sys/kern/sys_process.c

   1 /*
   2  * Copyright (c) 1994, Sean Eric Fagan
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions
   7  * are met:
   8  * 1. Redistributions of source code must retain the above copyright
   9  *    notice, this list of conditions and the following disclaimer.
  10  * 2. Redistributions in binary form must reproduce the above copyright
  11  *    notice, this list of conditions and the following disclaimer in the
  12  *    documentation and/or other materials provided with the distribution.
  13  * 3. All advertising materials mentioning features or use of this software
  14  *    must display the following acknowledgement:
  15  *      This product includes software developed by Sean Eric Fagan.
  16  * 4. The name of the author may not be used to endorse or promote products
  17  *    derived from this software without specific prior written permission.
  18  *
  19  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  20  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  21  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  22  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  23  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  24  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  25  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  26  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  27  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  28  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  29  * SUCH DAMAGE.
  30  *
  31  * $FreeBSD: src/sys/kern/sys_process.c,v 1.51.2.6 2003/01/08 03:06:45 kan Exp $
  32  * $DragonFly: src/sys/kern/sys_process.c,v 1.30 2007/02/19 01:14:23 corecode Exp $
  33  */
  34
  35 #include <sys/param.h>
  36 #include <sys/systm.h>
  37 #include <sys/sysproto.h>
  38 #include <sys/proc.h>
  39 #include <sys/vnode.h>
  40 #include <sys/ptrace.h>
  41 #include <sys/reg.h>
  42 #include <sys/lock.h>
  43
  44 #include <vm/vm.h>
  45 #include <vm/pmap.h>
  46 #include <vm/vm_map.h>
  47 #include <vm/vm_page.h>
  48
  49 #include <sys/user.h>
  50 #include <vfs/procfs/procfs.h>
  51 #include <sys/thread2.h>
  52
  53 /* use the equivalent procfs code */
  54 #if 0
  55 static int
  56 pread (struct proc *procp, unsigned int addr, unsigned int *retval) {
  57         int             rv;
  58         vm_map_t        map, tmap;
  59         vm_object_t     object;
  60         vm_offset_t     kva = 0;
  61         int             page_offset;    /* offset into page */
  62         vm_offset_t     pageno;         /* page number */
  63         vm_map_entry_t  out_entry;
  64         vm_prot_t       out_prot;
  65         boolean_t       wired;
  66         vm_pindex_t     pindex;
  67
  68         /* Map page into kernel space */
  69
  70         map = &procp->p_vmspace->vm_map;
  71
  72         page_offset = addr - trunc_page(addr);
  73         pageno = trunc_page(addr);
  74
  75         tmap = map;
  76         rv = vm_map_lookup (&tmap, pageno, VM_PROT_READ, &out_entry,
  77                 &object, &pindex, &out_prot, &wired);
  78
  79         if (rv != KERN_SUCCESS)
  80                 return EINVAL;
  81
  82         vm_map_lookup_done (tmap, out_entry, 0);
  83
  84         /* Find space in kernel_map for the page we're interested in */
  85         rv = vm_map_find (&kernel_map, object, IDX_TO_OFF(pindex),
  86                           &kva, PAGE_SIZE,
  87                           0,
  88                           VM_MAPTYPE_NORMAL,
  89                           VM_PROT_ALL, VM_PROT_ALL,
  90                           0);
  91
  92         if (!rv) {
  93                 vm_object_reference (object);
  94
  95                 rv = vm_map_wire (&kernel_map, kva, kva + PAGE_SIZE, 0);
  96                 if (!rv) {
  97                         *retval = 0;
  98                         bcopy ((caddr_t)kva + page_offset,
  99                                retval, sizeof *retval);
 100                 }
 101                 vm_map_remove (&kernel_map, kva, kva + PAGE_SIZE);
 102         }
 103
 104         return rv;
 105 }
 106
 107 static int
 108 pwrite (struct proc *procp, unsigned int addr, unsigned int datum) {
 109         int             rv;
 110         vm_map_t        map, tmap;
 111         vm_object_t     object;
 112         vm_offset_t     kva = 0;
 113         int             page_offset;    /* offset into page */
 114         vm_offset_t     pageno;         /* page number */
 115         vm_map_entry_t  out_entry;
 116         vm_prot_t       out_prot;
 117         boolean_t       wired;
 118         vm_pindex_t     pindex;
 119         boolean_t       fix_prot = 0;
 120
 121         /* Map page into kernel space */
 122
 123         map = &procp->p_vmspace->vm_map;
 124
 125         page_offset = addr - trunc_page(addr);
 126         pageno = trunc_page(addr);
 127
 128         /*
 129          * Check the permissions for the area we're interested in.
 130          */
 131
 132         if (vm_map_check_protection (map, pageno, pageno + PAGE_SIZE,
 133                 VM_PROT_WRITE) == FALSE) {
 134                 /*
 135                  * If the page was not writable, we make it so.
 136                  * XXX It is possible a page may *not* be read/executable,
 137                  * if a process changes that!
 138                  */
 139                 fix_prot = 1;
 140                 /* The page isn't writable, so let's try making it so... */
 141                 if ((rv = vm_map_protect (map, pageno, pageno + PAGE_SIZE,
 142                         VM_PROT_ALL, 0)) != KERN_SUCCESS)
 143                   return EFAULT;        /* I guess... */
 144         }
 145
 146         /*
 147          * Now we need to get the page.  out_entry, out_prot, wired, and
 148          * single_use aren't used.  One would think the vm code would be
 149          * a *bit* nicer...  We use tmap because vm_map_lookup() can
 150          * change the map argument.
 151          */
 152
 153         tmap = map;
 154         rv = vm_map_lookup (&tmap, pageno, VM_PROT_WRITE, &out_entry,
 155                 &object, &pindex, &out_prot, &wired);
 156         if (rv != KERN_SUCCESS) {
 157                 return EINVAL;
 158         }
 159
 160         /*
 161          * Okay, we've got the page.  Let's release tmap.
 162          */
 163
 164         vm_map_lookup_done (tmap, out_entry, 0);
 165
 166         /*
 167          * Fault the page in...
 168          */
 169
 170         rv = vm_fault(map, pageno, VM_PROT_WRITE|VM_PROT_READ, FALSE);
 171         if (rv != KERN_SUCCESS)
 172                 return EFAULT;
 173
 174         /* Find space in kernel_map for the page we're interested in */
 175         rv = vm_map_find (&kernel_map, object, IDX_TO_OFF(pindex),
 176                           &kva, PAGE_SIZE,
 177                           0,
 178                           VM_MAPTYPE_NORMAL,
 179                           VM_PROT_ALL, VM_PROT_ALL,
 180                           0);
 181         if (!rv) {
 182                 vm_object_reference (object);
 183
 184                 rv = vm_map_wire (&kernel_map, kva, kva + PAGE_SIZE, 0);
 185                 if (!rv) {
 186                   bcopy (&datum, (caddr_t)kva + page_offset, sizeof datum);
 187                 }
 188                 vm_map_remove (&kernel_map, kva, kva + PAGE_SIZE);
 189         }
 190
 191         if (fix_prot)
 192                 vm_map_protect (map, pageno, pageno + PAGE_SIZE,
 193                         VM_PROT_READ|VM_PROT_EXECUTE, 0);
 194         return rv;
 195 }
 196 #endif
 197
 198 /*
 199  * Process debugging system call.
 200  */
 201 int
 202 sys_ptrace(struct ptrace_args *uap)
 203 {
 204         struct proc *p = curproc;
 205
 206         /*
 207          * XXX this obfuscation is to reduce stack usage, but the register
 208          * structs may be too large to put on the stack anyway.
 209          */
 210         union {
 211                 struct ptrace_io_desc piod;
 212                 struct dbreg dbreg;
 213                 struct fpreg fpreg;
 214                 struct reg reg;
 215         } r;
 216         void *addr;
 217         int error = 0;
 218
 219         addr = &r;
 220         switch (uap->req) {
 221         case PT_GETREGS:
 222         case PT_GETFPREGS:
 223 #ifdef PT_GETDBREGS
 224         case PT_GETDBREGS:
 225 #endif
 226                 break;
 227         case PT_SETREGS:
 228                 error = copyin(uap->addr, &r.reg, sizeof r.reg);
 229                 break;
 230         case PT_SETFPREGS:
 231                 error = copyin(uap->addr, &r.fpreg, sizeof r.fpreg);
 232                 break;
 233 #ifdef PT_SETDBREGS
 234         case PT_SETDBREGS:
 235                 error = copyin(uap->addr, &r.dbreg, sizeof r.dbreg);
 236                 break;
 237 #endif
 238         case PT_IO:
 239                 error = copyin(uap->addr, &r.piod, sizeof r.piod);
 240                 break;
 241         default:
 242                 addr = uap->addr;
 243         }
 244         if (error)
 245                 return (error);
 246
 247         error = kern_ptrace(p, uap->req, uap->pid, addr, uap->data,
 248                         &uap->sysmsg_result);
 249         if (error)
 250                 return (error);
 251
 252         switch (uap->req) {
 253         case PT_IO:
 254                 (void)copyout(&r.piod, uap->addr, sizeof r.piod);
 255                 break;
 256         case PT_GETREGS:
 257                 error = copyout(&r.reg, uap->addr, sizeof r.reg);
 258                 break;
 259         case PT_GETFPREGS:
 260                 error = copyout(&r.fpreg, uap->addr, sizeof r.fpreg);
 261                 break;
 262 #ifdef PT_GETDBREGS
 263         case PT_GETDBREGS:
 264                 error = copyout(&r.dbreg, uap->addr, sizeof r.dbreg);
 265                 break;
 266 #endif
 267         }
 268
 269         return (error);
 270 }
 271
 272 int
 273 kern_ptrace(struct proc *curp, int req, pid_t pid, void *addr, int data, int *res)
 274 {
 275         struct proc *p, *pp;
 276         struct lwp *lp;
 277         struct iovec iov;
 278         struct uio uio;
 279         struct ptrace_io_desc *piod;
 280         int error = 0;
 281         int write, tmp;
 282
 283         write = 0;
 284         if (req == PT_TRACE_ME) {
 285                 p = curp;
 286         } else {
 287                 if ((p = pfind(pid)) == NULL)
 288                         return ESRCH;
 289         }
 290         if (!PRISON_CHECK(curp->p_ucred, p->p_ucred))
 291                 return (ESRCH);
 292
 293         /* Can't trace a process that's currently exec'ing. */
 294         if ((p->p_flag & P_INEXEC) != 0)
 295                 return EAGAIN;
 296
 297         /*
 298          * Permissions check
 299          */
 300         switch (req) {
 301         case PT_TRACE_ME:
 302                 /* Always legal. */
 303                 break;
 304
 305         case PT_ATTACH:
 306                 /* Self */
 307                 if (p->p_pid == curp->p_pid)
 308                         return EINVAL;
 309
 310                 /* Already traced */
 311                 if (p->p_flag & P_TRACED)
 312                         return EBUSY;
 313
 314                 if (curp->p_flag & P_TRACED)
 315                         for (pp = curp->p_pptr; pp != NULL; pp = pp->p_pptr)
 316                                 if (pp == p)
 317                                         return (EINVAL);
 318
 319                 /* not owned by you, has done setuid (unless you're root) */
 320                 if ((p->p_ucred->cr_ruid != curp->p_ucred->cr_ruid) ||
 321                      (p->p_flag & P_SUGID)) {
 322                         if ((error = suser_cred(curp->p_ucred, 0)) != 0)
 323                                 return error;
 324                 }
 325
 326                 /* can't trace init when securelevel > 0 */
 327                 if (securelevel > 0 && p->p_pid == 1)
 328                         return EPERM;
 329
 330                 /* OK */
 331                 break;
 332
 333         case PT_READ_I:
 334         case PT_READ_D:
 335         case PT_WRITE_I:
 336         case PT_WRITE_D:
 337         case PT_IO:
 338         case PT_CONTINUE:
 339         case PT_KILL:
 340         case PT_STEP:
 341         case PT_DETACH:
 342 #ifdef PT_GETREGS
 343         case PT_GETREGS:
 344 #endif
 345 #ifdef PT_SETREGS
 346         case PT_SETREGS:
 347 #endif
 348 #ifdef PT_GETFPREGS
 349         case PT_GETFPREGS:
 350 #endif
 351 #ifdef PT_SETFPREGS
 352         case PT_SETFPREGS:
 353 #endif
 354 #ifdef PT_GETDBREGS
 355         case PT_GETDBREGS:
 356 #endif
 357 #ifdef PT_SETDBREGS
 358         case PT_SETDBREGS:
 359 #endif
 360                 /* not being traced... */
 361                 if ((p->p_flag & P_TRACED) == 0)
 362                         return EPERM;
 363
 364                 /* not being traced by YOU */
 365                 if (p->p_pptr != curp)
 366                         return EBUSY;
 367
 368                 /* not currently stopped */
 369                 if (p->p_stat != SSTOP ||
 370                     (p->p_flag & P_WAITED) == 0) {
 371                         return EBUSY;
 372                 }
 373
 374                 /* OK */
 375                 break;
 376
 377         default:
 378                 return EINVAL;
 379         }
 380
 381         /* XXX lwp */
 382         lp = FIRST_LWP_IN_PROC(p);
 383 #ifdef FIX_SSTEP
 384         /*
 385          * Single step fixup ala procfs
 386          */
 387         FIX_SSTEP(lp);
 388 #endif
 389
 390         /*
 391          * Actually do the requests
 392          */
 393
 394         *res = 0;
 395
 396         switch (req) {
 397         case PT_TRACE_ME:
 398                 /* set my trace flag and "owner" so it can read/write me */
 399                 p->p_flag |= P_TRACED;
 400                 p->p_oppid = p->p_pptr->p_pid;
 401                 return 0;
 402
 403         case PT_ATTACH:
 404                 /* security check done above */
 405                 p->p_flag |= P_TRACED;
 406                 p->p_oppid = p->p_pptr->p_pid;
 407                 if (p->p_pptr != curp)
 408                         proc_reparent(p, curp);
 409                 data = SIGSTOP;
 410                 goto sendsig;   /* in PT_CONTINUE below */
 411
 412         case PT_STEP:
 413         case PT_CONTINUE:
 414         case PT_DETACH:
 415                 /* Zero means do not send any signal */
 416                 if (data < 0 || data > _SIG_MAXSIG)
 417                         return EINVAL;
 418
 419                 LWPHOLD(lp);
 420
 421                 if (req == PT_STEP) {
 422                         if ((error = ptrace_single_step (lp))) {
 423                                 LWPRELE(lp);
 424                                 return error;
 425                         }
 426                 }
 427
 428                 if (addr != (void *)1) {
 429                         if ((error = ptrace_set_pc (lp,
 430                             (u_long)(uintfptr_t)addr))) {
 431                                 LWPRELE(lp);
 432                                 return error;
 433                         }
 434                 }
 435                 LWPRELE(lp);
 436
 437                 if (req == PT_DETACH) {
 438                         /* reset process parent */
 439                         if (p->p_oppid != p->p_pptr->p_pid) {
 440                                 struct proc *pp;
 441
 442                                 pp = pfind(p->p_oppid);
 443                                 proc_reparent(p, pp ? pp : initproc);
 444                         }
 445
 446                         p->p_flag &= ~(P_TRACED | P_WAITED);
 447                         p->p_oppid = 0;
 448
 449                         /* should we send SIGCHLD? */
 450                 }
 451
 452         sendsig:
 453                 /*
 454                  * Deliver or queue signal.  If the process is stopped
 455                  * force it to be SACTIVE again.
 456                  */
 457                 crit_enter();
 458                 if (p->p_stat == SSTOP) {
 459                         p->p_xstat = data;
 460                         lp->lwp_flag |= LWP_BREAKTSLEEP;
 461                         proc_unstop(p);
 462                 } else if (data) {
 463                         ksignal(p, data);
 464                 }
 465                 crit_exit();
 466                 return 0;
 467
 468         case PT_WRITE_I:
 469         case PT_WRITE_D:
 470                 write = 1;
 471                 /* fallthrough */
 472         case PT_READ_I:
 473         case PT_READ_D:
 474                 /*
 475                  * NOTE! uio_offset represents the offset in the target
 476                  * process.  The iov is in the current process (the guy
 477                  * making the ptrace call) so uio_td must be the current
 478                  * process (though for a SYSSPACE transfer it doesn't
 479                  * really matter).
 480                  */
 481                 tmp = 0;
 482                 /* write = 0 set above */
 483                 iov.iov_base = write ? (caddr_t)&data : (caddr_t)&tmp;
 484                 iov.iov_len = sizeof(int);
 485                 uio.uio_iov = &iov;
 486                 uio.uio_iovcnt = 1;
 487                 uio.uio_offset = (off_t)(uintptr_t)addr;
 488                 uio.uio_resid = sizeof(int);
 489                 uio.uio_segflg = UIO_SYSSPACE;
 490                 uio.uio_rw = write ? UIO_WRITE : UIO_READ;
 491                 uio.uio_td = curthread;
 492                 error = procfs_domem(curp, lp, NULL, &uio);
 493                 if (uio.uio_resid != 0) {
 494                         /*
 495                          * XXX procfs_domem() doesn't currently return ENOSPC,
 496                          * so I think write() can bogusly return 0.
 497                          * XXX what happens for short writes?  We don't want
 498                          * to write partial data.
 499                          * XXX procfs_domem() returns EPERM for other invalid
 500                          * addresses.  Convert this to EINVAL.  Does this
 501                          * clobber returns of EPERM for other reasons?
 502                          */
 503                         if (error == 0 || error == ENOSPC || error == EPERM)
 504                                 error = EINVAL; /* EOF */
 505                 }
 506                 if (!write)
 507                         *res = tmp;
 508                 return (error);
 509
 510         case PT_IO:
 511                 /*
 512                  * NOTE! uio_offset represents the offset in the target
 513                  * process.  The iov is in the current process (the guy
 514                  * making the ptrace call) so uio_td must be the current
 515                  * process.
 516                  */
 517                 piod = addr;
 518                 iov.iov_base = piod->piod_addr;
 519                 iov.iov_len = piod->piod_len;
 520                 uio.uio_iov = &iov;
 521                 uio.uio_iovcnt = 1;
 522                 uio.uio_offset = (off_t)(uintptr_t)piod->piod_offs;
 523                 uio.uio_resid = piod->piod_len;
 524                 uio.uio_segflg = UIO_USERSPACE;
 525                 uio.uio_td = curthread;
 526                 switch (piod->piod_op) {
 527                 case PIOD_READ_D:
 528                 case PIOD_READ_I:
 529                         uio.uio_rw = UIO_READ;
 530                         break;
 531                 case PIOD_WRITE_D:
 532                 case PIOD_WRITE_I:
 533                         uio.uio_rw = UIO_WRITE;
 534                         break;
 535                 default:
 536                         return (EINVAL);
 537                 }
 538                 error = procfs_domem(curp, lp, NULL, &uio);
 539                 piod->piod_len -= uio.uio_resid;
 540                 return (error);
 541
 542         case PT_KILL:
 543                 data = SIGKILL;
 544                 goto sendsig;   /* in PT_CONTINUE above */
 545
 546 #ifdef PT_SETREGS
 547         case PT_SETREGS:
 548                 write = 1;
 549                 /* fallthrough */
 550 #endif /* PT_SETREGS */
 551 #ifdef PT_GETREGS
 552         case PT_GETREGS:
 553                 /* write = 0 above */
 554 #endif /* PT_SETREGS */
 555 #if defined(PT_SETREGS) || defined(PT_GETREGS)
 556                 if (!procfs_validregs(lp))      /* no P_SYSTEM procs please */
 557                         return EINVAL;
 558                 else {
 559                         iov.iov_base = addr;
 560                         iov.iov_len = sizeof(struct reg);
 561                         uio.uio_iov = &iov;
 562                         uio.uio_iovcnt = 1;
 563                         uio.uio_offset = 0;
 564                         uio.uio_resid = sizeof(struct reg);
 565                         uio.uio_segflg = UIO_SYSSPACE;
 566                         uio.uio_rw = write ? UIO_WRITE : UIO_READ;
 567                         uio.uio_td = curthread;
 568                         return (procfs_doregs(curp, lp, NULL, &uio));
 569                 }
 570 #endif /* defined(PT_SETREGS) || defined(PT_GETREGS) */
 571
 572 #ifdef PT_SETFPREGS
 573         case PT_SETFPREGS:
 574                 write = 1;
 575                 /* fallthrough */
 576 #endif /* PT_SETFPREGS */
 577 #ifdef PT_GETFPREGS
 578         case PT_GETFPREGS:
 579                 /* write = 0 above */
 580 #endif /* PT_SETFPREGS */
 581 #if defined(PT_SETFPREGS) || defined(PT_GETFPREGS)
 582                 if (!procfs_validfpregs(lp))    /* no P_SYSTEM procs please */
 583                         return EINVAL;
 584                 else {
 585                         iov.iov_base = addr;
 586                         iov.iov_len = sizeof(struct fpreg);
 587                         uio.uio_iov = &iov;
 588                         uio.uio_iovcnt = 1;
 589                         uio.uio_offset = 0;
 590                         uio.uio_resid = sizeof(struct fpreg);
 591                         uio.uio_segflg = UIO_SYSSPACE;
 592                         uio.uio_rw = write ? UIO_WRITE : UIO_READ;
 593                         uio.uio_td = curthread;
 594                         return (procfs_dofpregs(curp, lp, NULL, &uio));
 595                 }
 596 #endif /* defined(PT_SETFPREGS) || defined(PT_GETFPREGS) */
 597
 598 #ifdef PT_SETDBREGS
 599         case PT_SETDBREGS:
 600                 write = 1;
 601                 /* fallthrough */
 602 #endif /* PT_SETDBREGS */
 603 #ifdef PT_GETDBREGS
 604         case PT_GETDBREGS:
 605                 /* write = 0 above */
 606 #endif /* PT_SETDBREGS */
 607 #if defined(PT_SETDBREGS) || defined(PT_GETDBREGS)
 608                 if (!procfs_validdbregs(lp))    /* no P_SYSTEM procs please */
 609                         return EINVAL;
 610                 else {
 611                         iov.iov_base = addr;
 612                         iov.iov_len = sizeof(struct dbreg);
 613                         uio.uio_iov = &iov;
 614                         uio.uio_iovcnt = 1;
 615                         uio.uio_offset = 0;
 616                         uio.uio_resid = sizeof(struct dbreg);
 617                         uio.uio_segflg = UIO_SYSSPACE;
 618                         uio.uio_rw = write ? UIO_WRITE : UIO_READ;
 619                         uio.uio_td = curthread;
 620                         return (procfs_dodbregs(curp, lp, NULL, &uio));
 621                 }
 622 #endif /* defined(PT_SETDBREGS) || defined(PT_GETDBREGS) */
 623
 624         default:
 625                 break;
 626         }
 627
 628         return 0;
 629 }
 630
 631 int
 632 trace_req(struct proc *p)
 633 {
 634         return 1;
 635 }
 636
 637 /*
 638  * stopevent()
 639  *
 640  * Stop a process because of a procfs event.  Stay stopped until p->p_step
 641  * is cleared (cleared by PIOCCONT in procfs).
 642  *
 643  * MPSAFE
 644  */
 645 void
 646 stopevent(struct proc *p, unsigned int event, unsigned int val)
 647 {
 648         p->p_step = 1;
 649
 650         do {
 651                 crit_enter();
 652                 wakeup(&p->p_stype);    /* Wake up any PIOCWAIT'ing procs */
 653                 p->p_xstat = val;
 654                 p->p_stype = event;     /* Which event caused the stop? */
 655                 tsleep(&p->p_step, 0, "stopevent", 0);
 656                 crit_exit();
 657         } while (p->p_step);
 658 }
 659