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 old, new;
 100
 101         do {
 102                 old = r->ref;
 103                 new = old+1;
 104         } while(!cmpswap(&r->ref, old, new));
 105         return new;
 106 }
 107
 108 long
 109 decref(Ref *r)
 110 {
 111         long old, new;
 112
 113         do {
 114                 old = r->ref;
 115                 if(old <= 0)
 116                         panic("decref pc=%#p", getcallerpc(&r));
 117                 new = old-1;
 118         } while(!cmpswap(&r->ref, old, new));
 119         return new;
 120 }
 121
 122 /*
 123  * Rather than strncpy, which zeros the rest of the buffer, kstrcpy
 124  * truncates if necessary, always zero terminates, does not zero fill,
 125  * and puts ... at the end of the string if it's too long.  Usually used to
 126  * save a string in up->genbuf;
 127  */
 128 void
 129 kstrcpy(char *s, char *t, int ns)
 130 {
 131         int nt;
 132
 133         nt = strlen(t);
 134         if(nt < ns){
 135                 memmove(s, t, nt);
 136                 s[nt] = '\0';
 137                 return;
 138         }
 139         /* too long, truncate */
 140         nt = ns-1;
 141         memmove(s, t, nt);
 142         s[nt] = '\0';
 143         /* append ... if there is space */
 144         ns -= 4;
 145         if(ns < 0)
 146                 return;
 147         /* look for first byte of UTF-8 sequence by skipping continuation bytes */
 148         while(ns>0 && (s[--ns]&0xC0)==0x80)
 149                 ;
 150         strcpy(s+ns, "...");
 151 }
 152
 153 int
 154 emptystr(char *s)
 155 {
 156         if(s == nil)
 157                 return 1;
 158         if(s[0] == '\0')
 159                 return 1;
 160         return 0;
 161 }
 162
 163 /*
 164  * Atomically replace *p with copy of s
 165  */
 166 void
 167 kstrdup(char **p, char *s)
 168 {
 169         int n;
 170         char *t, *prev;
 171
 172         n = strlen(s);
 173         /* if it's a user, we can wait for memory; if not, something's very wrong */
 174         if(up != nil)
 175                 t = smalloc(n+1);
 176         else{
 177                 t = malloc(n+1);
 178                 if(t == nil)
 179                         panic("kstrdup: no memory");
 180         }
 181         setmalloctag(t, getcallerpc(&p));
 182         memmove(t, s, n);
 183         t[n] = '\0';
 184         prev = *p;
 185         *p = t;
 186         free(prev);
 187 }
 188
 189 void
 190 chandevreset(void)
 191 {
 192         int i;
 193
 194         todinit();      /* avoid later reentry causing infinite recursion */
 195         for(i=0; devtab[i] != nil; i++)
 196                 devtab[i]->reset();
 197 }
 198
 199 static void closeproc(void*);
 200
 201 void
 202 chandevinit(void)
 203 {
 204         int i;
 205
 206         for(i=0; devtab[i] != nil; i++)
 207                 devtab[i]->init();
 208         kproc("closeproc", closeproc, nil);
 209 }
 210
 211 void
 212 chandevshutdown(void)
 213 {
 214         int i;
 215
 216         /* shutdown in reverse order */
 217         for(i=0; devtab[i] != nil; i++)
 218                 ;
 219         for(i--; i >= 0; i--)
 220                 devtab[i]->shutdown();
 221 }
 222
 223 Chan*
 224 newchan(void)
 225 {
 226         Chan *c;
 227
 228         lock(&chanalloc);
 229         c = chanalloc.free;
 230         if(c != nil){
 231                 chanalloc.free = c->next;
 232                 c->next = nil;
 233         } else {
 234                 unlock(&chanalloc);
 235                 c = smalloc(sizeof(Chan));
 236                 lock(&chanalloc);
 237                 c->link = chanalloc.list;
 238                 chanalloc.list = c;
 239         }
 240         if(c->fid == 0)
 241                 c->fid = ++chanalloc.fid;
 242         unlock(&chanalloc);
 243
 244         /* if you get an error before associating with a dev,
 245            close calls rootclose, a nop */
 246         c->type = 0;
 247         c->flag = 0;
 248         c->ref = 1;
 249         c->dev = 0;
 250         c->offset = 0;
 251         c->devoffset = 0;
 252         c->iounit = 0;
 253         c->umh = nil;
 254         c->umc = nil;
 255         c->uri = 0;
 256         c->dri = 0;
 257         c->dirrock = nil;
 258         c->nrock = 0;
 259         c->mrock = 0;
 260         c->ismtpt = 0;
 261         c->mcp = nil;
 262         c->mux = nil;
 263         c->aux = nil;
 264         c->mchan = nil;
 265         memset(&c->mqid, 0, sizeof(c->mqid));
 266         c->path = nil;
 267
 268         return c;
 269 }
 270
 271 Ref npath;
 272
 273 Path*
 274 newpath(char *s)
 275 {
 276         int i;
 277         Path *p;
 278
 279         p = smalloc(sizeof(Path));
 280         i = strlen(s);
 281         p->len = i;
 282         p->alen = i+PATHSLOP;
 283         p->s = smalloc(p->alen);
 284         memmove(p->s, s, i+1);
 285         p->ref = 1;
 286         incref(&npath);
 287
 288         /*
 289          * Cannot use newpath for arbitrary names because the mtpt
 290          * array will not be populated correctly.  The names #/ and / are
 291          * allowed, but other names with / in them draw warnings.
 292          */
 293         if(strchr(s, '/') != nil && strcmp(s, "#/") != 0 && strcmp(s, "/") != 0)
 294                 print("newpath: %s from %#p\n", s, getcallerpc(&s));
 295
 296         p->mlen = 1;
 297         p->malen = PATHMSLOP;
 298         p->mtpt = smalloc(p->malen*sizeof p->mtpt[0]);
 299         return p;
 300 }
 301
 302 static Path*
 303 copypath(Path *p)
 304 {
 305         int i;
 306         Path *pp;
 307
 308         pp = smalloc(sizeof(Path));
 309         pp->ref = 1;
 310         incref(&npath);
 311         DBG("copypath %s %p => %p\n", p->s, p, pp);
 312
 313         pp->len = p->len;
 314         pp->alen = p->alen;
 315         pp->s = smalloc(p->alen);
 316         memmove(pp->s, p->s, p->len+1);
 317
 318         pp->mlen = p->mlen;
 319         pp->malen = p->malen;
 320         pp->mtpt = smalloc(p->malen*sizeof pp->mtpt[0]);
 321         for(i=0; i<pp->mlen; i++){
 322                 pp->mtpt[i] = p->mtpt[i];
 323                 if(pp->mtpt[i] != nil)
 324                         incref(pp->mtpt[i]);
 325         }
 326
 327         return pp;
 328 }
 329
 330 void
 331 pathclose(Path *p)
 332 {
 333         int i;
 334
 335         if(p == nil)
 336                 return;
 337 //XXX
 338         DBG("pathclose %p %s ref=%ld =>", p, p->s, p->ref);
 339         for(i=0; i<p->mlen; i++)
 340                 DBG(" %p", p->mtpt[i]);
 341         DBG("\n");
 342
 343         if(decref(p))
 344                 return;
 345         decref(&npath);
 346         free(p->s);
 347         for(i=0; i<p->mlen; i++)
 348                 if(p->mtpt[i] != nil)
 349                         cclose(p->mtpt[i]);
 350         free(p->mtpt);
 351         free(p);
 352 }
 353
 354 /*
 355  * In place, rewrite name to compress multiple /, eliminate ., and process ..
 356  * (Really only called to remove a trailing .. that has been added.
 357  * Otherwise would need to update n->mtpt as well.)
 358  */
 359 static void
 360 fixdotdotname(Path *p)
 361 {
 362         char *r;
 363
 364         if(p->s[0] == '#'){
 365                 r = strchr(p->s, '/');
 366                 if(r == nil)
 367                         return;
 368                 cleanname(r);
 369
 370                 /*
 371                  * The correct name is #i rather than #i/,
 372                  * but the correct name of #/ is #/.
 373                  */
 374                 if(strcmp(r, "/")==0 && p->s[1] != '/')
 375                         *r = '\0';
 376         }else
 377                 cleanname(p->s);
 378         p->len = strlen(p->s);
 379 }
 380
 381 static Path*
 382 uniquepath(Path *p)
 383 {
 384         Path *new;
 385
 386         if(p->ref > 1){
 387                 /* copy on write */
 388                 new = copypath(p);
 389                 pathclose(p);
 390                 p = new;
 391         }
 392         return p;
 393 }
 394
 395 static Path*
 396 addelem(Path *p, char *s, Chan *from)
 397 {
 398         char *t;
 399         int a, i;
 400         Chan *c, **tt;
 401
 402         if(s[0]=='.' && s[1]=='\0')
 403                 return p;
 404
 405         p = uniquepath(p);
 406
 407         i = strlen(s);
 408         if(p->len+1+i+1 > p->alen){
 409                 a = p->len+1+i+1 + PATHSLOP;
 410                 t = smalloc(a);
 411                 memmove(t, p->s, p->len+1);
 412                 free(p->s);
 413                 p->s = t;
 414                 p->alen = a;
 415         }
 416         /* don't insert extra slash if one is present */
 417         if(p->len>0 && p->s[p->len-1]!='/' && s[0]!='/')
 418                 p->s[p->len++] = '/';
 419         memmove(p->s+p->len, s, i+1);
 420         p->len += i;
 421         if(isdotdot(s)){
 422                 fixdotdotname(p);
 423                 DBG("addelem %s .. => rm %p\n", p->s, p->mtpt[p->mlen-1]);
 424                 if(p->mlen > 1 && (c = p->mtpt[--p->mlen]) != nil){
 425                         p->mtpt[p->mlen] = nil;
 426                         cclose(c);
 427                 }
 428         }else{
 429                 if(p->mlen >= p->malen){
 430                         p->malen = p->mlen+1+PATHMSLOP;
 431                         tt = smalloc(p->malen*sizeof tt[0]);
 432                         memmove(tt, p->mtpt, p->mlen*sizeof tt[0]);
 433                         free(p->mtpt);
 434                         p->mtpt = tt;
 435                 }
 436                 DBG("addelem %s %s => add %p\n", p->s, s, from);
 437                 p->mtpt[p->mlen++] = from;
 438                 if(from != nil)
 439                         incref(from);
 440         }
 441         return p;
 442 }
 443
 444 void
 445 chanfree(Chan *c)
 446 {
 447         c->flag = CFREE;
 448
 449         if(c->dirrock != nil){
 450                 free(c->dirrock);
 451                 c->dirrock = nil;
 452                 c->nrock = 0;
 453                 c->mrock = 0;
 454         }
 455         if(c->umh != nil){
 456                 putmhead(c->umh);
 457                 c->umh = nil;
 458         }
 459         if(c->umc != nil){
 460                 cclose(c->umc);
 461                 c->umc = nil;
 462         }
 463         if(c->mux != nil){
 464                 muxclose(c->mux);
 465                 c->mux = nil;
 466         }
 467         if(c->mchan != nil){
 468                 cclose(c->mchan);
 469                 c->mchan = nil;
 470         }
 471
 472         pathclose(c->path);
 473         c->path = nil;
 474
 475         lock(&chanalloc);
 476         c->next = chanalloc.free;
 477         chanalloc.free = c;
 478         unlock(&chanalloc);
 479 }
 480
 481 /*
 482  * Queue a chan to be closed by one of the clunk procs.
 483  */
 484 struct {
 485         Chan *head;
 486         Chan *tail;
 487         ulong nqueued;
 488         ulong nclosed;
 489         Lock l;
 490         QLock q;
 491         Rendez r;
 492 } clunkq;
 493
 494 static int
 495 clunkwork(void*)
 496 {
 497         return clunkq.head != nil;
 498 }
 499
 500 static void
 501 closechanq(Chan *c)
 502 {
 503         lock(&clunkq.l);
 504         clunkq.nqueued++;
 505         c->next = nil;
 506         if(clunkq.head)
 507                 clunkq.tail->next = c;
 508         else
 509                 clunkq.head = c;
 510         clunkq.tail = c;
 511         unlock(&clunkq.l);
 512         wakeup(&clunkq.r);
 513 }
 514
 515 static Chan*
 516 closechandeq(void)
 517 {
 518         Chan *c;
 519
 520         lock(&clunkq.l);
 521         c = clunkq.head;
 522         if(c != nil) {
 523                 clunkq.head = c->next;
 524                 clunkq.nclosed++;
 525         }
 526         unlock(&clunkq.l);
 527         return c;
 528 }
 529
 530 static void
 531 closeproc(void *)
 532 {
 533         Chan *c;
 534
 535         for(;;){
 536                 c = closechandeq();
 537                 if(c == nil) {
 538                         qlock(&clunkq.q);
 539                         if(!waserror()) {
 540                                 tsleep(&clunkq.r, clunkwork, nil, 500);
 541                                 poperror();
 542                         }
 543                         c = closechandeq();
 544                         if(c == nil) {
 545                                 if(clunkq.q.head != nil) {
 546                                         qunlock(&clunkq.q);
 547                                         pexit("no work", 1);
 548                                 }
 549                                 qunlock(&clunkq.q);
 550                                 continue;
 551                         }
 552                         if(clunkq.q.head == nil) {
 553                                 if(!waserror()) {
 554                                         kproc("closeproc", closeproc, nil);
 555                                         poperror();
 556                                 }
 557                         }
 558                         qunlock(&clunkq.q);
 559                 }
 560                 if(!waserror()){
 561                         devtab[c->type]->close(c);
 562                         poperror();
 563                 }
 564                 chanfree(c);
 565         }
 566 }
 567
 568 void
 569 cclose(Chan *c)
 570 {
 571         if(c == nil || c->ref < 1 || c->flag&CFREE)
 572                 panic("cclose %#p", getcallerpc(&c));
 573
 574         DBG("cclose %p name=%s ref=%ld\n", c, chanpath(c), c->ref);
 575
 576         if(decref(c))
 577                 return;
 578
 579         if(devtab[c->type]->dc == L'M')
 580         if((c->flag&(CRCLOSE|CCACHE)) == CCACHE)
 581         if((c->qid.type&(QTEXCL|QTMOUNT|QTAUTH)) == 0)
 582         if((clunkq.nqueued - clunkq.nclosed) < 64){
 583                 closechanq(c);
 584                 return;
 585         }
 586
 587         if(!waserror()){
 588                 devtab[c->type]->close(c);
 589                 poperror();
 590         }
 591         chanfree(c);
 592 }
 593
 594 void
 595 ccloseq(Chan *c)
 596 {
 597         if(c == nil || c->ref < 1 || c->flag&CFREE)
 598                 panic("ccloseq %#p", getcallerpc(&c));
 599
 600         DBG("ccloseq %p name=%s ref=%ld\n", c, chanpath(c), c->ref);
 601
 602         if(decref(c))
 603                 return;
 604
 605         closechanq(c);
 606 }
 607
 608 /*
 609  * Make sure we have the only copy of c.  (Copy on write.)
 610  */
 611 Chan*
 612 cunique(Chan *c)
 613 {
 614         Chan *nc;
 615
 616         if(c->ref != 1){
 617                 nc = cclone(c);
 618                 cclose(c);
 619                 c = nc;
 620         }
 621
 622         return c;
 623 }
 624
 625 int
 626 eqqid(Qid a, Qid b)
 627 {
 628         return a.path==b.path && a.vers==b.vers;
 629 }
 630
 631 int
 632 eqchan(Chan *a, Chan *b, int skipvers)
 633 {
 634         if(a->qid.path != b->qid.path)
 635                 return 0;
 636         if(!skipvers && a->qid.vers!=b->qid.vers)
 637                 return 0;
 638         if(a->type != b->type)
 639                 return 0;
 640         if(a->dev != b->dev)
 641                 return 0;
 642         return 1;
 643 }
 644
 645 int
 646 eqchantdqid(Chan *a, int type, int dev, Qid qid, int skipvers)
 647 {
 648         if(a->qid.path != qid.path)
 649                 return 0;
 650         if(!skipvers && a->qid.vers!=qid.vers)
 651                 return 0;
 652         if(a->type != type)
 653                 return 0;
 654         if(a->dev != dev)
 655                 return 0;
 656         return 1;
 657 }
 658
 659 Mhead*
 660 newmhead(Chan *from)
 661 {
 662         Mhead *mh;
 663
 664         mh = smalloc(sizeof(Mhead));
 665         mh->ref = 1;
 666         mh->from = from;
 667         incref(from);
 668         return mh;
 669 }
 670
 671 int
 672 cmount(Chan **newp, Chan *old, int flag, char *spec)
 673 {
 674         int order, flg;
 675         Chan *new;
 676         Mhead *m, **l, *mh;
 677         Mount *nm, *f, *um, **h;
 678         Pgrp *pg;
 679
 680         if(QTDIR & (old->qid.type^(*newp)->qid.type))
 681                 error(Emount);
 682
 683         if(old->umh)
 684                 print("cmount: unexpected umh, caller %#p\n", getcallerpc(&newp));
 685
 686         order = flag&MORDER;
 687
 688         if((old->qid.type&QTDIR)==0 && order != MREPL)
 689                 error(Emount);
 690
 691         new = *newp;
 692         mh = new->umh;
 693
 694         /*
 695          * Not allowed to bind when the old directory is itself a union.
 696          * (Maybe it should be allowed, but I don't see what the semantics
 697          * would be.)
 698          *
 699          * We need to check mh->mount->next to tell unions apart from
 700          * simple mount points, so that things like
 701          *      mount -c fd /root
 702          *      bind -c /root /
 703          * work.
 704          *
 705          * The check of mount->mflag allows things like
 706          *      mount fd /root
 707          *      bind -c /root /
 708          *
 709          * This is far more complicated than it should be, but I don't
 710          * see an easier way at the moment.
 711          */
 712         if((flag&MCREATE) && mh && mh->mount
 713         && (mh->mount->next || !(mh->mount->mflag&MCREATE)))
 714                 error(Emount);
 715
 716         pg = up->pgrp;
 717         wlock(&pg->ns);
 718
 719         l = &MOUNTH(pg, old->qid);
 720         for(m = *l; m; m = m->hash){
 721                 if(eqchan(m->from, old, 1))
 722                         break;
 723                 l = &m->hash;
 724         }
 725
 726         if(m == nil){
 727                 /*
 728                  *  nothing mounted here yet.  create a mount
 729                  *  head and add to the hash table.
 730                  */
 731                 m = newmhead(old);
 732                 *l = m;
 733
 734                 /*
 735                  *  if this is a union mount, add the old
 736                  *  node to the mount chain.
 737                  */
 738                 if(order != MREPL)
 739                         m->mount = newmount(m, old, 0, 0);
 740         }
 741         wlock(&m->lock);
 742         if(waserror()){
 743                 wunlock(&m->lock);
 744                 nexterror();
 745         }
 746         wunlock(&pg->ns);
 747
 748         nm = newmount(m, new, flag, spec);
 749         if(mh != nil && mh->mount != nil){
 750                 /*
 751                  *  copy a union when binding it onto a directory
 752                  */
 753                 flg = order;
 754                 if(order == MREPL)
 755                         flg = MAFTER;
 756                 h = &nm->next;
 757                 um = mh->mount;
 758                 for(um = um->next; um; um = um->next){
 759                         f = newmount(m, um->to, flg, um->spec);
 760                         *h = f;
 761                         h = &f->next;
 762                 }
 763         }
 764
 765         if(m->mount && order == MREPL){
 766                 mountfree(m->mount);
 767                 m->mount = 0;
 768         }
 769
 770         if(flag & MCREATE)
 771                 nm->mflag |= MCREATE;
 772
 773         if(m->mount && order == MAFTER){
 774                 for(f = m->mount; f->next; f = f->next)
 775                         ;
 776                 f->next = nm;
 777         }else{
 778                 for(f = nm; f->next; f = f->next)
 779                         ;
 780                 f->next = m->mount;
 781                 m->mount = nm;
 782         }
 783
 784         wunlock(&m->lock);
 785         poperror();
 786         return nm->mountid;
 787 }
 788
 789 void
 790 cunmount(Chan *mnt, Chan *mounted)
 791 {
 792         Pgrp *pg;
 793         Mhead *m, **l;
 794         Mount *f, **p;
 795
 796         if(mnt->umh)    /* should not happen */
 797                 print("cunmount newp extra umh %p has %p\n", mnt, mnt->umh);
 798
 799         /*
 800          * It _can_ happen that mounted->umh is non-nil,
 801          * because mounted is the result of namec(Aopen)
 802          * (see sysfile.c:/^sysunmount).
 803          * If we open a union directory, it will have a umh.
 804          * Although surprising, this is okay, since the
 805          * cclose will take care of freeing the umh.
 806          */
 807
 808         pg = up->pgrp;
 809         wlock(&pg->ns);
 810
 811         l = &MOUNTH(pg, mnt->qid);
 812         for(m = *l; m; m = m->hash){
 813                 if(eqchan(m->from, mnt, 1))
 814                         break;
 815                 l = &m->hash;
 816         }
 817
 818         if(m == 0){
 819                 wunlock(&pg->ns);
 820                 error(Eunmount);
 821         }
 822
 823         wlock(&m->lock);
 824         if(mounted == 0){
 825                 *l = m->hash;
 826                 wunlock(&pg->ns);
 827                 mountfree(m->mount);
 828                 m->mount = nil;
 829                 cclose(m->from);
 830                 wunlock(&m->lock);
 831                 putmhead(m);
 832                 return;
 833         }
 834
 835         p = &m->mount;
 836         for(f = *p; f; f = f->next){
 837                 /* BUG: Needs to be 2 pass */
 838                 if(eqchan(f->to, mounted, 1) ||
 839                   (f->to->mchan && eqchan(f->to->mchan, mounted, 1))){
 840                         *p = f->next;
 841                         f->next = 0;
 842                         mountfree(f);
 843                         if(m->mount == nil){
 844                                 *l = m->hash;
 845                                 cclose(m->from);
 846                                 wunlock(&m->lock);
 847                                 wunlock(&pg->ns);
 848                                 putmhead(m);
 849                                 return;
 850                         }
 851                         wunlock(&m->lock);
 852                         wunlock(&pg->ns);
 853                         return;
 854                 }
 855                 p = &f->next;
 856         }
 857         wunlock(&m->lock);
 858         wunlock(&pg->ns);
 859         error(Eunion);
 860 }
 861
 862 Chan*
 863 cclone(Chan *c)
 864 {
 865         Chan *nc;
 866         Walkqid *wq;
 867
 868         if(c == nil || c->ref < 1 || c->flag&CFREE)
 869                 panic("cclone: %#p", getcallerpc(&c));
 870         wq = devtab[c->type]->walk(c, nil, nil, 0);
 871         if(wq == nil)
 872                 error("clone failed");
 873         nc = wq->clone;
 874         free(wq);
 875         if((nc->path = c->path) != nil)
 876                 incref(c->path);
 877         return nc;
 878 }
 879
 880 /* also used by sysfile.c:/^mountfix */
 881 int
 882 findmount(Chan **cp, Mhead **mp, int type, int dev, Qid qid)
 883 {
 884         Pgrp *pg;
 885         Mhead *m;
 886
 887         pg = up->pgrp;
 888         rlock(&pg->ns);
 889         for(m = MOUNTH(pg, qid); m; m = m->hash){
 890                 rlock(&m->lock);
 891                 if(m->from == nil){
 892                         print("m %p m->from 0\n", m);
 893                         runlock(&m->lock);
 894                         continue;
 895                 }
 896                 if(eqchantdqid(m->from, type, dev, qid, 1)){
 897                         runlock(&pg->ns);
 898                         if(mp != nil){
 899                                 incref(m);
 900                                 if(*mp != nil)
 901                                         putmhead(*mp);
 902                                 *mp = m;
 903                         }
 904                         if(*cp != nil)
 905                                 cclose(*cp);
 906                         incref(m->mount->to);
 907                         *cp = m->mount->to;
 908                         runlock(&m->lock);
 909                         return 1;
 910                 }
 911                 runlock(&m->lock);
 912         }
 913
 914         runlock(&pg->ns);
 915         return 0;
 916 }
 917
 918 /*
 919  * Calls findmount but also updates path.
 920  */
 921 static int
 922 domount(Chan **cp, Mhead **mp, Path **path)
 923 {
 924         Chan **lc;
 925         Path *p;
 926
 927         if(findmount(cp, mp, (*cp)->type, (*cp)->dev, (*cp)->qid) == 0)
 928                 return 0;
 929
 930         if(path != nil){
 931                 p = *path;
 932                 p = uniquepath(p);
 933                 if(p->mlen <= 0)
 934                         print("domount: path %s has mlen==%d\n", p->s, p->mlen);
 935                 else{
 936                         lc = &p->mtpt[p->mlen-1];
 937 DBG("domount %p %s => add %p (was %p)\n", p, p->s, (*mp)->from, p->mtpt[p->mlen-1]);
 938                         incref((*mp)->from);
 939                         if(*lc != nil)
 940                                 cclose(*lc);
 941                         *lc = (*mp)->from;
 942                 }
 943                 *path = p;
 944         }
 945         return 1;
 946 }
 947
 948 /*
 949  * If c is the right-hand-side of a mount point, returns the left hand side.
 950  * Changes name to reflect the fact that we've uncrossed the mountpoint,
 951  * so name had better be ours to change!
 952  */
 953 static Chan*
 954 undomount(Chan *c, Path *path)
 955 {
 956         Chan *nc;
 957
 958         if(path->ref != 1 || path->mlen == 0)
 959                 print("undomount: path %s ref %ld mlen %d caller %#p\n",
 960                         path->s, path->ref, path->mlen, getcallerpc(&c));
 961
 962         if(path->mlen > 0 && (nc = path->mtpt[path->mlen-1]) != nil){
 963 DBG("undomount %p %s => remove %p\n", path, path->s, nc);
 964                 cclose(c);
 965                 path->mtpt[path->mlen-1] = nil;
 966                 c = nc;
 967         }
 968         return c;
 969 }
 970
 971 /*
 972  * Call dev walk but catch errors.
 973  */
 974 static Walkqid*
 975 ewalk(Chan *c, Chan *nc, char **name, int nname)
 976 {
 977         Walkqid *wq;
 978
 979         if(waserror())
 980                 return nil;
 981         wq = devtab[c->type]->walk(c, nc, name, nname);
 982         poperror();
 983         return wq;
 984 }
 985
 986 /*
 987  * Either walks all the way or not at all.  No partial results in *cp.
 988  * *nerror is the number of names to display in an error message.
 989  */
 990 static char Edoesnotexist[] = "does not exist";
 991 int
 992 walk(Chan **cp, char **names, int nnames, int nomount, int *nerror)
 993 {
 994         int dev, didmount, dotdot, i, n, nhave, ntry, type;
 995         Chan *c, *nc, *mtpt;
 996         Path *path;
 997         Mhead *mh, *nmh;
 998         Mount *f;
 999         Walkqid *wq;
1000
1001         c = *cp;
1002         incref(c);
1003         path = c->path;
1004         incref(path);
1005         mh = nil;
1006
1007         /*
1008          * While we haven't gotten all the way down the path:
1009          *    1. step through a mount point, if any
1010          *    2. send a walk request for initial dotdot or initial prefix without dotdot
1011          *    3. move to the first mountpoint along the way.
1012          *    4. repeat.
1013          *
1014          * Each time through the loop:
1015          *
1016          *      If didmount==0, c is on the undomount side of the mount point.
1017          *      If didmount==1, c is on the domount side of the mount point.
1018          *      Either way, c's full path is path.
1019          */
1020         didmount = 0;
1021         for(nhave=0; nhave<nnames; nhave+=n){
1022                 if((c->qid.type&QTDIR)==0){
1023                         if(nerror)
1024                                 *nerror = nhave;
1025                         pathclose(path);
1026                         cclose(c);
1027                         kstrcpy(up->errstr, Enotdir, ERRMAX);
1028                         if(mh != nil)
1029                                 putmhead(mh);
1030                         return -1;
1031                 }
1032                 ntry = nnames - nhave;
1033                 if(ntry > MAXWELEM)
1034                         ntry = MAXWELEM;
1035                 dotdot = 0;
1036                 for(i=0; i<ntry; i++){
1037                         if(isdotdot(names[nhave+i])){
1038                                 if(i==0){
1039                                         dotdot = 1;
1040                                         ntry = 1;
1041                                 }else
1042                                         ntry = i;
1043                                 break;
1044                         }
1045                 }
1046
1047                 if(!dotdot && !nomount && !didmount)
1048                         domount(&c, &mh, &path);
1049
1050                 type = c->type;
1051                 dev = c->dev;
1052
1053                 if((wq = ewalk(c, nil, names+nhave, ntry)) == nil){
1054                         /* try a union mount, if any */
1055                         if(mh && !nomount){
1056                                 /*
1057                                  * mh->mount->to == c, so start at mh->mount->next
1058                                  */
1059                                 rlock(&mh->lock);
1060                                 if((f = mh->mount) != nil)
1061                                         f = f->next;
1062                                 for(; f != nil; f = f->next)
1063                                         if((wq = ewalk(f->to, nil, names+nhave, ntry)) != nil){
1064                                                 type = f->to->type;
1065                                                 dev = f->to->dev;
1066                                                 break;
1067                                         }
1068                                 runlock(&mh->lock);
1069                         }
1070                         if(wq == nil){
1071                                 cclose(c);
1072                                 pathclose(path);
1073                                 if(nerror)
1074                                         *nerror = nhave+1;
1075                                 if(mh != nil)
1076                                         putmhead(mh);
1077                                 return -1;
1078                         }
1079                 }
1080
1081                 didmount = 0;
1082                 if(dotdot){
1083                         assert(wq->nqid == 1);
1084                         assert(wq->clone != nil);
1085
1086                         path = addelem(path, "..", nil);
1087                         nc = undomount(wq->clone, path);
1088                         nmh = nil;
1089                         n = 1;
1090                 }else{
1091                         nc = nil;
1092                         nmh = nil;
1093                         if(!nomount){
1094                                 for(i=0; i<wq->nqid && i<ntry-1; i++){
1095                                         if(findmount(&nc, &nmh, type, dev, wq->qid[i])){
1096                                                 didmount = 1;
1097                                                 break;
1098                                         }
1099                                 }
1100                         }
1101                         if(nc == nil){  /* no mount points along path */
1102                                 if(wq->clone == nil){
1103                                         cclose(c);
1104                                         pathclose(path);
1105                                         if(wq->nqid==0 || (wq->qid[wq->nqid-1].type&QTDIR)){
1106                                                 if(nerror)
1107                                                         *nerror = nhave+wq->nqid+1;
1108                                                 kstrcpy(up->errstr, Edoesnotexist, ERRMAX);
1109                                         }else{
1110                                                 if(nerror)
1111                                                         *nerror = nhave+wq->nqid;
1112                                                 kstrcpy(up->errstr, Enotdir, ERRMAX);
1113                                         }
1114                                         free(wq);
1115                                         if(mh != nil)
1116                                                 putmhead(mh);
1117                                         return -1;
1118                                 }
1119                                 n = wq->nqid;
1120                                 nc = wq->clone;
1121                         }else{          /* stopped early, at a mount point */
1122                                 didmount = 1;
1123                                 if(wq->clone != nil){
1124                                         cclose(wq->clone);
1125                                         wq->clone = nil;
1126                                 }
1127                                 n = i+1;
1128                         }
1129                         for(i=0; i<n; i++){
1130                                 mtpt = nil;
1131                                 if(i==n-1 && nmh!=nil)
1132                                         mtpt = nmh->from;
1133                                 path = addelem(path, names[nhave+i], mtpt);
1134                         }
1135                 }
1136                 cclose(c);
1137                 c = nc;
1138                 putmhead(mh);
1139                 mh = nmh;
1140                 free(wq);
1141         }
1142
1143         putmhead(mh);
1144
1145         c = cunique(c);
1146
1147         if(c->umh != nil){      //BUG
1148                 print("walk umh\n");
1149                 putmhead(c->umh);
1150                 c->umh = nil;
1151         }
1152
1153         pathclose(c->path);
1154         c->path = path;
1155
1156         cclose(*cp);
1157         *cp = c;
1158         if(nerror)
1159                 *nerror = nhave;
1160         return 0;
1161 }
1162
1163 /*
1164  * c is a mounted non-creatable directory.  find a creatable one.
1165  */
1166 Chan*
1167 createdir(Chan *c, Mhead *m)
1168 {
1169         Chan *nc;
1170         Mount *f;
1171
1172         rlock(&m->lock);
1173         if(waserror()){
1174                 runlock(&m->lock);
1175                 nexterror();
1176         }
1177         for(f = m->mount; f; f = f->next){
1178                 if(f->to != nil && (f->mflag&MCREATE) != 0){
1179                         nc = cclone(f->to);
1180                         runlock(&m->lock);
1181                         poperror();
1182                         cclose(c);
1183                         return nc;
1184                 }
1185         }
1186         error(Enocreate);
1187         return 0;
1188 }
1189
1190 void
1191 saveregisters(void)
1192 {
1193 }
1194
1195 static void
1196 growparse(Elemlist *e)
1197 {
1198         char **new;
1199         int *inew;
1200         enum { Delta = 8 };
1201
1202         if(e->nelems % Delta == 0){
1203                 new = smalloc((e->nelems+Delta) * sizeof(char*));
1204                 memmove(new, e->elems, e->nelems*sizeof(char*));
1205                 free(e->elems);
1206                 e->elems = new;
1207                 inew = smalloc((e->nelems+Delta+1) * sizeof(int));
1208                 memmove(inew, e->off, (e->nelems+1)*sizeof(int));
1209                 free(e->off);
1210                 e->off = inew;
1211         }
1212 }
1213
1214 /*
1215  * The name is known to be valid.
1216  * Copy the name so slashes can be overwritten.
1217  * An empty string will set nelem=0.
1218  * A path ending in / or /. or /.//./ etc. will have
1219  * e.mustbedir = 1, so that we correctly
1220  * reject, e.g., "/adm/users/." when /adm/users is a file
1221  * rather than a directory.
1222  */
1223 static void
1224 parsename(char *aname, Elemlist *e)
1225 {
1226         char *name, *slash;
1227
1228         kstrdup(&e->name, aname);
1229         name = e->name;
1230         e->nelems = 0;
1231         e->elems = nil;
1232         e->off = smalloc(sizeof(int));
1233         e->off[0] = skipslash(name) - name;
1234         for(;;){
1235                 name = skipslash(name);
1236                 if(*name == '\0'){
1237                         e->off[e->nelems] = name+strlen(name) - e->name;
1238                         e->mustbedir = 1;
1239                         break;
1240                 }
1241                 growparse(e);
1242                 e->elems[e->nelems++] = name;
1243                 slash = utfrune(name, '/');
1244                 if(slash == nil){
1245                         e->off[e->nelems] = name+strlen(name) - e->name;
1246                         e->mustbedir = 0;
1247                         break;
1248                 }
1249                 e->off[e->nelems] = slash - e->name;
1250                 *slash++ = '\0';
1251                 name = slash;
1252         }
1253
1254         if(0 && chandebug){
1255                 int i;
1256
1257                 print("parsename %s:", e->name);
1258                 for(i=0; i<=e->nelems; i++)
1259                         print(" %d", e->off[i]);
1260                 print("\n");
1261         }
1262 }
1263
1264 void*
1265 memrchr(void *va, int c, long n)
1266 {
1267         uchar *a, *e;
1268
1269         a = va;
1270         for(e=a+n-1; e>a; e--)
1271                 if(*e == c)
1272                         return e;
1273         return nil;
1274 }
1275
1276 void
1277 namelenerror(char *aname, int len, char *err)
1278 {
1279         char *ename, *name, *next;
1280         int i, errlen;
1281
1282         /*
1283          * If the name is short enough, just use the whole thing.
1284          */
1285         errlen = strlen(err);
1286         if(len < ERRMAX/3 || len+errlen < 2*ERRMAX/3)
1287                 snprint(up->genbuf, sizeof up->genbuf, "%.*s",
1288                         utfnlen(aname, len), aname);
1289         else{
1290                 /*
1291                  * Print a suffix of the name, but try to get a little info.
1292                  */
1293                 ename = aname+len;
1294                 next = ename;
1295                 do{
1296                         name = next;
1297                         next = memrchr(aname, '/', name-aname);
1298                         if(next == nil)
1299                                 next = aname;
1300                         len = ename-next;
1301                 }while(len < ERRMAX/3 || len + errlen < 2*ERRMAX/3);
1302
1303                 /*
1304                  * If the name is ridiculously long, chop it.
1305                  */
1306                 if(name == ename){
1307                         name = ename-ERRMAX/4;
1308                         if(name <= aname)
1309                                 panic("bad math in namelenerror");
1310                         /* walk out of current UTF sequence */
1311                         for(i=0; (*name&0xC0)==0x80 && i<UTFmax; i++)
1312                                 name++;
1313                 }
1314                 snprint(up->genbuf, sizeof up->genbuf, "...%.*s",
1315                         utfnlen(name, ename-name), name);
1316         }
1317         snprint(up->errstr, ERRMAX, "%#q %s", up->genbuf, err);
1318         nexterror();
1319 }
1320
1321 void
1322 nameerror(char *name, char *err)
1323 {
1324         namelenerror(name, strlen(name), err);
1325 }
1326
1327 /*
1328  * Turn a name into a channel.
1329  * &name[0] is known to be a valid address.  It may be a kernel address.
1330  *
1331  * Opening with amode Aopen, Acreate, Aremove, or Aaccess guarantees
1332  * that the result will be the only reference to that particular fid.
1333  * This is necessary since we might pass the result to
1334  * devtab[]->remove().
1335  *
1336  * Opening Atodir or Amount does not guarantee this.
1337  *
1338  * Under certain circumstances, opening Aaccess will cause
1339  * an unnecessary clone in order to get a cunique Chan so it
1340  * can attach the correct name.  Sysstat and sys_stat need the
1341  * correct name so they can rewrite the stat info.
1342  */
1343 Chan*
1344 namec(char *aname, int amode, int omode, ulong perm)
1345 {
1346         int len, n, t, nomount;
1347         Chan *c, *cnew;
1348         Path *path;
1349         Elemlist e;
1350         Rune r;
1351         Mhead *m;
1352         char *createerr, tmperrbuf[ERRMAX];
1353         char *name;
1354
1355         if(aname[0] == '\0')
1356                 error("empty file name");
1357         aname = validnamedup(aname, 1);
1358         if(waserror()){
1359                 free(aname);
1360                 nexterror();
1361         }
1362         DBG("namec %s %d %d\n", aname, amode, omode);
1363         name = aname;
1364
1365         /*
1366          * Find the starting off point (the current slash, the root of
1367          * a device tree, or the current dot) as well as the name to
1368          * evaluate starting there.
1369          */
1370         nomount = 0;
1371         switch(name[0]){
1372         case '/':
1373                 c = up->slash;
1374                 incref(c);
1375                 break;
1376
1377         case '#':
1378                 nomount = 1;
1379                 up->genbuf[0] = '\0';
1380                 n = 0;
1381                 while(*name != '\0' && (*name != '/' || n < 2)){
1382                         if(n >= sizeof(up->genbuf)-1)
1383                                 error(Efilename);
1384                         up->genbuf[n++] = *name++;
1385                 }
1386                 up->genbuf[n] = '\0';
1387                 /*
1388                  *  noattach is sandboxing.
1389                  *
1390                  *  the OK exceptions are:
1391                  *      |  it only gives access to pipes you create
1392                  *      d  this process's file descriptors
1393                  *      e  this process's environment
1394                  *  the iffy exceptions are:
1395                  *      c  time and pid, but also cons and consctl
1396                  *      p  control of your own processes (and unfortunately
1397                  *         any others left unprotected)
1398                  */
1399                 n = chartorune(&r, up->genbuf+1)+1;
1400                 /* actually / is caught by parsing earlier */
1401                 if(utfrune("M", r) != nil)
1402                         error(Enoattach);
1403                 if(up->pgrp->noattach && utfrune("|decp", r)==nil)
1404                         error(Enoattach);
1405                 t = devno(r, 1);
1406                 if(t == -1)
1407                         error(Ebadsharp);
1408                 c = devtab[t]->attach(up->genbuf+n);
1409                 break;
1410
1411         default:
1412                 c = up->dot;
1413                 incref(c);
1414                 break;
1415         }
1416
1417         e.aname = aname;
1418         e.prefix = name - aname;
1419         e.name = nil;
1420         e.elems = nil;
1421         e.off = nil;
1422         e.nelems = 0;
1423         e.nerror = 0;
1424         if(waserror()){
1425                 cclose(c);
1426                 free(e.name);
1427                 free(e.elems);
1428                 /*
1429                  * Prepare nice error, showing first e.nerror elements of name.
1430                  */
1431                 if(e.nerror == 0)
1432                         nexterror();
1433                 strcpy(tmperrbuf, up->errstr);
1434                 if(e.off[e.nerror]==0)
1435                         print("nerror=%d but off=%d\n",
1436                                 e.nerror, e.off[e.nerror]);
1437                 if(0 && chandebug)
1438                         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]);
1439                 len = e.prefix+e.off[e.nerror];
1440                 free(e.off);
1441                 namelenerror(aname, len, tmperrbuf);
1442         }
1443
1444         /*
1445          * Build a list of elements in the name.
1446          */
1447         parsename(name, &e);
1448
1449         /*
1450          * On create, ....
1451          */
1452         if(amode == Acreate){
1453                 /* perm must have DMDIR if last element is / or /. */
1454                 if(e.mustbedir && !(perm&DMDIR)){
1455                         e.nerror = e.nelems;
1456                         error("create without DMDIR");
1457                 }
1458
1459                 /* don't try to walk the last path element just yet. */
1460                 if(e.nelems == 0)
1461                         error(Eexist);
1462                 e.nelems--;
1463         }
1464
1465         if(walk(&c, e.elems, e.nelems, nomount, &e.nerror) < 0){
1466                 if(e.nerror < 0 || e.nerror > e.nelems){
1467                         print("namec %s walk error nerror=%d\n", aname, e.nerror);
1468                         e.nerror = 0;
1469                 }
1470                 nexterror();
1471         }
1472
1473         if(e.mustbedir && !(c->qid.type&QTDIR))
1474                 error("not a directory");
1475
1476         if(amode == Aopen && (omode&3) == OEXEC && (c->qid.type&QTDIR))
1477                 error("cannot exec directory");
1478
1479         switch(amode){
1480         case Abind:
1481                 /* no need to maintain path - cannot dotdot an Abind */
1482                 m = nil;
1483                 if(!nomount)
1484                         domount(&c, &m, nil);
1485                 if(c->umh != nil)
1486                         putmhead(c->umh);
1487                 c->umh = m;
1488                 break;
1489
1490         case Aaccess:
1491         case Aremove:
1492         case Aopen:
1493         Open:
1494                 /* save&update the name; domount might change c */
1495                 path = c->path;
1496                 incref(path);
1497                 if(waserror()){
1498                         pathclose(path);
1499                         nexterror();
1500                 }
1501                 m = nil;
1502                 if(!nomount)
1503                         domount(&c, &m, &path);
1504
1505                 /* our own copy to open or remove */
1506                 c = cunique(c);
1507
1508                 /* now it's our copy anyway, we can put the name back */
1509                 pathclose(c->path);
1510                 c->path = path;
1511                 poperror();
1512
1513                 /* record whether c is on a mount point */
1514                 c->ismtpt = m!=nil;
1515
1516                 switch(amode){
1517                 case Aaccess:
1518                 case Aremove:
1519                         putmhead(m);
1520                         break;
1521
1522                 case Aopen:
1523                 case Acreate:
1524 if(c->umh != nil){
1525         print("cunique umh Open\n");
1526         putmhead(c->umh);
1527         c->umh = nil;
1528 }
1529                         /* only save the mount head if it's a multiple element union */
1530                         if(m != nil && m->mount != nil && m->mount->next != nil)
1531                                 c->umh = m;
1532                         else
1533                                 putmhead(m);
1534
1535                         /* save registers else error() in open has wrong value of c saved */
1536                         saveregisters();
1537
1538                         c = devtab[c->type]->open(c, omode&~OCEXEC);
1539
1540                         if(omode & OCEXEC)
1541                                 c->flag |= CCEXEC;
1542                         if(omode & ORCLOSE)
1543                                 c->flag |= CRCLOSE;
1544                         break;
1545                 }
1546                 break;
1547
1548         case Atodir:
1549                 /*
1550                  * Directories (e.g. for cd) are left before the mount point,
1551                  * so one may mount on / or . and see the effect.
1552                  */
1553                 if(!(c->qid.type & QTDIR))
1554                         error(Enotdir);
1555                 break;
1556
1557         case Amount:
1558                 /*
1559                  * When mounting on an already mounted upon directory,
1560                  * one wants subsequent mounts to be attached to the
1561                  * original directory, not the replacement.  Don't domount.
1562                  */
1563                 break;
1564
1565         case Acreate:
1566                 /*
1567                  * We've already walked all but the last element.
1568                  * If the last exists, try to open it OTRUNC.
1569                  * If omode&OEXCL is set, just give up.
1570                  */
1571                 e.nelems++;
1572                 e.nerror++;
1573                 if(walk(&c, e.elems+e.nelems-1, 1, nomount, nil) == 0){
1574                         if(omode&OEXCL)
1575                                 error(Eexist);
1576                         omode |= OTRUNC;
1577                         goto Open;
1578                 }
1579
1580                 /*
1581                  * The semantics of the create(2) system call are that if the
1582                  * file exists and can be written, it is to be opened with truncation.
1583                  * On the other hand, the create(5) message fails if the file exists.
1584                  * If we get two create(2) calls happening simultaneously,
1585                  * they might both get here and send create(5) messages, but only
1586                  * one of the messages will succeed.  To provide the expected create(2)
1587                  * semantics, the call with the failed message needs to try the above
1588                  * walk again, opening for truncation.  This correctly solves the
1589                  * create/create race, in the sense that any observable outcome can
1590                  * be explained as one happening before the other.
1591                  * The create/create race is quite common.  For example, it happens
1592                  * when two rc subshells simultaneously update the same
1593                  * environment variable.
1594                  *
1595                  * The implementation still admits a create/create/remove race:
1596                  * (A) walk to file, fails
1597                  * (B) walk to file, fails
1598                  * (A) create file, succeeds, returns
1599                  * (B) create file, fails
1600                  * (A) remove file, succeeds, returns
1601                  * (B) walk to file, return failure.
1602                  *
1603                  * This is hardly as common as the create/create race, and is really
1604                  * not too much worse than what might happen if (B) got a hold of a
1605                  * file descriptor and then the file was removed -- either way (B) can't do
1606                  * anything with the result of the create call.  So we don't care about this race.
1607                  *
1608                  * Applications that care about more fine-grained decision of the races
1609                  * can use the OEXCL flag to get at the underlying create(5) semantics;
1610                  * by default we provide the common case.
1611                  *
1612                  * We need to stay behind the mount point in case we
1613                  * need to do the first walk again (should the create fail).
1614                  *
1615                  * We also need to cross the mount point and find the directory
1616                  * in the union in which we should be creating.
1617                  *
1618                  * The channel staying behind is c, the one moving forward is cnew.
1619                  */
1620                 m = nil;
1621                 cnew = nil;     /* is this assignment necessary? */
1622                 if(!waserror()){        /* try create */
1623                         if(!nomount && findmount(&cnew, &m, c->type, c->dev, c->qid))
1624                                 cnew = createdir(cnew, m);
1625                         else{
1626                                 cnew = c;
1627                                 incref(cnew);
1628                         }
1629
1630                         /*
1631                          * We need our own copy of the Chan because we're
1632                          * about to send a create, which will move it.  Once we have
1633                          * our own copy, we can fix the name, which might be wrong
1634                          * if findmount gave us a new Chan.
1635                          */
1636                         cnew = cunique(cnew);
1637                         pathclose(cnew->path);
1638                         cnew->path = c->path;
1639                         incref(cnew->path);
1640
1641                         cnew = devtab[cnew->type]->create(cnew, e.elems[e.nelems-1], omode&~(OEXCL|OCEXEC), perm);
1642                         poperror();
1643                         if(omode & OCEXEC)
1644                                 cnew->flag |= CCEXEC;
1645                         if(omode & ORCLOSE)
1646                                 cnew->flag |= CRCLOSE;
1647                         if(m != nil)
1648                                 putmhead(m);
1649                         cclose(c);
1650                         c = cnew;
1651                         c->path = addelem(c->path, e.elems[e.nelems-1], nil);
1652                         break;
1653                 }
1654
1655                 /* create failed */
1656                 cclose(cnew);
1657                 if(m != nil)
1658                         putmhead(m);
1659                 if(omode & OEXCL)
1660                         nexterror();
1661                 /* save error */
1662                 createerr = up->errstr;
1663                 up->errstr = tmperrbuf;
1664                 /* note: we depend that walk does not error */
1665                 if(walk(&c, e.elems+e.nelems-1, 1, nomount, nil) < 0){
1666                         up->errstr = createerr;
1667                         error(createerr);       /* report true error */
1668                 }
1669                 up->errstr = createerr;
1670                 omode |= OTRUNC;
1671                 goto Open;
1672
1673         default:
1674                 panic("unknown namec access %d", amode);
1675         }
1676
1677         /* place final element in genbuf for e.g. exec */
1678         if(e.nelems > 0)
1679                 kstrcpy(up->genbuf, e.elems[e.nelems-1], sizeof up->genbuf);
1680         else
1681                 kstrcpy(up->genbuf, ".", sizeof up->genbuf);
1682         free(e.name);
1683         free(e.elems);
1684         free(e.off);
1685         poperror();     /* e c */
1686         free(aname);
1687         poperror();     /* aname */
1688
1689         return c;
1690 }
1691
1692 /*
1693  * name is valid. skip leading / and ./ as much as possible
1694  */
1695 char*
1696 skipslash(char *name)
1697 {
1698         while(name[0]=='/' || (name[0]=='.' && (name[1]==0 || name[1]=='/')))
1699                 name++;
1700         return name;
1701 }
1702
1703 char isfrog[256]={
1704         /*NUL*/ 1, 1, 1, 1, 1, 1, 1, 1,
1705         /*BKS*/ 1, 1, 1, 1, 1, 1, 1, 1,
1706         /*DLE*/ 1, 1, 1, 1, 1, 1, 1, 1,
1707         /*CAN*/ 1, 1, 1, 1, 1, 1, 1, 1,
1708         ['/']   1,
1709         [0x7f]  1,
1710 };
1711
1712 /*
1713  * Check that the name
1714  *  a) is in valid memory.
1715  *  b) is shorter than 2^16 bytes, so it can fit in a 9P string field.
1716  *  c) contains no frogs.
1717  * The first byte is known to be addressible by the requester, so the
1718  * routine works for kernel and user memory both.
1719  * The parameter slashok flags whether a slash character is an error
1720  * or a valid character.
1721  *
1722  * The parameter dup flags whether the string should be copied
1723  * out of user space before being scanned the second time.
1724  * (Otherwise a malicious thread could remove the NUL, causing us
1725  * to access unchecked addresses.)
1726  */
1727 static char*
1728 validname0(char *aname, int slashok, int dup, uintptr pc)
1729 {
1730         char *ename, *name, *s;
1731         int c, n;
1732         Rune r;
1733
1734         name = aname;
1735         if((uintptr)name < KZERO){
1736                 if(!dup)
1737                         print("warning: validname called from %#p with user pointer", pc);
1738                 ename = vmemchr(name, 0, (1<<16));
1739         }else
1740                 ename = memchr(name, 0, (1<<16));
1741
1742         if(ename==nil || ename-name>=(1<<16))
1743                 error("name too long");
1744
1745         s = nil;
1746         if(dup){
1747                 n = ename-name;
1748                 s = smalloc(n+1);
1749                 memmove(s, name, n);
1750                 s[n] = 0;
1751                 aname = s;
1752                 name = s;
1753                 setmalloctag(s, pc);
1754         }
1755
1756         while(*name){
1757                 /* all characters above '~' are ok */
1758                 c = *(uchar*)name;
1759                 if(c >= Runeself)
1760                         name += chartorune(&r, name);
1761                 else{
1762                         if(isfrog[c])
1763                                 if(!slashok || c!='/'){
1764                                         snprint(up->genbuf, sizeof(up->genbuf), "%s: %q", Ebadchar, aname);
1765                                         free(s);
1766                                         error(up->genbuf);
1767                         }
1768                         name++;
1769                 }
1770         }
1771         return s;
1772 }
1773
1774 void
1775 validname(char *aname, int slashok)
1776 {
1777         validname0(aname, slashok, 0, getcallerpc(&aname));
1778 }
1779
1780 char*
1781 validnamedup(char *aname, int slashok)
1782 {
1783         return validname0(aname, slashok, 1, getcallerpc(&aname));
1784 }
1785
1786 void
1787 isdir(Chan *c)
1788 {
1789         if(c->qid.type & QTDIR)
1790                 return;
1791         error(Enotdir);
1792 }
1793
1794 /*
1795  * This is necessary because there are many
1796  * pointers to the top of a given mount list:
1797  *
1798  *      - the mhead in the namespace hash table
1799  *      - the mhead in chans returned from findmount:
1800  *        used in namec and then by unionread.
1801  *      - the mhead in chans returned from createdir:
1802  *        used in the open/create race protect, which is gone.
1803  *
1804  * The RWlock in the Mhead protects the mount list it contains.
1805  * The mount list is deleted when we cunmount.
1806  * The RWlock ensures that nothing is using the mount list at that time.
1807  *
1808  * It is okay to replace c->mh with whatever you want as
1809  * long as you are sure you have a unique reference to it.
1810  *
1811  * This comment might belong somewhere else.
1812  */
1813 void
1814 putmhead(Mhead *m)
1815 {
1816         if(m != nil && decref(m) == 0){
1817                 m->mount = (Mount*)0xCafeBeef;
1818                 free(m);
1819         }
1820 }
1821