectx, ldt_restorectx,
        ldt_installctx, ldt_savectx, ldt_freectx) == 0);

    installpctx(targ, targ, ldt_savectx, ldt_restorectx,
        ldt_installctx, ldt_savectx, ldt_freectx);

    /*
     * We've just disabled fast system call and return instructions; take
     * the slow path out to make sure we don't try to use one to return
     * back to user. We must set t_post_sys for every thread in the
     * process to make sure none of them escape out via fast return.
     */

    mutex_enter(&targ->p_lock);
    t = targ->p_tlist;
    do {
        t->t_post_sys = 1;
    } while ((t = t->t_forw) != targ->p_tlist);
    mutex_exit(&targ->p_lock);
}

int
setdscr(struct ssd *ssd)
{
    ushort_t seli;      /* selector index */
    user_desc_t *dscrp; /* descriptor pointer */
    proc_t  *pp = ttoproc(curthread);

    /*
     * LDT segments: executable and data at DPL 3 only.
     */
    if (!SELISLDT(ssd->sel) || !SELISUPL(ssd->sel))
        return (EINVAL);

    /*
     * check the selector index.
     */
    seli = SELTOIDX(ssd->sel);
    if (seli >= MAXNLDT || seli < LDT_UDBASE)
        return (EINVAL);

    mutex_enter(&pp->p_ldtlock);

    /*
     * If this is the first time for this process then setup a
     * private LDT for it.
     */
    if (pp->p_ldt == NULL) {
        kpreempt_disable();
        setup_ldt(pp);

        /*
         * Now that this process has a private LDT, the use of
         * the syscall/sysret and sysenter/sysexit instructions
         * is forbidden for this processes because they destroy
         * the contents of %cs and %ss segment registers.
         *
         * Explicity disable them here and add a context handler
         * to the process. Note that disabling
         * them here means we can't use sysret or sysexit on
         * the way out of this system call - so we force this
         * thread to take the slow path (which doesn't make use
         * of sysenter or sysexit) back out.
         */

        ldt_installctx(pp, NULL);

        cpu_fast_syscall_disable(NULL);

        ASSERT(curthread->t_post_sys != 0);
        wr_ldtr(ULDT_SEL);
        kpreempt_enable();
    }

    if (ldt_map(pp, seli) == NULL) {
        mutex_exit(&pp->p_ldtlock);
        return (ENOMEM);
    }

    ASSERT(seli <= pp->p_ldtlimit);
    dscrp = &pp->p_ldt[seli];

    /*
     * On the 64-bit kernel, this is where things get more subtle.
     * Recall that in the 64-bit kernel, when we enter the kernel we
     * deliberately -don't- reload the segment selectors we came in on
     * for %ds, %es, %fs or %gs. Messing with selectors is expensive,
     * and the underlying descriptors are essentially ignored by the
     * hardware in long mode - except for the base that we override with
     * the gsbase MSRs.
     *
     * However, there's one unfortunate issue with this rosy picture --
     * a descriptor that's not marked as 'present' will still generate
     * an #np when loading a segment register.
     *
     * Consider this case.  An lwp creates a harmless LDT entry, points
     * one of it's segment registers at it, then tells the kernel (here)
     * to delete it.  In the 32-bit kernel, the #np will happen on the
     * way back to userland where we reload the segment registers, and be
     * handled in kern_gpfault().  In the 64-bit kernel, the same thing
     * will happen in the normal case too.  However, if we're trying to
     * use a debugger that wants to save and restore the segment registers,
     * and the debugger things that we have valid segment registers, we
     * have the problem that the debugger will try and restore the
     * segment register that points at the now 'not present' descriptor
     * and will take a #np right there.
     *
     * We should obviously fix the debugger to be paranoid about
     * -not- restoring segment registers that point to bad descriptors;
     * however we can prevent the problem here if we check to see if any
     * of the segment registers are still pointing at the thing we're
     * destroying; if they are, return an error instead. (That also seems
     * a lot better failure mode than SIGKILL and a core file
     * from kern_gpfault() too.)
     */
    if (SI86SSD_PRES(ssd) == 0) {
        kthread_t *t;
        int bad = 0;

        /*
         * Look carefully at the segment registers of every lwp
         * in the process (they're all stopped by our caller).
         * If we're about to invalidate a descriptor that's still
         * being referenced by *any* of them, return an error,
         * rather than having them #gp on their way out of the kernel.
         */
        ASSERT(pp->p_lwprcnt == 1);

        mutex_enter(&pp->p_lock);
        t = pp->p_tlist;
        do {
            klwp_t *lwp = ttolwp(t);
            struct regs *rp = lwp->lwp_regs;
#if defined(__amd64)
            pcb_t *pcb = &lwp->lwp_pcb;
#endif

            if (ssd->sel == rp->r_cs || ssd->sel == rp->r_ss) {
                bad = 1;
                break;
            }

#if defined(__amd64)
            if (pcb->pcb_flags & RUPDATE_PENDING) {
                if (ssd->sel == pcb->pcb_ds ||
                    ssd->sel == pcb->pcb_es ||
                    ssd->sel == pcb->pcb_fs ||
                    ssd->sel == pcb->pcb_gs) {
                    bad = 1;
                    break;
                }
            } else
#endif
            {
                if (ssd->sel == rp->r_ds ||
                    ssd->sel == rp->r_es ||
                    ssd->sel == rp->r_fs ||
                    ssd->sel == rp->r_gs) {
                    bad = 1;
                    break;
                }
            }

        } while ((t = t->t_forw) != pp->p_tlist);
        mutex_exit(&pp->p_lock);

        if (bad) {
            mutex_exit(&pp->p_ldtlock);
            return (EBUSY);
        }
    }

    /*
     * If acc1 is zero, clear the descriptor (including the 'present' bit)
     */
    if (ssd->acc1 == 0) {
        bzero(dscrp, sizeof (*dscrp));
        mutex_exit(&pp->p_ldtlock);
        return (0);
    }

    /*
     * Check segment type, allow segment not present and
     * only user DPL (3).
     */
    if (SI86SSD_DPL(ssd) != SEL_UPL) {
        mutex_exit(&pp->p_ldtlock);
        return (EINVAL);
    }

#if defined(__amd64)
    /*
     * Do not allow 32-bit applications to create 64-bit mode code
     * segments.
     */
    if (SI86SSD_ISUSEG(ssd) && ((SI86SSD_TYPE(ssd) >> 3) & 1) == 1 &&
        SI86SSD_ISLONG(ssd)) {
        mutex_exit(&pp->p_ldtlock);
        return (EINVAL);
    }
#endif /* __amd64 */

    /*
     * Set up a code or data user segment descriptor.
     */
    if (SI86SSD_ISUSEG(ssd)) {
        ssd_to_usd(ssd, dscrp);
        mutex_exit(&pp->p_ldtlock);
        return (0);
    }

    /*
     * Allow a call gate only if the destination is in the LDT.
     */
    if (SI86SSD_TYPE(ssd) == SDT_SYSCGT && SELISLDT(ssd->ls)) {
        ssd_to_sgd(ssd, (gate_desc_t *)dscrp);
        mutex_exit(&pp->p_ldtlock);
        return (0);
    }

    mutex_exit(&pp->p_ldtlock);
    return (EINVAL);
}

/*
 * Allocate a private LDT for this process and initialize it with the
 * default entries.
 */
static void
setup_ldt(proc_t *pp)
{
    user_desc_t *ldtp;  /* descriptor pointer */
    pgcnt_t npages = btopr(MAXNLDT * sizeof (user_desc_t));

    /*
     * Allocate maximum virtual space we need for this LDT.
     */
    ldtp = vmem_alloc(heap_arena, ptob(npages), VM_SLEEP);

    /*
     * Allocate the minimum number of physical pages for LDT.
     */
    (void) segkmem_xalloc(NULL, ldtp, MINNLDT * sizeof (user_desc_t),
        VM_SLEEP, 0, segkmem_page_create, NULL);

    bzero(ldtp, ptob(btopr(MINNLDT * sizeof (user_desc_t))));

    kpreempt_disable();

    /* Update proc structure. XXX - need any locks here??? */

    set_syssegd(&pp->p_ldt_desc, ldtp, MINNLDT * sizeof (user_desc_t) - 1,
        SDT_SYSLDT, SEL_KPL);

    pp->p_ldtlimit = MINNLDT - 1;
    pp->p_ldt = ldtp;
    if (pp == curproc)
        *((system_desc_t *)&CPU->cpu_gdt[GDT_LDT]) = pp->p_ldt_desc;

    kpreempt_enable();
}

/*
 * Map the page corresponding to the selector entry. If the page is
 * already mapped then it simply returns with the pointer to the entry.
 * Otherwise it allocates a physical page for it and returns the pointer
 * to the entry.  Returns 0 for errors.
 */
static void *
ldt_map(proc_t *pp, uint_t seli)
{
    caddr_t ent0_addr = (caddr_t)&pp->p_ldt[0];
    caddr_t ent_addr = (caddr_t)&pp->p_ldt[seli];
    volatile caddr_t page = (caddr_t)((uintptr_t)ent0_addr & (~PAGEOFFSET));
    caddr_t epage = (caddr_t)((uintptr_t)ent_addr & (~PAGEOFFSET));
    on_trap_data_t otd;

    ASSERT(pp->p_ldt != NULL);

    if (seli <= pp->p_ldtlimit)
        return (ent_addr);

    /*
     * We are increasing the size of the process's LDT.
     * Make sure this and all intervening pages are mapped.
     */
    while (page <= epage) {
        if (!on_trap(&otd, OT_DATA_ACCESS))
            (void) *(volatile int *)page;   /* peek at the page */
        else {      /* Allocate a physical page */
            (void) segkmem_xalloc(NULL, page, PAGESIZE, VM_SLEEP, 0,
                segkmem_page_create, NULL);
            bzero(page, PAGESIZE);
        }
        no_trap();
        page += PAGESIZE;
    }

    /* XXX - need any locks to update proc_t or gdt ??? */

    ASSERT(curproc == pp);

    kpreempt_disable();
    pp->p_ldtlimit = seli;
    SYSSEGD_SETLIMIT(&pp->p_ldt_desc, (seli+1) * sizeof (user_desc_t) -1);

    ldt_load();
    kpreempt_enable();

    return (ent_addr);
}

/*
 * Free up the kernel memory used for LDT of this process.
 */
static void
ldt_free(proc_t *pp)
{
    on_trap_data_t otd;
    caddr_t start, end;
    volatile caddr_t addr;

    ASSERT(pp->p_ldt != NULL);

    mutex_enter(&pp->p_ldtlock);
    start = (caddr_t)pp->p_ldt; /* beginning of the LDT */
    end = start + (pp->p_ldtlimit * sizeof (user_desc_t));

    /* Free the physical page(s) used for mapping LDT */
    for (addr = start; addr <= end; addr += PAGESIZE) {
        if (!on_trap(&otd, OT_DATA_ACCESS)) {
            /* peek at the address */
            (void) *(volatile int *)addr;
            segkmem_free(NULL, addr, PAGESIZE);
        }
    }
    no_trap();

    /* Free up the virtual address space used for this LDT */
    vmem_free(heap_arena, pp->p_ldt,
        ptob(btopr(MAXNLDT * sizeof (user_desc_t))));
    kpreempt_disable();
    pp->p_ldt = NULL;
    pp->p_ldt_desc = zero_sdesc;
    pp->p_ldtlimit = 0;

    if (pp == curproc)
        ldt_unload();
    kpreempt_enable();
    mutex_exit(&pp->p_ldtlock);
}

/*
 * On fork copy new ldt for child.
 */
void
ldt_dup(proc_t *pp, proc_t *cp)
{
    on_trap_data_t otd;
    caddr_t start, end;
    volatile caddr_t addr, caddr;
    int minsize;

    ASSERT(pp->p_ldt);

    setup_ldt(cp);

    mutex_enter(&pp->p_ldtlock);
    cp->p_ldtlimit = pp->p_ldtlimit;
    SYSSEGD_SETLIMIT(&cp->p_ldt_desc,
        (pp->p_ldtlimit+1) * sizeof (user_desc_t) -1);
    start = (caddr_t)pp->p_ldt; /* beginning of the LDT */
    end = start + (pp->p_ldtlimit * sizeof (user_desc_t));
    caddr = (caddr_t)cp->p_ldt; /* child LDT start */

    minsize = ((MINNLDT * sizeof (user_desc_t)) + PAGESIZE) & ~PAGEOFFSET;
    /* Walk thru the physical page(s) used for parent's LDT */
    for (addr = start; addr <= end; addr += PAGESIZE, caddr += PAGESIZE) {
        if (!on_trap(&otd, OT_DATA_ACCESS)) {
            (void) *(volatile int *)addr; /* peek at the address */
            /* allocate a page if necessary */
            if (caddr >= ((caddr_t)cp->p_ldt + minsize)) {
                (void) segkmem_xalloc(NULL, caddr, PAGESIZE,
                    VM_SLEEP, 0, segkmem_page_create, NULL);
            }
            bcopy(addr, caddr, PAGESIZE);
        }
    }
    no_trap();
    mutex_exit(&pp->p_ldtlock);
}