lib/tempname.c

   1 /* Copyright (C) 1991-2020 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #if !_LIBC
  19 # include <libc-config.h>
  20 # include "tempname.h"
  21 #endif
  22
  23 #include <sys/types.h>
  24 #include <assert.h>
  25
  26 #include <errno.h>
  27
  28 #include <stdio.h>
  29 #ifndef P_tmpdir
  30 # define P_tmpdir "/tmp"
  31 #endif
  32 #ifndef TMP_MAX
  33 # define TMP_MAX 238328
  34 #endif
  35 #ifndef __GT_FILE
  36 # define __GT_FILE      0
  37 # define __GT_DIR       1
  38 # define __GT_NOCREATE  2
  39 #endif
  40 #if !_LIBC && (GT_FILE != __GT_FILE || GT_DIR != __GT_DIR       \
  41                || GT_NOCREATE != __GT_NOCREATE)
  42 # error report this to bug-gnulib@gnu.org
  43 #endif
  44
  45 #include <stddef.h>
  46 #include <stdlib.h>
  47 #include <string.h>
  48
  49 #include <fcntl.h>
  50 #include <stdalign.h>
  51 #include <stdint.h>
  52 #include <sys/random.h>
  53 #include <sys/stat.h>
  54 #include <time.h>
  55
  56 #if _LIBC
  57 # define struct_stat64 struct stat64
  58 # define __secure_getenv __libc_secure_getenv
  59 #else
  60 # define struct_stat64 struct stat
  61 # define __gen_tempname gen_tempname
  62 # define __mkdir mkdir
  63 # define __open open
  64 # define __lxstat64(version, file, buf) lstat (file, buf)
  65 # define __getrandom getrandom
  66 # define __clock_gettime64 clock_gettime
  67 # define __timespec64 timespec
  68 #endif
  69
  70 /* Use getrandom if it works, falling back on a 64-bit linear
  71    congruential generator that starts with Var's value
  72    mixed in with a clock's low-order bits if available.  */
  73 typedef uint_fast64_t random_value;
  74 #define RANDOM_VALUE_MAX UINT_FAST64_MAX
  75 #define BASE_62_DIGITS 10 /* 62**10 < UINT_FAST64_MAX */
  76 #define BASE_62_POWER (62LL * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62 * 62)
  77
  78 static random_value
  79 random_bits (random_value var)
  80 {
  81   random_value r;
  82   if (__getrandom (&r, sizeof r, 0) == sizeof r)
  83     return r;
  84 #if _LIBC || (defined CLOCK_MONOTONIC && HAVE_CLOCK_GETTIME)
  85   /* Add entropy if getrandom is not supported.  */
  86   struct __timespec64 tv;
  87   __clock_gettime64 (CLOCK_MONOTONIC, &tv);
  88   var ^= tv.tv_nsec;
  89 #endif
  90   return 2862933555777941757 * var + 3037000493;
  91 }
  92
  93 #if _LIBC
  94 /* Return nonzero if DIR is an existent directory.  */
  95 static int
  96 direxists (const char *dir)
  97 {
  98   struct_stat64 buf;
  99   return __xstat64 (_STAT_VER, dir, &buf) == 0 && S_ISDIR (buf.st_mode);
 100 }
 101
 102 /* Path search algorithm, for tmpnam, tmpfile, etc.  If DIR is
 103    non-null and exists, uses it; otherwise uses the first of $TMPDIR,
 104    P_tmpdir, /tmp that exists.  Copies into TMPL a template suitable
 105    for use with mk[s]temp.  Will fail (-1) if DIR is non-null and
 106    doesn't exist, none of the searched dirs exists, or there's not
 107    enough space in TMPL. */
 108 int
 109 __path_search (char *tmpl, size_t tmpl_len, const char *dir, const char *pfx,
 110                int try_tmpdir)
 111 {
 112   const char *d;
 113   size_t dlen, plen;
 114
 115   if (!pfx || !pfx[0])
 116     {
 117       pfx = "file";
 118       plen = 4;
 119     }
 120   else
 121     {
 122       plen = strlen (pfx);
 123       if (plen > 5)
 124         plen = 5;
 125     }
 126
 127   if (try_tmpdir)
 128     {
 129       d = __secure_getenv ("TMPDIR");
 130       if (d != NULL && direxists (d))
 131         dir = d;
 132       else if (dir != NULL && direxists (dir))
 133         /* nothing */ ;
 134       else
 135         dir = NULL;
 136     }
 137   if (dir == NULL)
 138     {
 139       if (direxists (P_tmpdir))
 140         dir = P_tmpdir;
 141       else if (strcmp (P_tmpdir, "/tmp") != 0 && direxists ("/tmp"))
 142         dir = "/tmp";
 143       else
 144         {
 145           __set_errno (ENOENT);
 146           return -1;
 147         }
 148     }
 149
 150   dlen = strlen (dir);
 151   while (dlen > 1 && dir[dlen - 1] == '/')
 152     dlen--;                     /* remove trailing slashes */
 153
 154   /* check we have room for "${dir}/${pfx}XXXXXX\0" */
 155   if (tmpl_len < dlen + 1 + plen + 6 + 1)
 156     {
 157       __set_errno (EINVAL);
 158       return -1;
 159     }
 160
 161   sprintf (tmpl, "%.*s/%.*sXXXXXX", (int) dlen, dir, (int) plen, pfx);
 162   return 0;
 163 }
 164 #endif /* _LIBC */
 165
 166 #if _LIBC
 167 static int try_tempname_len (char *, int, void *, int (*) (char *, void *),
 168                              size_t);
 169 #endif
 170
 171 static int
 172 try_file (char *tmpl, void *flags)
 173 {
 174   int *openflags = flags;
 175   return __open (tmpl,
 176                  (*openflags & ~O_ACCMODE)
 177                  | O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
 178 }
 179
 180 static int
 181 try_dir (char *tmpl, void *flags _GL_UNUSED)
 182 {
 183   return __mkdir (tmpl, S_IRUSR | S_IWUSR | S_IXUSR);
 184 }
 185
 186 static int
 187 try_nocreate (char *tmpl, void *flags _GL_UNUSED)
 188 {
 189   struct_stat64 st;
 190
 191   if (__lxstat64 (_STAT_VER, tmpl, &st) == 0 || errno == EOVERFLOW)
 192     __set_errno (EEXIST);
 193   return errno == ENOENT ? 0 : -1;
 194 }
 195
 196 /* These are the characters used in temporary file names.  */
 197 static const char letters[] =
 198 "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
 199
 200 /* Generate a temporary file name based on TMPL.  TMPL must match the
 201    rules for mk[s]temp (i.e., end in at least X_SUFFIX_LEN "X"s,
 202    possibly with a suffix).
 203    The name constructed does not exist at the time of the call to
 204    this function.  TMPL is overwritten with the result.
 205
 206    KIND may be one of:
 207    __GT_NOCREATE:       simply verify that the name does not exist
 208                         at the time of the call.
 209    __GT_FILE:           create the file using open(O_CREAT|O_EXCL)
 210                         and return a read-write fd.  The file is mode 0600.
 211    __GT_DIR:            create a directory, which will be mode 0700.
 212
 213    We use a clever algorithm to get hard-to-predict names. */
 214 #ifdef _LIBC
 215 static
 216 #endif
 217 int
 218 gen_tempname_len (char *tmpl, int suffixlen, int flags, int kind,
 219                   size_t x_suffix_len)
 220 {
 221   static int (*const tryfunc[]) (char *, void *) =
 222     {
 223       [__GT_FILE] = try_file,
 224       [__GT_DIR] = try_dir,
 225       [__GT_NOCREATE] = try_nocreate
 226     };
 227   return try_tempname_len (tmpl, suffixlen, &flags, tryfunc[kind],
 228                            x_suffix_len);
 229 }
 230
 231 #ifdef _LIBC
 232 static
 233 #endif
 234 int
 235 try_tempname_len (char *tmpl, int suffixlen, void *args,
 236                   int (*tryfunc) (char *, void *), size_t x_suffix_len)
 237 {
 238   size_t len;
 239   char *XXXXXX;
 240   unsigned int count;
 241   int fd = -1;
 242   int save_errno = errno;
 243
 244   /* A lower bound on the number of temporary files to attempt to
 245      generate.  The maximum total number of temporary file names that
 246      can exist for a given template is 62**6.  It should never be
 247      necessary to try all of these combinations.  Instead if a reasonable
 248      number of names is tried (we define reasonable as 62**3) fail to
 249      give the system administrator the chance to remove the problems.
 250      This value requires that X_SUFFIX_LEN be at least 3.  */
 251 #define ATTEMPTS_MIN (62 * 62 * 62)
 252
 253   /* The number of times to attempt to generate a temporary file.  To
 254      conform to POSIX, this must be no smaller than TMP_MAX.  */
 255 #if ATTEMPTS_MIN < TMP_MAX
 256   unsigned int attempts = TMP_MAX;
 257 #else
 258   unsigned int attempts = ATTEMPTS_MIN;
 259 #endif
 260
 261   /* A random variable.  The initial value is used only the for fallback path
 262      on 'random_bits' on 'getrandom' failure.  Its initial value tries to use
 263      some entropy from the ASLR and ignore possible bits from the stack
 264      alignment.  */
 265   random_value v = ((uintptr_t) &v) / alignof (max_align_t);
 266
 267   /* How many random base-62 digits can currently be extracted from V.  */
 268   int vdigits = 0;
 269
 270   /* Least unfair value for V.  If V is less than this, V can generate
 271      BASE_62_DIGITS digits fairly.  Otherwise it might be biased.  */
 272   random_value const unfair_min
 273     = RANDOM_VALUE_MAX - RANDOM_VALUE_MAX % BASE_62_POWER;
 274
 275   len = strlen (tmpl);
 276   if (len < x_suffix_len + suffixlen
 277       || strspn (&tmpl[len - x_suffix_len - suffixlen], "X") < x_suffix_len)
 278     {
 279       __set_errno (EINVAL);
 280       return -1;
 281     }
 282
 283   /* This is where the Xs start.  */
 284   XXXXXX = &tmpl[len - x_suffix_len - suffixlen];
 285
 286   for (count = 0; count < attempts; ++count)
 287     {
 288       for (size_t i = 0; i < x_suffix_len; i++)
 289         {
 290           if (vdigits == 0)
 291             {
 292               do
 293                 v = random_bits (v);
 294               while (unfair_min <= v);
 295
 296               vdigits = BASE_62_DIGITS;
 297             }
 298
 299           XXXXXX[i] = letters[v % 62];
 300           v /= 62;
 301           vdigits--;
 302         }
 303
 304       fd = tryfunc (tmpl, args);
 305       if (fd >= 0)
 306         {
 307           __set_errno (save_errno);
 308           return fd;
 309         }
 310       else if (errno != EEXIST)
 311         return -1;
 312     }
 313
 314   /* We got out of the loop because we ran out of combinations to try.  */
 315   __set_errno (EEXIST);
 316   return -1;
 317 }
 318
 319 int
 320 __gen_tempname (char *tmpl, int suffixlen, int flags, int kind)
 321 {
 322   return gen_tempname_len (tmpl, suffixlen, flags, kind, 6);
 323 }
 324
 325 #if !_LIBC
 326 int
 327 try_tempname (char *tmpl, int suffixlen, void *args,
 328               int (*tryfunc) (char *, void *))
 329 {
 330   return try_tempname_len (tmpl, suffixlen, args, tryfunc, 6);
 331 }
 332 #endif