sys/src/9/port/chan.c

   1 #include        "u.h"
   2 #include        "../port/lib.h"
   3 #include        "mem.h"
   4 #include        "dat.h"
   5 #include        "fns.h"
   6 #include        "../port/error.h"
   7
   8 int chandebug=0;                /* toggled by sysr1 */
   9 #define DBG if(chandebug)iprint
  10
  11 enum
  12 {
  13         PATHSLOP        = 20,
  14         PATHMSLOP       = 20,
  15 };
  16
  17 static struct Chanalloc
  18 {
  19         Lock;
  20         int     fid;
  21         Chan    *free;
  22         Chan    *list;
  23 }chanalloc;
  24
  25 typedef struct Elemlist Elemlist;
  26
  27 struct Elemlist
  28 {
  29         char    *aname; /* original name */
  30         char    *name;  /* copy of name, so '/' can be overwritten */
  31         int     nelems;
  32         char    **elems;
  33         int     *off;
  34         int     mustbedir;
  35         int     nerror;
  36         int     prefix;
  37 };
  38
  39 #define SEP(c) ((c) == 0 || (c) == '/')
  40
  41 static void
  42 dumpmount(void)         /* DEBUGGING */
  43 {
  44         Pgrp *pg;
  45         Mount *t;
  46         Mhead **h, **he, *f;
  47
  48         if(up == nil){
  49                 print("no process for dumpmount\n");
  50                 return;
  51         }
  52         pg = up->pgrp;
  53         if(pg == nil){
  54                 print("no pgrp for dumpmount\n");
  55                 return;
  56         }
  57         rlock(&pg->ns);
  58         if(waserror()){
  59                 runlock(&pg->ns);
  60                 nexterror();
  61         }
  62
  63         he = &pg->mnthash[MNTHASH];
  64         for(h = pg->mnthash; h < he; h++){
  65                 for(f = *h; f; f = f->hash){
  66                         print("head: %#p: %s %#llux.%lud %C %lud -> \n", f,
  67                                 f->from->path->s, f->from->qid.path,
  68                                 f->from->qid.vers, devtab[f->from->type]->dc,
  69                                 f->from->dev);
  70                         for(t = f->mount; t; t = t->next)
  71                                 print("\t%#p: %s (umh %#p) (path %#.8llux dev %C %lud)\n", t, t->to->path->s, t->to->umh, t->to->qid.path, devtab[t->to->type]->dc, t->to->dev);
  72                 }
  73         }
  74         poperror();
  75         runlock(&pg->ns);
  76 }
  77
  78 char*
  79 chanpath(Chan *c)
  80 {
  81         if(c == nil)
  82                 return "<nil chan>";
  83         if(c->path == nil)
  84                 return "<nil path>";
  85         if(c->path->s == nil)
  86                 return "<nil path.s>";
  87         return c->path->s;
  88 }
  89
  90 int
  91 isdotdot(char *p)
  92 {
  93         return p[0]=='.' && p[1]=='.' && p[2]=='\0';
  94 }
  95
  96 long
  97 incref(Ref *r)
  98 {
  99         long x;
 100
 101         lock(r);
 102         x = ++r->ref;
 103         unlock(r);
 104         return x;
 105 }
 106
 107 long
 108 decref(Ref *r)
 109 {
 110         long x;
 111
 112         lock(r);
 113         x = --r->ref;
 114         unlock(r);
 115         if(x < 0)
 116                 panic("decref pc=%#p", getcallerpc(&r));
 117         return x;
 118 }
 119
 120 /*
 121  * Rather than strncpy, which zeros the rest of the buffer, kstrcpy
 122  * truncates if necessary, always zero terminates, does not zero fill,
 123  * and puts ... at the end of the string if it's too long.  Usually used to
 124  * save a string in up->genbuf;
 125  */
 126 void
 127 kstrcpy(char *s, char *t, int ns)
 128 {
 129         int nt;
 130
 131         nt = strlen(t);
 132         if(nt < ns){
 133                 memmove(s, t, nt);
 134                 s[nt] = '\0';
 135                 return;
 136         }
 137         /* too long, truncate */
 138         nt = ns-1;
 139         memmove(s, t, nt);
 140         s[nt] = '\0';
 141         /* append ... if there is space */
 142         ns -= 4;
 143         if(ns < 0)
 144                 return;
 145         /* look for first byte of UTF-8 sequence by skipping continuation bytes */
 146         while(ns>0 && (s[--ns]&0xC0)==0x80)
 147                 ;
 148         strcpy(s+ns, "...");
 149 }
 150
 151 int
 152 emptystr(char *s)
 153 {
 154         if(s == nil)
 155                 return 1;
 156         if(s[0] == '\0')
 157                 return 1;
 158         return 0;
 159 }
 160
 161 /*
 162  * Atomically replace *p with copy of s
 163  */
 164 void
 165 kstrdup(char **p, char *s)
 166 {
 167         int n;
 168         char *t, *prev;
 169
 170         n = strlen(s);
 171         /* if it's a user, we can wait for memory; if not, something's very wrong */
 172         if(up != nil)
 173                 t = smalloc(n+1);
 174         else{
 175                 t = malloc(n+1);
 176                 if(t == nil)
 177                         panic("kstrdup: no memory");
 178         }
 179         setmalloctag(t, getcallerpc(&p));
 180         memmove(t, s, n);
 181         t[n] = '\0';
 182         prev = *p;
 183         *p = t;
 184         free(prev);
 185 }
 186
 187 void
 188 chandevreset(void)
 189 {
 190         int i;
 191
 192         todinit();      /* avoid later reentry causing infinite recursion */
 193         for(i=0; devtab[i] != nil; i++)
 194                 devtab[i]->reset();
 195 }
 196
 197 void
 198 chandevinit(void)
 199 {
 200         int i;
 201
 202         for(i=0; devtab[i] != nil; i++)
 203                 devtab[i]->init();
 204 }
 205
 206 void
 207 chandevshutdown(void)
 208 {
 209         int i;
 210
 211         /* shutdown in reverse order */
 212         for(i=0; devtab[i] != nil; i++)
 213                 ;
 214         for(i--; i >= 0; i--)
 215                 devtab[i]->shutdown();
 216 }
 217
 218 Chan*
 219 newchan(void)
 220 {
 221         Chan *c;
 222
 223         lock(&chanalloc);
 224         c = chanalloc.free;
 225         if(c != 0){
 226                 chanalloc.free = c->next;
 227                 c->next = 0;
 228         }
 229         unlock(&chanalloc);
 230
 231         if(c == nil){
 232                 c = smalloc(sizeof(Chan));
 233                 lock(&chanalloc);
 234                 c->fid = ++chanalloc.fid;
 235                 c->link = chanalloc.list;
 236                 chanalloc.list = c;
 237                 unlock(&chanalloc);
 238         }
 239
 240         /* if you get an error before associating with a dev,
 241            close calls rootclose, a nop */
 242         c->type = 0;
 243         c->flag = 0;
 244         c->ref = 1;
 245         c->dev = 0;
 246         c->offset = 0;
 247         c->devoffset = 0;
 248         c->iounit = 0;
 249         c->umh = 0;
 250         c->uri = 0;
 251         c->dri = 0;
 252         c->aux = 0;
 253         c->mchan = 0;
 254         c->mcp = 0;
 255         c->mux = 0;
 256         memset(&c->mqid, 0, sizeof(c->mqid));
 257         c->path = 0;
 258         c->ismtpt = 0;
 259
 260         return c;
 261 }
 262
 263 Ref npath;
 264
 265 Path*
 266 newpath(char *s)
 267 {
 268         int i;
 269         Path *p;
 270
 271         p = smalloc(sizeof(Path));
 272         i = strlen(s);
 273         p->len = i;
 274         p->alen = i+PATHSLOP;
 275         p->s = smalloc(p->alen);
 276         memmove(p->s, s, i+1);
 277         p->ref = 1;
 278         incref(&npath);
 279
 280         /*
 281          * Cannot use newpath for arbitrary names because the mtpt
 282          * array will not be populated correctly.  The names #/ and / are
 283          * allowed, but other names with / in them draw warnings.
 284          */
 285         if(strchr(s, '/') && strcmp(s, "#/") != 0 && strcmp(s, "/") != 0)
 286                 print("newpath: %s from %#p\n", s, getcallerpc(&s));
 287
 288         p->mlen = 1;
 289         p->malen = PATHMSLOP;
 290         p->mtpt = smalloc(p->malen*sizeof p->mtpt[0]);
 291         return p;
 292 }
 293
 294 static Path*
 295 copypath(Path *p)
 296 {
 297         int i;
 298         Path *pp;
 299
 300         pp = smalloc(sizeof(Path));
 301         pp->ref = 1;
 302         incref(&npath);
 303         DBG("copypath %s %p => %p\n", p->s, p, pp);
 304
 305         pp->len = p->len;
 306         pp->alen = p->alen;
 307         pp->s = smalloc(p->alen);
 308         memmove(pp->s, p->s, p->len+1);
 309
 310         pp->mlen = p->mlen;
 311         pp->malen = p->malen;
 312         pp->mtpt = smalloc(p->malen*sizeof pp->mtpt[0]);
 313         for(i=0; i<pp->mlen; i++){
 314                 pp->mtpt[i] = p->mtpt[i];
 315                 if(pp->mtpt[i])
 316                         incref(pp->mtpt[i]);
 317         }
 318
 319         return pp;
 320 }
 321
 322 void
 323 pathclose(Path *p)
 324 {
 325         int i;
 326
 327         if(p == nil)
 328                 return;
 329 //XXX
 330         DBG("pathclose %p %s ref=%ld =>", p, p->s, p->ref);
 331         for(i=0; i<p->mlen; i++)
 332                 DBG(" %p", p->mtpt[i]);
 333         DBG("\n");
 334
 335         if(decref(p))
 336                 return;
 337         decref(&npath);
 338         free(p->s);
 339         for(i=0; i<p->mlen; i++)
 340                 if(p->mtpt[i])
 341                         cclose(p->mtpt[i]);
 342         free(p->mtpt);
 343         free(p);
 344 }
 345
 346 /*
 347  * In place, rewrite name to compress multiple /, eliminate ., and process ..
 348  * (Really only called to remove a trailing .. that has been added.
 349  * Otherwise would need to update n->mtpt as well.)
 350  */
 351 static void
 352 fixdotdotname(Path *p)
 353 {
 354         char *r;
 355
 356         if(p->s[0] == '#'){
 357                 r = strchr(p->s, '/');
 358                 if(r == nil)
 359                         return;
 360                 cleanname(r);
 361
 362                 /*
 363                  * The correct name is #i rather than #i/,
 364                  * but the correct name of #/ is #/.
 365                  */
 366                 if(strcmp(r, "/")==0 && p->s[1] != '/')
 367                         *r = '\0';
 368         }else
 369                 cleanname(p->s);
 370         p->len = strlen(p->s);
 371 }
 372
 373 static Path*
 374 uniquepath(Path *p)
 375 {
 376         Path *new;
 377
 378         if(p->ref > 1){
 379                 /* copy on write */
 380                 new = copypath(p);
 381                 pathclose(p);
 382                 p = new;
 383         }
 384         return p;
 385 }
 386
 387 static Path*
 388 addelem(Path *p, char *s, Chan *from)
 389 {
 390         char *t;
 391         int a, i;
 392         Chan *c, **tt;
 393
 394         if(s[0]=='.' && s[1]=='\0')
 395                 return p;
 396
 397         p = uniquepath(p);
 398
 399         i = strlen(s);
 400         if(p->len+1+i+1 > p->alen){
 401                 a = p->len+1+i+1 + PATHSLOP;
 402                 t = smalloc(a);
 403                 memmove(t, p->s, p->len+1);
 404                 free(p->s);
 405                 p->s = t;
 406                 p->alen = a;
 407         }
 408         /* don't insert extra slash if one is present */
 409         if(p->len>0 && p->s[p->len-1]!='/' && s[0]!='/')
 410                 p->s[p->len++] = '/';
 411         memmove(p->s+p->len, s, i+1);
 412         p->len += i;
 413         if(isdotdot(s)){
 414                 fixdotdotname(p);
 415                 DBG("addelem %s .. => rm %p\n", p->s, p->mtpt[p->mlen-1]);
 416                 if(p->mlen>1 && (c = p->mtpt[--p->mlen])){
 417                         p->mtpt[p->mlen] = nil;
 418                         cclose(c);
 419                 }
 420         }else{
 421                 if(p->mlen >= p->malen){
 422                         p->malen = p->mlen+1+PATHMSLOP;
 423                         tt = smalloc(p->malen*sizeof tt[0]);
 424                         memmove(tt, p->mtpt, p->mlen*sizeof tt[0]);
 425                         free(p->mtpt);
 426                         p->mtpt = tt;
 427                 }
 428                 DBG("addelem %s %s => add %p\n", p->s, s, from);
 429                 p->mtpt[p->mlen++] = from;
 430                 if(from)
 431                         incref(from);
 432         }
 433         return p;
 434 }
 435
 436 void
 437 chanfree(Chan *c)
 438 {
 439         c->flag = CFREE;
 440
 441         if(c->dirrock != nil){
 442                 free(c->dirrock);
 443                 c->dirrock = 0;
 444                 c->nrock = 0;
 445                 c->mrock = 0;
 446         }
 447         if(c->umh != nil){
 448                 putmhead(c->umh);
 449                 c->umh = nil;
 450         }
 451         if(c->umc != nil){
 452                 cclose(c->umc);
 453                 c->umc = nil;
 454         }
 455         if(c->mux != nil){
 456                 muxclose(c->mux);
 457                 c->mux = nil;
 458         }
 459         if(c->mchan != nil){
 460                 cclose(c->mchan);
 461                 c->mchan = nil;
 462         }
 463
 464         pathclose(c->path);
 465         c->path = nil;
 466
 467         lock(&chanalloc);
 468         c->next = chanalloc.free;
 469         chanalloc.free = c;
 470         unlock(&chanalloc);
 471 }
 472
 473 /*
 474  * Queue a chan to be closed by one of the clunk procs.
 475  */
 476 struct {
 477         Chan *head;
 478         Chan *tail;
 479         ulong nqueued;
 480         ulong nclosed;
 481         Lock l;
 482         QLock q;
 483         Rendez r;
 484 } clunkq;
 485
 486 static int
 487 clunkwork(void*)
 488 {
 489         return clunkq.head != nil;
 490 }
 491
 492 static void
 493 closeproc(void*)
 494 {
 495         Chan *c;
 496
 497         for(;;){
 498                 if(clunkq.head == nil){
 499                         if(!waserror()){
 500                                 tsleep(&clunkq.r, clunkwork, nil, 5000);
 501                                 poperror();
 502                         }
 503                 }
 504                 qunlock(&clunkq.q);
 505                 lock(&clunkq.l);
 506                 c = clunkq.head;
 507                 if(c == nil){
 508                         unlock(&clunkq.l);
 509                         pexit("no work", 1);
 510                 }
 511                 clunkq.head = c->next;
 512                 clunkq.nclosed++;
 513                 unlock(&clunkq.l);
 514                 if(!waserror()){
 515                         devtab[c->type]->close(c);
 516                         poperror();
 517                 }
 518                 chanfree(c);
 519                 qlock(&clunkq.q);
 520         }
 521 }
 522
 523 static void
 524 closechanq(Chan *c)
 525 {
 526         lock(&clunkq.l);
 527         clunkq.nqueued++;
 528         c->next = nil;
 529         if(clunkq.head)
 530                 clunkq.tail->next = c;
 531         else
 532                 clunkq.head = c;
 533         clunkq.tail = c;
 534         unlock(&clunkq.l);
 535
 536         if(up != 0 && palloc.Lock.p != up && canqlock(&clunkq.q)){
 537                 c = up->dot;
 538                 up->dot = nil;
 539                 if(!waserror()){
 540                         kproc("closeproc", closeproc, nil);
 541                         poperror();
 542                 }
 543                 up->dot = c;
 544         }else
 545                 wakeup(&clunkq.r);
 546 }
 547
 548 void
 549 cclose(Chan *c)
 550 {
 551         if(c == nil || c->ref < 1 || c->flag&CFREE)
 552                 panic("cclose %#p", getcallerpc(&c));
 553
 554         DBG("cclose %p name=%s ref=%ld\n", c, chanpath(c), c->ref);
 555
 556         if(decref(c))
 557                 return;
 558
 559         if(devtab[c->type]->dc == L'M')
 560         if((c->flag&(CRCLOSE|CCACHE)) == CCACHE)
 561         if((c->qid.type&(QTEXCL|QTMOUNT|QTAUTH)) == 0)
 562         if((clunkq.nqueued - clunkq.nclosed) < 64){
 563                 closechanq(c);
 564                 return;
 565         }
 566
 567         if(!waserror()){
 568                 devtab[c->type]->close(c);
 569                 poperror();
 570         }
 571         chanfree(c);
 572 }
 573
 574 void
 575 ccloseq(Chan *c)
 576 {
 577         if(c == nil || c->ref < 1 || c->flag&CFREE)
 578                 panic("ccloseq %#p", getcallerpc(&c));
 579
 580         DBG("ccloseq %p name=%s ref=%ld\n", c, chanpath(c), c->ref);
 581
 582         if(decref(c))
 583                 return;
 584
 585         closechanq(c);
 586 }
 587
 588 /*
 589  * Make sure we have the only copy of c.  (Copy on write.)
 590  */
 591 Chan*
 592 cunique(Chan *c)
 593 {
 594         Chan *nc;
 595
 596         if(c->ref != 1){
 597                 nc = cclone(c);
 598                 cclose(c);
 599                 c = nc;
 600         }
 601
 602         return c;
 603 }
 604
 605 int
 606 eqqid(Qid a, Qid b)
 607 {
 608         return a.path==b.path && a.vers==b.vers;
 609 }
 610
 611 int
 612 eqchan(Chan *a, Chan *b, int skipvers)
 613 {
 614         if(a->qid.path != b->qid.path)
 615                 return 0;
 616         if(!skipvers && a->qid.vers!=b->qid.vers)
 617                 return 0;
 618         if(a->type != b->type)
 619                 return 0;
 620         if(a->dev != b->dev)
 621                 return 0;
 622         return 1;
 623 }
 624
 625 int
 626 eqchantdqid(Chan *a, int type, int dev, Qid qid, int skipvers)
 627 {
 628         if(a->qid.path != qid.path)
 629                 return 0;
 630         if(!skipvers && a->qid.vers!=qid.vers)
 631                 return 0;
 632         if(a->type != type)
 633                 return 0;
 634         if(a->dev != dev)
 635                 return 0;
 636         return 1;
 637 }
 638
 639 Mhead*
 640 newmhead(Chan *from)
 641 {
 642         Mhead *mh;
 643
 644         mh = smalloc(sizeof(Mhead));
 645         mh->ref = 1;
 646         mh->from = from;
 647         incref(from);
 648         return mh;
 649 }
 650
 651 int
 652 cmount(Chan **newp, Chan *old, int flag, char *spec)
 653 {
 654         int order, flg;
 655         Chan *new;
 656         Mhead *m, **l, *mh;
 657         Mount *nm, *f, *um, **h;
 658         Pgrp *pg;
 659
 660         if(QTDIR & (old->qid.type^(*newp)->qid.type))
 661                 error(Emount);
 662
 663         if(old->umh)
 664                 print("cmount: unexpected umh, caller %#p\n", getcallerpc(&newp));
 665
 666         order = flag&MORDER;
 667
 668         if((old->qid.type&QTDIR)==0 && order != MREPL)
 669                 error(Emount);
 670
 671         new = *newp;
 672         mh = new->umh;
 673
 674         /*
 675          * Not allowed to bind when the old directory is itself a union.
 676          * (Maybe it should be allowed, but I don't see what the semantics
 677          * would be.)
 678          *
 679          * We need to check mh->mount->next to tell unions apart from
 680          * simple mount points, so that things like
 681          *      mount -c fd /root
 682          *      bind -c /root /
 683          * work.
 684          *
 685          * The check of mount->mflag allows things like
 686          *      mount fd /root
 687          *      bind -c /root /
 688          *
 689          * This is far more complicated than it should be, but I don't
 690          * see an easier way at the moment.
 691          */
 692         if((flag&MCREATE) && mh && mh->mount
 693         && (mh->mount->next || !(mh->mount->mflag&MCREATE)))
 694                 error(Emount);
 695
 696         pg = up->pgrp;
 697         wlock(&pg->ns);
 698
 699         l = &MOUNTH(pg, old->qid);
 700         for(m = *l; m; m = m->hash){
 701                 if(eqchan(m->from, old, 1))
 702                         break;
 703                 l = &m->hash;
 704         }
 705
 706         if(m == nil){
 707                 /*
 708                  *  nothing mounted here yet.  create a mount
 709                  *  head and add to the hash table.
 710                  */
 711                 m = newmhead(old);
 712                 *l = m;
 713
 714                 /*
 715                  *  if this is a union mount, add the old
 716                  *  node to the mount chain.
 717                  */
 718                 if(order != MREPL)
 719                         m->mount = newmount(m, old, 0, 0);
 720         }
 721         wlock(&m->lock);
 722         if(waserror()){
 723                 wunlock(&m->lock);
 724                 nexterror();
 725         }
 726         wunlock(&pg->ns);
 727
 728         nm = newmount(m, new, flag, spec);
 729         if(mh != nil && mh->mount != nil){
 730                 /*
 731                  *  copy a union when binding it onto a directory
 732                  */
 733                 flg = order;
 734                 if(order == MREPL)
 735                         flg = MAFTER;
 736                 h = &nm->next;
 737                 um = mh->mount;
 738                 for(um = um->next; um; um = um->next){
 739                         f = newmount(m, um->to, flg, um->spec);
 740                         *h = f;
 741                         h = &f->next;
 742                 }
 743         }
 744
 745         if(m->mount && order == MREPL){
 746                 mountfree(m->mount);
 747                 m->mount = 0;
 748         }
 749
 750         if(flag & MCREATE)
 751                 nm->mflag |= MCREATE;
 752
 753         if(m->mount && order == MAFTER){
 754                 for(f = m->mount; f->next; f = f->next)
 755                         ;
 756                 f->next = nm;
 757         }else{
 758                 for(f = nm; f->next; f = f->next)
 759                         ;
 760                 f->next = m->mount;
 761                 m->mount = nm;
 762         }
 763
 764         wunlock(&m->lock);
 765         poperror();
 766         return nm->mountid;
 767 }
 768
 769 void
 770 cunmount(Chan *mnt, Chan *mounted)
 771 {
 772         Pgrp *pg;
 773         Mhead *m, **l;
 774         Mount *f, **p;
 775
 776         if(mnt->umh)    /* should not happen */
 777                 print("cunmount newp extra umh %p has %p\n", mnt, mnt->umh);
 778
 779         /*
 780          * It _can_ happen that mounted->umh is non-nil,
 781          * because mounted is the result of namec(Aopen)
 782          * (see sysfile.c:/^sysunmount).
 783          * If we open a union directory, it will have a umh.
 784          * Although surprising, this is okay, since the
 785          * cclose will take care of freeing the umh.
 786          */
 787
 788         pg = up->pgrp;
 789         wlock(&pg->ns);
 790
 791         l = &MOUNTH(pg, mnt->qid);
 792         for(m = *l; m; m = m->hash){
 793                 if(eqchan(m->from, mnt, 1))
 794                         break;
 795                 l = &m->hash;
 796         }
 797
 798         if(m == 0){
 799                 wunlock(&pg->ns);
 800                 error(Eunmount);
 801         }
 802
 803         wlock(&m->lock);
 804         if(mounted == 0){
 805                 *l = m->hash;
 806                 wunlock(&pg->ns);
 807                 mountfree(m->mount);
 808                 m->mount = nil;
 809                 cclose(m->from);
 810                 wunlock(&m->lock);
 811                 putmhead(m);
 812                 return;
 813         }
 814
 815         p = &m->mount;
 816         for(f = *p; f; f = f->next){
 817                 /* BUG: Needs to be 2 pass */
 818                 if(eqchan(f->to, mounted, 1) ||
 819                   (f->to->mchan && eqchan(f->to->mchan, mounted, 1))){
 820                         *p = f->next;
 821                         f->next = 0;
 822                         mountfree(f);
 823                         if(m->mount == nil){
 824                                 *l = m->hash;
 825                                 cclose(m->from);
 826                                 wunlock(&m->lock);
 827                                 wunlock(&pg->ns);
 828                                 putmhead(m);
 829                                 return;
 830                         }
 831                         wunlock(&m->lock);
 832                         wunlock(&pg->ns);
 833                         return;
 834                 }
 835                 p = &f->next;
 836         }
 837         wunlock(&m->lock);
 838         wunlock(&pg->ns);
 839         error(Eunion);
 840 }
 841
 842 Chan*
 843 cclone(Chan *c)
 844 {
 845         Chan *nc;
 846         Walkqid *wq;
 847
 848         if(c == nil || c->ref < 1 || c->flag&CFREE)
 849                 panic("cclone: %#p", getcallerpc(&c));
 850         wq = devtab[c->type]->walk(c, nil, nil, 0);
 851         if(wq == nil)
 852                 error("clone failed");
 853         nc = wq->clone;
 854         free(wq);
 855         nc->path = c->path;
 856         if(c->path)
 857                 incref(c->path);
 858         return nc;
 859 }
 860
 861 /* also used by sysfile.c:/^mountfix */
 862 int
 863 findmount(Chan **cp, Mhead **mp, int type, int dev, Qid qid)
 864 {
 865         Pgrp *pg;
 866         Mhead *m;
 867
 868         pg = up->pgrp;
 869         rlock(&pg->ns);
 870         for(m = MOUNTH(pg, qid); m; m = m->hash){
 871                 rlock(&m->lock);
 872                 if(m->from == nil){
 873                         print("m %p m->from 0\n", m);
 874                         runlock(&m->lock);
 875                         continue;
 876                 }
 877                 if(eqchantdqid(m->from, type, dev, qid, 1)){
 878                         runlock(&pg->ns);
 879                         if(mp != nil){
 880                                 incref(m);
 881                                 if(*mp != nil)
 882                                         putmhead(*mp);
 883                                 *mp = m;
 884                         }
 885                         if(*cp != nil)
 886                                 cclose(*cp);
 887                         incref(m->mount->to);
 888                         *cp = m->mount->to;
 889                         runlock(&m->lock);
 890                         return 1;
 891                 }
 892                 runlock(&m->lock);
 893         }
 894
 895         runlock(&pg->ns);
 896         return 0;
 897 }
 898
 899 /*
 900  * Calls findmount but also updates path.
 901  */
 902 static int
 903 domount(Chan **cp, Mhead **mp, Path **path)
 904 {
 905         Chan **lc;
 906         Path *p;
 907
 908         if(findmount(cp, mp, (*cp)->type, (*cp)->dev, (*cp)->qid) == 0)
 909                 return 0;
 910
 911         if(path){
 912                 p = *path;
 913                 p = uniquepath(p);
 914                 if(p->mlen <= 0)
 915                         print("domount: path %s has mlen==%d\n", p->s, p->mlen);
 916                 else{
 917                         lc = &p->mtpt[p->mlen-1];
 918 DBG("domount %p %s => add %p (was %p)\n", p, p->s, (*mp)->from, p->mtpt[p->mlen-1]);
 919                         incref((*mp)->from);
 920                         if(*lc)
 921                                 cclose(*lc);
 922                         *lc = (*mp)->from;
 923                 }
 924                 *path = p;
 925         }
 926         return 1;
 927 }
 928
 929 /*
 930  * If c is the right-hand-side of a mount point, returns the left hand side.
 931  * Changes name to reflect the fact that we've uncrossed the mountpoint,
 932  * so name had better be ours to change!
 933  */
 934 static Chan*
 935 undomount(Chan *c, Path *path)
 936 {
 937         Chan *nc;
 938
 939         if(path->ref != 1 || path->mlen == 0)
 940                 print("undomount: path %s ref %ld mlen %d caller %#p\n",
 941                         path->s, path->ref, path->mlen, getcallerpc(&c));
 942
 943         if(path->mlen>0 && (nc=path->mtpt[path->mlen-1]) != nil){
 944 DBG("undomount %p %s => remove %p\n", path, path->s, nc);
 945                 cclose(c);
 946                 path->mtpt[path->mlen-1] = nil;
 947                 c = nc;
 948         }
 949         return c;
 950 }
 951
 952 /*
 953  * Call dev walk but catch errors.
 954  */
 955 static Walkqid*
 956 ewalk(Chan *c, Chan *nc, char **name, int nname)
 957 {
 958         Walkqid *wq;
 959
 960         if(waserror())
 961                 return nil;
 962         wq = devtab[c->type]->walk(c, nc, name, nname);
 963         poperror();
 964         return wq;
 965 }
 966
 967 /*
 968  * Either walks all the way or not at all.  No partial results in *cp.
 969  * *nerror is the number of names to display in an error message.
 970  */
 971 static char Edoesnotexist[] = "does not exist";
 972 int
 973 walk(Chan **cp, char **names, int nnames, int nomount, int *nerror)
 974 {
 975         int dev, didmount, dotdot, i, n, nhave, ntry, type;
 976         Chan *c, *nc, *mtpt;
 977         Path *path;
 978         Mhead *mh, *nmh;
 979         Mount *f;
 980         Walkqid *wq;
 981
 982         c = *cp;
 983         incref(c);
 984         path = c->path;
 985         incref(path);
 986         mh = nil;
 987
 988         /*
 989          * While we haven't gotten all the way down the path:
 990          *    1. step through a mount point, if any
 991          *    2. send a walk request for initial dotdot or initial prefix without dotdot
 992          *    3. move to the first mountpoint along the way.
 993          *    4. repeat.
 994          *
 995          * Each time through the loop:
 996          *
 997          *      If didmount==0, c is on the undomount side of the mount point.
 998          *      If didmount==1, c is on the domount side of the mount point.
 999          *      Either way, c's full path is path.
1000          */
1001         didmount = 0;
1002         for(nhave=0; nhave<nnames; nhave+=n){
1003                 if((c->qid.type&QTDIR)==0){
1004                         if(nerror)
1005                                 *nerror = nhave;
1006                         pathclose(path);
1007                         cclose(c);
1008                         kstrcpy(up->errstr, Enotdir, ERRMAX);
1009                         if(mh != nil)
1010                                 putmhead(mh);
1011                         return -1;
1012                 }
1013                 ntry = nnames - nhave;
1014                 if(ntry > MAXWELEM)
1015                         ntry = MAXWELEM;
1016                 dotdot = 0;
1017                 for(i=0; i<ntry; i++){
1018                         if(isdotdot(names[nhave+i])){
1019                                 if(i==0){
1020                                         dotdot = 1;
1021                                         ntry = 1;
1022                                 }else
1023                                         ntry = i;
1024                                 break;
1025                         }
1026                 }
1027
1028                 if(!dotdot && !nomount && !didmount)
1029                         domount(&c, &mh, &path);
1030
1031                 type = c->type;
1032                 dev = c->dev;
1033
1034                 if((wq = ewalk(c, nil, names+nhave, ntry)) == nil){
1035                         /* try a union mount, if any */
1036                         if(mh && !nomount){
1037                                 /*
1038                                  * mh->mount->to == c, so start at mh->mount->next
1039                                  */
1040                                 rlock(&mh->lock);
1041                                 if((f = mh->mount) != nil)
1042                                         f = f->next;
1043                                 for(; f != nil; f = f->next)
1044                                         if((wq = ewalk(f->to, nil, names+nhave, ntry)) != nil){
1045                                                 type = f->to->type;
1046                                                 dev = f->to->dev;
1047                                                 break;
1048                                         }
1049                                 runlock(&mh->lock);
1050                         }
1051                         if(wq == nil){
1052                                 cclose(c);
1053                                 pathclose(path);
1054                                 if(nerror)
1055                                         *nerror = nhave+1;
1056                                 if(mh != nil)
1057                                         putmhead(mh);
1058                                 return -1;
1059                         }
1060                 }
1061
1062                 didmount = 0;
1063                 if(dotdot){
1064                         assert(wq->nqid == 1);
1065                         assert(wq->clone != nil);
1066
1067                         path = addelem(path, "..", nil);
1068                         nc = undomount(wq->clone, path);
1069                         nmh = nil;
1070                         n = 1;
1071                 }else{
1072                         nc = nil;
1073                         nmh = nil;
1074                         if(!nomount){
1075                                 for(i=0; i<wq->nqid && i<ntry-1; i++){
1076                                         if(findmount(&nc, &nmh, type, dev, wq->qid[i])){
1077                                                 didmount = 1;
1078                                                 break;
1079                                         }
1080                                 }
1081                         }
1082                         if(nc == nil){  /* no mount points along path */
1083                                 if(wq->clone == nil){
1084                                         cclose(c);
1085                                         pathclose(path);
1086                                         if(wq->nqid==0 || (wq->qid[wq->nqid-1].type&QTDIR)){
1087                                                 if(nerror)
1088                                                         *nerror = nhave+wq->nqid+1;
1089                                                 kstrcpy(up->errstr, Edoesnotexist, ERRMAX);
1090                                         }else{
1091                                                 if(nerror)
1092                                                         *nerror = nhave+wq->nqid;
1093                                                 kstrcpy(up->errstr, Enotdir, ERRMAX);
1094                                         }
1095                                         free(wq);
1096                                         if(mh != nil)
1097                                                 putmhead(mh);
1098                                         return -1;
1099                                 }
1100                                 n = wq->nqid;
1101                                 nc = wq->clone;
1102                         }else{          /* stopped early, at a mount point */
1103                                 didmount = 1;
1104                                 if(wq->clone != nil){
1105                                         cclose(wq->clone);
1106                                         wq->clone = nil;
1107                                 }
1108                                 n = i+1;
1109                         }
1110                         for(i=0; i<n; i++){
1111                                 mtpt = nil;
1112                                 if(i==n-1 && nmh)
1113                                         mtpt = nmh->from;
1114                                 path = addelem(path, names[nhave+i], mtpt);
1115                         }
1116                 }
1117                 cclose(c);
1118                 c = nc;
1119                 putmhead(mh);
1120                 mh = nmh;
1121                 free(wq);
1122         }
1123
1124         putmhead(mh);
1125
1126         c = cunique(c);
1127
1128         if(c->umh != nil){      //BUG
1129                 print("walk umh\n");
1130                 putmhead(c->umh);
1131                 c->umh = nil;
1132         }
1133
1134         pathclose(c->path);
1135         c->path = path;
1136
1137         cclose(*cp);
1138         *cp = c;
1139         if(nerror)
1140                 *nerror = nhave;
1141         return 0;
1142 }
1143
1144 /*
1145  * c is a mounted non-creatable directory.  find a creatable one.
1146  */
1147 Chan*
1148 createdir(Chan *c, Mhead *m)
1149 {
1150         Chan *nc;
1151         Mount *f;
1152
1153         rlock(&m->lock);
1154         if(waserror()){
1155                 runlock(&m->lock);
1156                 nexterror();
1157         }
1158         for(f = m->mount; f; f = f->next){
1159                 if(f->mflag&MCREATE){
1160                         nc = cclone(f->to);
1161                         runlock(&m->lock);
1162                         poperror();
1163                         cclose(c);
1164                         return nc;
1165                 }
1166         }
1167         error(Enocreate);
1168         return 0;
1169 }
1170
1171 void
1172 saveregisters(void)
1173 {
1174 }
1175
1176 static void
1177 growparse(Elemlist *e)
1178 {
1179         char **new;
1180         int *inew;
1181         enum { Delta = 8 };
1182
1183         if(e->nelems % Delta == 0){
1184                 new = smalloc((e->nelems+Delta) * sizeof(char*));
1185                 memmove(new, e->elems, e->nelems*sizeof(char*));
1186                 free(e->elems);
1187                 e->elems = new;
1188                 inew = smalloc((e->nelems+Delta+1) * sizeof(int));
1189                 memmove(inew, e->off, (e->nelems+1)*sizeof(int));
1190                 free(e->off);
1191                 e->off = inew;
1192         }
1193 }
1194
1195 /*
1196  * The name is known to be valid.
1197  * Copy the name so slashes can be overwritten.
1198  * An empty string will set nelem=0.
1199  * A path ending in / or /. or /.//./ etc. will have
1200  * e.mustbedir = 1, so that we correctly
1201  * reject, e.g., "/adm/users/." when /adm/users is a file
1202  * rather than a directory.
1203  */
1204 static void
1205 parsename(char *aname, Elemlist *e)
1206 {
1207         char *name, *slash;
1208
1209         kstrdup(&e->name, aname);
1210         name = e->name;
1211         e->nelems = 0;
1212         e->elems = nil;
1213         e->off = smalloc(sizeof(int));
1214         e->off[0] = skipslash(name) - name;
1215         for(;;){
1216                 name = skipslash(name);
1217                 if(*name == '\0'){
1218                         e->off[e->nelems] = name+strlen(name) - e->name;
1219                         e->mustbedir = 1;
1220                         break;
1221                 }
1222                 growparse(e);
1223                 e->elems[e->nelems++] = name;
1224                 slash = utfrune(name, '/');
1225                 if(slash == nil){
1226                         e->off[e->nelems] = name+strlen(name) - e->name;
1227                         e->mustbedir = 0;
1228                         break;
1229                 }
1230                 e->off[e->nelems] = slash - e->name;
1231                 *slash++ = '\0';
1232                 name = slash;
1233         }
1234
1235         if(0 && chandebug){
1236                 int i;
1237
1238                 print("parsename %s:", e->name);
1239                 for(i=0; i<=e->nelems; i++)
1240                         print(" %d", e->off[i]);
1241                 print("\n");
1242         }
1243 }
1244
1245 void*
1246 memrchr(void *va, int c, long n)
1247 {
1248         uchar *a, *e;
1249
1250         a = va;
1251         for(e=a+n-1; e>a; e--)
1252                 if(*e == c)
1253                         return e;
1254         return nil;
1255 }
1256
1257 void
1258 namelenerror(char *aname, int len, char *err)
1259 {
1260         char *ename, *name, *next;
1261         int i, errlen;
1262
1263         /*
1264          * If the name is short enough, just use the whole thing.
1265          */
1266         errlen = strlen(err);
1267         if(len < ERRMAX/3 || len+errlen < 2*ERRMAX/3)
1268                 snprint(up->genbuf, sizeof up->genbuf, "%.*s",
1269                         utfnlen(aname, len), aname);
1270         else{
1271                 /*
1272                  * Print a suffix of the name, but try to get a little info.
1273                  */
1274                 ename = aname+len;
1275                 next = ename;
1276                 do{
1277                         name = next;
1278                         next = memrchr(aname, '/', name-aname);
1279                         if(next == nil)
1280                                 next = aname;
1281                         len = ename-next;
1282                 }while(len < ERRMAX/3 || len + errlen < 2*ERRMAX/3);
1283
1284                 /*
1285                  * If the name is ridiculously long, chop it.
1286                  */
1287                 if(name == ename){
1288                         name = ename-ERRMAX/4;
1289                         if(name <= aname)
1290                                 panic("bad math in namelenerror");
1291                         /* walk out of current UTF sequence */
1292                         for(i=0; (*name&0xC0)==0x80 && i<UTFmax; i++)
1293                                 name++;
1294                 }
1295                 snprint(up->genbuf, sizeof up->genbuf, "...%.*s",
1296                         utfnlen(name, ename-name), name);
1297         }
1298         snprint(up->errstr, ERRMAX, "%#q %s", up->genbuf, err);
1299         nexterror();
1300 }
1301
1302 void
1303 nameerror(char *name, char *err)
1304 {
1305         namelenerror(name, strlen(name), err);
1306 }
1307
1308 /*
1309  * Turn a name into a channel.
1310  * &name[0] is known to be a valid address.  It may be a kernel address.
1311  *
1312  * Opening with amode Aopen, Acreate, Aremove, or Aaccess guarantees
1313  * that the result will be the only reference to that particular fid.
1314  * This is necessary since we might pass the result to
1315  * devtab[]->remove().
1316  *
1317  * Opening Atodir or Amount does not guarantee this.
1318  *
1319  * Under certain circumstances, opening Aaccess will cause
1320  * an unnecessary clone in order to get a cunique Chan so it
1321  * can attach the correct name.  Sysstat and sys_stat need the
1322  * correct name so they can rewrite the stat info.
1323  */
1324 Chan*
1325 namec(char *aname, int amode, int omode, ulong perm)
1326 {
1327         int len, n, t, nomount;
1328         Chan *c, *cnew;
1329         Path *path;
1330         Elemlist e;
1331         Rune r;
1332         Mhead *m;
1333         char *createerr, tmperrbuf[ERRMAX];
1334         char *name;
1335
1336         if(aname[0] == '\0')
1337                 error("empty file name");
1338         aname = validnamedup(aname, 1);
1339         if(waserror()){
1340                 free(aname);
1341                 nexterror();
1342         }
1343         DBG("namec %s %d %d\n", aname, amode, omode);
1344         name = aname;
1345
1346         /*
1347          * Find the starting off point (the current slash, the root of
1348          * a device tree, or the current dot) as well as the name to
1349          * evaluate starting there.
1350          */
1351         nomount = 0;
1352         switch(name[0]){
1353         case '/':
1354                 c = up->slash;
1355                 incref(c);
1356                 break;
1357
1358         case '#':
1359                 nomount = 1;
1360                 up->genbuf[0] = '\0';
1361                 n = 0;
1362                 while(*name != '\0' && (*name != '/' || n < 2)){
1363                         if(n >= sizeof(up->genbuf)-1)
1364                                 error(Efilename);
1365                         up->genbuf[n++] = *name++;
1366                 }
1367                 up->genbuf[n] = '\0';
1368                 /*
1369                  *  noattach is sandboxing.
1370                  *
1371                  *  the OK exceptions are:
1372                  *      |  it only gives access to pipes you create
1373                  *      d  this process's file descriptors
1374                  *      e  this process's environment
1375                  *  the iffy exceptions are:
1376                  *      c  time and pid, but also cons and consctl
1377                  *      p  control of your own processes (and unfortunately
1378                  *         any others left unprotected)
1379                  */
1380                 n = chartorune(&r, up->genbuf+1)+1;
1381                 /* actually / is caught by parsing earlier */
1382                 if(utfrune("M", r))
1383                         error(Enoattach);
1384                 if(up->pgrp->noattach && utfrune("|decp", r)==nil)
1385                         error(Enoattach);
1386                 t = devno(r, 1);
1387                 if(t == -1)
1388                         error(Ebadsharp);
1389                 c = devtab[t]->attach(up->genbuf+n);
1390                 break;
1391
1392         default:
1393                 c = up->dot;
1394                 incref(c);
1395                 break;
1396         }
1397
1398         e.aname = aname;
1399         e.prefix = name - aname;
1400         e.name = nil;
1401         e.elems = nil;
1402         e.off = nil;
1403         e.nelems = 0;
1404         e.nerror = 0;
1405         if(waserror()){
1406                 cclose(c);
1407                 free(e.name);
1408                 free(e.elems);
1409                 /*
1410                  * Prepare nice error, showing first e.nerror elements of name.
1411                  */
1412                 if(e.nerror == 0)
1413                         nexterror();
1414                 strcpy(tmperrbuf, up->errstr);
1415                 if(e.off[e.nerror]==0)
1416                         print("nerror=%d but off=%d\n",
1417                                 e.nerror, e.off[e.nerror]);
1418                 if(0 && chandebug)
1419                         print("showing %d+%d/%d (of %d) of %s (%d %d)\n", e.prefix, e.off[e.nerror], e.nerror, e.nelems, aname, e.off[0], e.off[1]);
1420                 len = e.prefix+e.off[e.nerror];
1421                 free(e.off);
1422                 namelenerror(aname, len, tmperrbuf);
1423         }
1424
1425         /*
1426          * Build a list of elements in the name.
1427          */
1428         parsename(name, &e);
1429
1430         /*
1431          * On create, ....
1432          */
1433         if(amode == Acreate){
1434                 /* perm must have DMDIR if last element is / or /. */
1435                 if(e.mustbedir && !(perm&DMDIR)){
1436                         e.nerror = e.nelems;
1437                         error("create without DMDIR");
1438                 }
1439
1440                 /* don't try to walk the last path element just yet. */
1441                 if(e.nelems == 0)
1442                         error(Eexist);
1443                 e.nelems--;
1444         }
1445
1446         if(walk(&c, e.elems, e.nelems, nomount, &e.nerror) < 0){
1447                 if(e.nerror < 0 || e.nerror > e.nelems){
1448                         print("namec %s walk error nerror=%d\n", aname, e.nerror);
1449                         e.nerror = 0;
1450                 }
1451                 nexterror();
1452         }
1453
1454         if(e.mustbedir && !(c->qid.type&QTDIR))
1455                 error("not a directory");
1456
1457         if(amode == Aopen && (omode&3) == OEXEC && (c->qid.type&QTDIR))
1458                 error("cannot exec directory");
1459
1460         switch(amode){
1461         case Abind:
1462                 /* no need to maintain path - cannot dotdot an Abind */
1463                 m = nil;
1464                 if(!nomount)
1465                         domount(&c, &m, nil);
1466                 if(c->umh != nil)
1467                         putmhead(c->umh);
1468                 c->umh = m;
1469                 break;
1470
1471         case Aaccess:
1472         case Aremove:
1473         case Aopen:
1474         Open:
1475                 /* save&update the name; domount might change c */
1476                 path = c->path;
1477                 incref(path);
1478                 if(waserror()){
1479                         pathclose(path);
1480                         nexterror();
1481                 }
1482                 m = nil;
1483                 if(!nomount)
1484                         domount(&c, &m, &path);
1485
1486                 /* our own copy to open or remove */
1487                 c = cunique(c);
1488
1489                 /* now it's our copy anyway, we can put the name back */
1490                 pathclose(c->path);
1491                 c->path = path;
1492                 poperror();
1493
1494                 /* record whether c is on a mount point */
1495                 c->ismtpt = m!=nil;
1496
1497                 switch(amode){
1498                 case Aaccess:
1499                 case Aremove:
1500                         putmhead(m);
1501                         break;
1502
1503                 case Aopen:
1504                 case Acreate:
1505 if(c->umh != nil){
1506         print("cunique umh Open\n");
1507         putmhead(c->umh);
1508         c->umh = nil;
1509 }
1510                         /* only save the mount head if it's a multiple element union */
1511                         if(m && m->mount && m->mount->next)
1512                                 c->umh = m;
1513                         else
1514                                 putmhead(m);
1515
1516                         /* save registers else error() in open has wrong value of c saved */
1517                         saveregisters();
1518
1519                         c = devtab[c->type]->open(c, omode&~OCEXEC);
1520
1521                         if(omode & OCEXEC)
1522                                 c->flag |= CCEXEC;
1523                         if(omode & ORCLOSE)
1524                                 c->flag |= CRCLOSE;
1525                         break;
1526                 }
1527                 break;
1528
1529         case Atodir:
1530                 /*
1531                  * Directories (e.g. for cd) are left before the mount point,
1532                  * so one may mount on / or . and see the effect.
1533                  */
1534                 if(!(c->qid.type & QTDIR))
1535                         error(Enotdir);
1536                 break;
1537
1538         case Amount:
1539                 /*
1540                  * When mounting on an already mounted upon directory,
1541                  * one wants subsequent mounts to be attached to the
1542                  * original directory, not the replacement.  Don't domount.
1543                  */
1544                 break;
1545
1546         case Acreate:
1547                 /*
1548                  * We've already walked all but the last element.
1549                  * If the last exists, try to open it OTRUNC.
1550                  * If omode&OEXCL is set, just give up.
1551                  */
1552                 e.nelems++;
1553                 e.nerror++;
1554                 if(walk(&c, e.elems+e.nelems-1, 1, nomount, nil) == 0){
1555                         if(omode&OEXCL)
1556                                 error(Eexist);
1557                         omode |= OTRUNC;
1558                         goto Open;
1559                 }
1560
1561                 /*
1562                  * The semantics of the create(2) system call are that if the
1563                  * file exists and can be written, it is to be opened with truncation.
1564                  * On the other hand, the create(5) message fails if the file exists.
1565                  * If we get two create(2) calls happening simultaneously,
1566                  * they might both get here and send create(5) messages, but only
1567                  * one of the messages will succeed.  To provide the expected create(2)
1568                  * semantics, the call with the failed message needs to try the above
1569                  * walk again, opening for truncation.  This correctly solves the
1570                  * create/create race, in the sense that any observable outcome can
1571                  * be explained as one happening before the other.
1572                  * The create/create race is quite common.  For example, it happens
1573                  * when two rc subshells simultaneously update the same
1574                  * environment variable.
1575                  *
1576                  * The implementation still admits a create/create/remove race:
1577                  * (A) walk to file, fails
1578                  * (B) walk to file, fails
1579                  * (A) create file, succeeds, returns
1580                  * (B) create file, fails
1581                  * (A) remove file, succeeds, returns
1582                  * (B) walk to file, return failure.
1583                  *
1584                  * This is hardly as common as the create/create race, and is really
1585                  * not too much worse than what might happen if (B) got a hold of a
1586                  * file descriptor and then the file was removed -- either way (B) can't do
1587                  * anything with the result of the create call.  So we don't care about this race.
1588                  *
1589                  * Applications that care about more fine-grained decision of the races
1590                  * can use the OEXCL flag to get at the underlying create(5) semantics;
1591                  * by default we provide the common case.
1592                  *
1593                  * We need to stay behind the mount point in case we
1594                  * need to do the first walk again (should the create fail).
1595                  *
1596                  * We also need to cross the mount point and find the directory
1597                  * in the union in which we should be creating.
1598                  *
1599                  * The channel staying behind is c, the one moving forward is cnew.
1600                  */
1601                 m = nil;
1602                 cnew = nil;     /* is this assignment necessary? */
1603                 if(!waserror()){        /* try create */
1604                         if(!nomount && findmount(&cnew, &m, c->type, c->dev, c->qid))
1605                                 cnew = createdir(cnew, m);
1606                         else{
1607                                 cnew = c;
1608                                 incref(cnew);
1609                         }
1610
1611                         /*
1612                          * We need our own copy of the Chan because we're
1613                          * about to send a create, which will move it.  Once we have
1614                          * our own copy, we can fix the name, which might be wrong
1615                          * if findmount gave us a new Chan.
1616                          */
1617                         cnew = cunique(cnew);
1618                         pathclose(cnew->path);
1619                         cnew->path = c->path;
1620                         incref(cnew->path);
1621
1622                         cnew = devtab[cnew->type]->create(cnew, e.elems[e.nelems-1], omode&~(OEXCL|OCEXEC), perm);
1623                         poperror();
1624                         if(omode & OCEXEC)
1625                                 cnew->flag |= CCEXEC;
1626                         if(omode & ORCLOSE)
1627                                 cnew->flag |= CRCLOSE;
1628                         if(m)
1629                                 putmhead(m);
1630                         cclose(c);
1631                         c = cnew;
1632                         c->path = addelem(c->path, e.elems[e.nelems-1], nil);
1633                         break;
1634                 }
1635
1636                 /* create failed */
1637                 cclose(cnew);
1638                 if(m)
1639                         putmhead(m);
1640                 if(omode & OEXCL)
1641                         nexterror();
1642                 /* save error */
1643                 createerr = up->errstr;
1644                 up->errstr = tmperrbuf;
1645                 /* note: we depend that walk does not error */
1646                 if(walk(&c, e.elems+e.nelems-1, 1, nomount, nil) < 0){
1647                         up->errstr = createerr;
1648                         error(createerr);       /* report true error */
1649                 }
1650                 up->errstr = createerr;
1651                 omode |= OTRUNC;
1652                 goto Open;
1653
1654         default:
1655                 panic("unknown namec access %d", amode);
1656         }
1657
1658         /* place final element in genbuf for e.g. exec */
1659         if(e.nelems > 0)
1660                 kstrcpy(up->genbuf, e.elems[e.nelems-1], sizeof up->genbuf);
1661         else
1662                 kstrcpy(up->genbuf, ".", sizeof up->genbuf);
1663         free(e.name);
1664         free(e.elems);
1665         free(e.off);
1666         poperror();     /* e c */
1667         free(aname);
1668         poperror();     /* aname */
1669
1670         return c;
1671 }
1672
1673 /*
1674  * name is valid. skip leading / and ./ as much as possible
1675  */
1676 char*
1677 skipslash(char *name)
1678 {
1679         while(name[0]=='/' || (name[0]=='.' && (name[1]==0 || name[1]=='/')))
1680                 name++;
1681         return name;
1682 }
1683
1684 char isfrog[256]={
1685         /*NUL*/ 1, 1, 1, 1, 1, 1, 1, 1,
1686         /*BKS*/ 1, 1, 1, 1, 1, 1, 1, 1,
1687         /*DLE*/ 1, 1, 1, 1, 1, 1, 1, 1,
1688         /*CAN*/ 1, 1, 1, 1, 1, 1, 1, 1,
1689         ['/']   1,
1690         [0x7f]  1,
1691 };
1692
1693 /*
1694  * Check that the name
1695  *  a) is in valid memory.
1696  *  b) is shorter than 2^16 bytes, so it can fit in a 9P string field.
1697  *  c) contains no frogs.
1698  * The first byte is known to be addressible by the requester, so the
1699  * routine works for kernel and user memory both.
1700  * The parameter slashok flags whether a slash character is an error
1701  * or a valid character.
1702  *
1703  * The parameter dup flags whether the string should be copied
1704  * out of user space before being scanned the second time.
1705  * (Otherwise a malicious thread could remove the NUL, causing us
1706  * to access unchecked addresses.)
1707  */
1708 static char*
1709 validname0(char *aname, int slashok, int dup, ulong pc)
1710 {
1711         char *ename, *name, *s;
1712         int c, n;
1713         Rune r;
1714
1715         name = aname;
1716         if((ulong)name < KZERO){
1717                 if(!dup)
1718                         print("warning: validname called from %#p with user pointer", pc);
1719                 ename = vmemchr(name, 0, (1<<16));
1720         }else
1721                 ename = memchr(name, 0, (1<<16));
1722
1723         if(ename==nil || ename-name>=(1<<16))
1724                 error("name too long");
1725
1726         s = nil;
1727         if(dup){
1728                 n = ename-name;
1729                 s = smalloc(n+1);
1730                 memmove(s, name, n);
1731                 s[n] = 0;
1732                 aname = s;
1733                 name = s;
1734                 setmalloctag(s, pc);
1735         }
1736
1737         while(*name){
1738                 /* all characters above '~' are ok */
1739                 c = *(uchar*)name;
1740                 if(c >= Runeself)
1741                         name += chartorune(&r, name);
1742                 else{
1743                         if(isfrog[c])
1744                                 if(!slashok || c!='/'){
1745                                         snprint(up->genbuf, sizeof(up->genbuf), "%s: %q", Ebadchar, aname);
1746                                         free(s);
1747                                         error(up->genbuf);
1748                         }
1749                         name++;
1750                 }
1751         }
1752         return s;
1753 }
1754
1755 void
1756 validname(char *aname, int slashok)
1757 {
1758         validname0(aname, slashok, 0, getcallerpc(&aname));
1759 }
1760
1761 char*
1762 validnamedup(char *aname, int slashok)
1763 {
1764         return validname0(aname, slashok, 1, getcallerpc(&aname));
1765 }
1766
1767 void
1768 isdir(Chan *c)
1769 {
1770         if(c->qid.type & QTDIR)
1771                 return;
1772         error(Enotdir);
1773 }
1774
1775 /*
1776  * This is necessary because there are many
1777  * pointers to the top of a given mount list:
1778  *
1779  *      - the mhead in the namespace hash table
1780  *      - the mhead in chans returned from findmount:
1781  *        used in namec and then by unionread.
1782  *      - the mhead in chans returned from createdir:
1783  *        used in the open/create race protect, which is gone.
1784  *
1785  * The RWlock in the Mhead protects the mount list it contains.
1786  * The mount list is deleted when we cunmount.
1787  * The RWlock ensures that nothing is using the mount list at that time.
1788  *
1789  * It is okay to replace c->mh with whatever you want as
1790  * long as you are sure you have a unique reference to it.
1791  *
1792  * This comment might belong somewhere else.
1793  */
1794 void
1795 putmhead(Mhead *m)
1796 {
1797         if(m && decref(m) == 0){
1798                 m->mount = (Mount*)0xCafeBeef;
1799                 free(m);
1800         }
1801 }
1802